<!-- doc/src/sgml/config.sgml -->

<chapter id="runtime-config">
  <title>Server Configuration</title>

  <indexterm>
   <primary>configuration</primary>
   <secondary>of the server</secondary>
  </indexterm>

  <para>
   There are many configuration parameters that affect the behavior of
   the database system. In the first section of this chapter we
   describe how to interact with configuration parameters. The subsequent sections
   discuss each parameter in detail.
  </para>

  <sect1 id="config-setting">
   <title>Setting Parameters</title>

   <sect2 id="config-setting-names-values">
    <title>Parameter Names and Values</title>

    <para>
     All parameter names are case-insensitive. Every parameter takes a
     value of one of five types: boolean, string, integer, floating point,
     or enumerated (enum).  The type determines the syntax for setting the
     parameter:
    </para>

    <itemizedlist>
     <listitem>
      <para>
       <emphasis>Boolean:</emphasis>
       Values can be written as
       <literal>on</literal>,
       <literal>off</literal>,
       <literal>true</literal>,
       <literal>false</literal>,
       <literal>yes</literal>,
       <literal>no</literal>,
       <literal>1</literal>,
       <literal>0</literal>
       (all case-insensitive) or any unambiguous prefix of one of these.
      </para>
     </listitem>

     <listitem>
      <para>
       <emphasis>String:</emphasis>
       In general, enclose the value in single quotes, doubling any single
       quotes within the value.  Quotes can usually be omitted if the value
       is a simple number or identifier, however.
      </para>
     </listitem>

     <listitem>
      <para>
       <emphasis>Numeric (integer and floating point):</emphasis>
       A decimal point is permitted only for floating-point parameters.
       Do not use thousands separators.  Quotes are not required.
      </para>
     </listitem>

     <listitem>
      <para>
       <emphasis>Numeric with Unit:</emphasis>
       Some numeric parameters have an implicit unit, because they describe
       quantities of memory or time. The unit might be bytes, kilobytes, blocks
       (typically eight kilobytes), milliseconds, seconds, or minutes.
       An unadorned numeric value for one of these settings will use the
       setting's default unit, which can be learned from
       <structname>pg_settings</structname>.<structfield>unit</structfield>.
       For convenience, settings can be given with a unit specified explicitly,
       for example <literal>'120 ms'</literal> for a time value, and they will be
       converted to whatever the parameter's actual unit is.  Note that the
       value must be written as a string (with quotes) to use this feature.
       The unit name is case-sensitive, and there can be whitespace between
       the numeric value and the unit.

       <itemizedlist>
        <listitem>
         <para>
          Valid memory units are <literal>B</literal> (bytes),
          <literal>kB</literal> (kilobytes),
          <literal>MB</literal> (megabytes), <literal>GB</literal>
          (gigabytes), and <literal>TB</literal> (terabytes).
          The multiplier for memory units is 1024, not 1000.
         </para>
        </listitem>

        <listitem>
         <para>
          Valid time units are <literal>ms</literal> (milliseconds),
          <literal>s</literal> (seconds), <literal>min</literal> (minutes),
          <literal>h</literal> (hours), and <literal>d</literal> (days).
         </para>
        </listitem>
       </itemizedlist>
      </para>
     </listitem>

     <listitem>
      <para>
       <emphasis>Enumerated:</emphasis>
       Enumerated-type parameters are written in the same way as string
       parameters, but are restricted to have one of a limited set of
       values.  The values allowable for such a parameter can be found from
       <structname>pg_settings</structname>.<structfield>enumvals</structfield>.
       Enum parameter values are case-insensitive.
      </para>
     </listitem>
    </itemizedlist>
   </sect2>

   <sect2 id="config-setting-configuration-file">
    <title>Parameter Interaction via the Configuration File</title>

    <para>
     The most fundamental way to set these parameters is to edit the file
     <filename>postgresql.conf</filename><indexterm><primary>postgresql.conf</primary></indexterm>,
     which is normally kept in the data directory.  A default copy is
     installed when the database cluster directory is initialized.
     An example of what this file might look like is:
<programlisting>
# This is a comment
log_connections = yes
log_destination = 'syslog'
search_path = '"$user", public'
shared_buffers = 128MB
</programlisting>
     One parameter is specified per line. The equal sign between name and
     value is optional. Whitespace is insignificant (except within a quoted
     parameter value) and blank lines are
     ignored. Hash marks (<literal>#</literal>) designate the remainder
     of the line as a comment.  Parameter values that are not simple
     identifiers or numbers must be single-quoted.  To embed a single
     quote in a parameter value, write either two quotes (preferred)
     or backslash-quote.
     If the file contains multiple entries for the same parameter,
     all but the last one are ignored.
    </para>

    <para>
     Parameters set in this way provide default values for the cluster.
     The settings seen by active sessions will be these values unless they
     are overridden.  The following sections describe ways in which the
     administrator or user can override these defaults.
    </para>

    <para>
     <indexterm>
      <primary>SIGHUP</primary>
     </indexterm>
     The configuration file is reread whenever the main server process
     receives a <systemitem>SIGHUP</systemitem> signal; this signal is most easily
     sent by running <literal>pg_ctl reload</literal> from the command line or by
     calling the SQL function <function>pg_reload_conf()</function>. The main
     server process also propagates this signal to all currently running
     server processes, so that existing sessions also adopt the new values
     (this will happen after they complete any currently-executing client
     command).  Alternatively, you can
     send the signal to a single server process directly.  Some parameters
     can only be set at server start; any changes to their entries in the
     configuration file will be ignored until the server is restarted.
     Invalid parameter settings in the configuration file are likewise
     ignored (but logged) during <systemitem>SIGHUP</systemitem> processing.
    </para>

    <para>
     In addition to <filename>postgresql.conf</filename>,
     a <productname>PostgreSQL</productname> data directory contains a file
     <filename>postgresql.auto.conf</filename><indexterm><primary>postgresql.auto.conf</primary></indexterm>,
     which has the same format as <filename>postgresql.conf</filename> but
     is intended to be edited automatically not manually.  This file holds
     settings provided through the <xref linkend="sql-altersystem"/> command.
     This file is read whenever <filename>postgresql.conf</filename> is,
     and its settings take effect in the same way.  Settings
     in <filename>postgresql.auto.conf</filename> override those
     in <filename>postgresql.conf</filename>.
    </para>

    <para>
     External tools may also
     modify <filename>postgresql.auto.conf</filename>.  It is not
     recommended to do this while the server is running, since a
     concurrent <command>ALTER SYSTEM</command> command could overwrite
     such changes.  Such tools might simply append new settings to the end,
     or they might choose to remove duplicate settings and/or comments
     (as <command>ALTER SYSTEM</command> will).
    </para>

    <para>
     The system view
     <link linkend="view-pg-file-settings"><structname>pg_file_settings</structname></link>
     can be helpful for pre-testing changes to the configuration files, or for
     diagnosing problems if a <systemitem>SIGHUP</systemitem> signal did not have the
     desired effects.
    </para>
   </sect2>

   <sect2 id="config-setting-sql-command-interaction">
    <title>Parameter Interaction via SQL</title>

     <para>
      <productname>PostgreSQL</productname> provides three SQL
      commands to establish configuration defaults.
      The already-mentioned <xref linkend="sql-altersystem"/> command
      provides a SQL-accessible means of changing global defaults; it is
      functionally equivalent to editing <filename>postgresql.conf</filename>.
      In addition, there are two commands that allow setting of defaults
      on a per-database or per-role basis:
     </para>

     <itemizedlist>
     <listitem>
      <para>
       The <xref linkend="sql-alterdatabase"/> command allows global
       settings to be overridden on a per-database basis.
      </para>
     </listitem>

     <listitem>
      <para>
       The <xref linkend="sql-alterrole"/> command allows both global and
       per-database settings to be overridden with user-specific values.
      </para>
     </listitem>
    </itemizedlist>

     <para>
      Values set with <command>ALTER DATABASE</command> and <command>ALTER ROLE</command>
      are applied only when starting a fresh database session.  They
      override values obtained from the configuration files or server
      command line, and constitute defaults for the rest of the session.
      Note that some settings cannot be changed after server start, and
      so cannot be set with these commands (or the ones listed below).
    </para>

     <para>
      Once a client is connected to the database, <productname>PostgreSQL</productname>
      provides two additional SQL commands (and equivalent functions) to
      interact with session-local configuration settings:
    </para>

    <itemizedlist>
     <listitem>
     <para>
      The <xref linkend="sql-show"/> command allows inspection of the
      current value of all parameters.  The corresponding function is
      <function>current_setting(setting_name text)</function>.
     </para>
     </listitem>

     <listitem>
      <para>
       The <xref linkend="sql-set"/> command allows modification of the
       current value of those parameters that can be set locally to a
       session; it has no effect on other sessions.
       The corresponding function is
       <function>set_config(setting_name, new_value, is_local)</function>.
      </para>
     </listitem>
    </itemizedlist>

    <para>
     In addition, the system view <link
     linkend="view-pg-settings"><structname>pg_settings</structname></link> can be
     used to view and change session-local values:
    </para>

    <itemizedlist>
     <listitem>
      <para>
       Querying this view is similar to using <command>SHOW ALL</command> but
       provides more detail.  It is also more flexible, since it's possible
       to specify filter conditions or join against other relations.
      </para>
     </listitem>

     <listitem>
      <para>
       Using <xref linkend="sql-update"/> on this view, specifically
       updating the <structname>setting</structname> column, is the equivalent
       of issuing <command>SET</command> commands.  For example, the equivalent of
<programlisting>
SET configuration_parameter TO DEFAULT;
</programlisting>
       is:
<programlisting>
UPDATE pg_settings SET setting = reset_val WHERE name = 'configuration_parameter';
</programlisting>
      </para>
     </listitem>
    </itemizedlist>

   </sect2>

   <sect2>
    <title>Parameter Interaction via the Shell</title>

     <para>
      In addition to setting global defaults or attaching
      overrides at the database or role level, you can pass settings to
      <productname>PostgreSQL</productname> via shell facilities.
      Both the server and <application>libpq</application> client library
      accept parameter values via the shell.
     </para>

     <itemizedlist>
      <listitem>
      <para>
       During server startup, parameter settings can be
       passed to the <command>postgres</command> command via the
       <option>-c</option> command-line parameter.  For example,
<programlisting>
postgres -c log_connections=yes -c log_destination='syslog'
</programlisting>
       Settings provided in this way override those set via
       <filename>postgresql.conf</filename> or <command>ALTER SYSTEM</command>,
       so they cannot be changed globally without restarting the server.
     </para>
    </listitem>

    <listitem>
     <para>
      When starting a client session via <application>libpq</application>,
      parameter settings can be
      specified using the <envar>PGOPTIONS</envar> environment variable.
      Settings established in this way constitute defaults for the life
      of the session, but do not affect other sessions.
      For historical reasons, the format of <envar>PGOPTIONS</envar> is
      similar to that used when launching the <command>postgres</command>
      command; specifically, the <option>-c</option> flag must be specified.
      For example,
<programlisting>
env PGOPTIONS="-c geqo=off -c statement_timeout=5min" psql
</programlisting>
     </para>

     <para>
      Other clients and libraries might provide their own mechanisms,
      via the shell or otherwise, that allow the user to alter session
      settings without direct use of SQL commands.
     </para>
    </listitem>
   </itemizedlist>

   </sect2>

   <sect2 id="config-includes">
    <title>Managing Configuration File Contents</title>

     <para>
      <productname>PostgreSQL</productname> provides several features for breaking
      down complex <filename>postgresql.conf</filename> files into sub-files.
      These features are especially useful when managing multiple servers
      with related, but not identical, configurations.
     </para>

     <para>
      <indexterm>
       <primary><literal>include</literal></primary>
       <secondary>in configuration file</secondary>
      </indexterm>
      In addition to individual parameter settings,
      the <filename>postgresql.conf</filename> file can contain <firstterm>include
      directives</firstterm>, which specify another file to read and process as if
      it were inserted into the configuration file at this point.  This
      feature allows a configuration file to be divided into physically
      separate parts.  Include directives simply look like:
<programlisting>
include 'filename'
</programlisting>
      If the file name is not an absolute path, it is taken as relative to
      the directory containing the referencing configuration file.
      Inclusions can be nested.
     </para>

     <para>
      <indexterm>
       <primary><literal>include_if_exists</literal></primary>
       <secondary>in configuration file</secondary>
      </indexterm>
      There is also an <literal>include_if_exists</literal> directive, which acts
      the same as the <literal>include</literal> directive, except
      when the referenced file does not exist or cannot be read.  A regular
      <literal>include</literal> will consider this an error condition, but
      <literal>include_if_exists</literal> merely logs a message and continues
      processing the referencing configuration file.
     </para>

     <para>
      <indexterm>
       <primary><literal>include_dir</literal></primary>
       <secondary>in configuration file</secondary>
      </indexterm>
      The <filename>postgresql.conf</filename> file can also contain
      <literal>include_dir</literal> directives, which specify an entire
      directory of configuration files to include.  These look like
<programlisting>
include_dir 'directory'
</programlisting>
      Non-absolute directory names are taken as relative to the directory
      containing the referencing configuration file.  Within the specified
      directory, only non-directory files whose names end with the
      suffix <literal>.conf</literal> will be included.  File names that
      start with the <literal>.</literal> character are also ignored, to
      prevent mistakes since such files are hidden on some platforms.  Multiple
      files within an include directory are processed in file name order
      (according to C locale rules, i.e., numbers before letters, and
      uppercase letters before lowercase ones).
     </para>

     <para>
      Include files or directories can be used to logically separate portions
      of the database configuration, rather than having a single large
      <filename>postgresql.conf</filename> file.  Consider a company that has two
      database servers, each with a different amount of memory.  There are
      likely elements of the configuration both will share, for things such
      as logging.  But memory-related parameters on the server will vary
      between the two.  And there might be server specific customizations,
      too.  One way to manage this situation is to break the custom
      configuration changes for your site into three files.  You could add
      this to the end of your <filename>postgresql.conf</filename> file to include
      them:
<programlisting>
include 'shared.conf'
include 'memory.conf'
include 'server.conf'
</programlisting>
      All systems would have the same <filename>shared.conf</filename>.  Each
      server with a particular amount of memory could share the
      same <filename>memory.conf</filename>; you might have one for all servers
      with 8GB of RAM, another for those having 16GB.  And
      finally <filename>server.conf</filename> could have truly server-specific
      configuration information in it.
     </para>

     <para>
      Another possibility is to create a configuration file directory and
      put this information into files there. For example, a <filename>conf.d</filename>
      directory could be referenced at the end of <filename>postgresql.conf</filename>:
<programlisting>
include_dir 'conf.d'
</programlisting>
      Then you could name the files in the <filename>conf.d</filename> directory
      like this:
<programlisting>
00shared.conf
01memory.conf
02server.conf
</programlisting>
       This naming convention establishes a clear order in which these
       files will be loaded.  This is important because only the last
       setting encountered for a particular parameter while the server is
       reading configuration files will be used.  In this example,
       something set in <filename>conf.d/02server.conf</filename> would override a
       value set in <filename>conf.d/01memory.conf</filename>.
     </para>

     <para>
      You might instead use this approach to naming the files
      descriptively:
<programlisting>
00shared.conf
01memory-8GB.conf
02server-foo.conf
</programlisting>
      This sort of arrangement gives a unique name for each configuration file
      variation.  This can help eliminate ambiguity when several servers have
      their configurations all stored in one place, such as in a version
      control repository.  (Storing database configuration files under version
      control is another good practice to consider.)
     </para>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-file-locations">
    <title>File Locations</title>

     <para>
      In addition to the <filename>postgresql.conf</filename> file
      already mentioned, <productname>PostgreSQL</productname> uses
      two other manually-edited configuration files, which control
      client authentication (their use is discussed in <xref
      linkend="client-authentication"/>).  By default, all three
      configuration files are stored in the database cluster's data
      directory.  The parameters described in this section allow the
      configuration files to be placed elsewhere.  (Doing so can ease
      administration.  In particular it is often easier to ensure that
      the configuration files are properly backed-up when they are
      kept separate.)
     </para>

     <variablelist>
     <varlistentry id="guc-data-directory" xreflabel="data_directory">
      <term><varname>data_directory</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>data_directory</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
         Specifies the directory to use for data storage.
         This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-config-file" xreflabel="config_file">
      <term><varname>config_file</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>config_file</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
         Specifies the main server configuration file
         (customarily called <filename>postgresql.conf</filename>).
         This parameter can only be set on the <command>postgres</command> command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-hba-file" xreflabel="hba_file">
      <term><varname>hba_file</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>hba_file</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
         Specifies the configuration file for host-based authentication
         (customarily called <filename>pg_hba.conf</filename>).
         This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ident-file" xreflabel="ident_file">
      <term><varname>ident_file</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>ident_file</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
         Specifies the configuration file for user name mapping
         (customarily called <filename>pg_ident.conf</filename>).
         This parameter can only be set at server start.
         See also <xref linkend="auth-username-maps"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-external-pid-file" xreflabel="external_pid_file">
      <term><varname>external_pid_file</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>external_pid_file</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the name of an additional process-ID (PID) file that the
        server should create for use by server administration programs.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     </variablelist>

     <para>
      In a default installation, none of the above parameters are set
      explicitly.  Instead, the
      data directory is specified by the <option>-D</option> command-line
      option or the <envar>PGDATA</envar> environment variable, and the
      configuration files are all found within the data directory.
     </para>

     <para>
      If you wish to keep the configuration files elsewhere than the
      data directory, the <command>postgres</command> <option>-D</option>
      command-line option or <envar>PGDATA</envar> environment variable
      must point to the directory containing the configuration files,
      and the <varname>data_directory</varname> parameter must be set in
      <filename>postgresql.conf</filename> (or on the command line) to show
      where the data directory is actually located.  Notice that
      <varname>data_directory</varname> overrides <option>-D</option> and
      <envar>PGDATA</envar> for the location
      of the data directory, but not for the location of the configuration
      files.
     </para>

     <para>
      If you wish, you can specify the configuration file names and locations
      individually using the parameters <varname>config_file</varname>,
      <varname>hba_file</varname> and/or <varname>ident_file</varname>.
      <varname>config_file</varname> can only be specified on the
      <command>postgres</command> command line, but the others can be
      set within the main configuration file.  If all three parameters plus
      <varname>data_directory</varname> are explicitly set, then it is not necessary
      to specify <option>-D</option> or <envar>PGDATA</envar>.
     </para>

     <para>
      When setting any of these parameters, a relative path will be interpreted
      with respect to the directory in which <command>postgres</command>
      is started.
     </para>
   </sect1>

   <sect1 id="runtime-config-connection">
    <title>Connections and Authentication</title>

    <sect2 id="runtime-config-connection-settings">
     <title>Connection Settings</title>

     <variablelist>

     <varlistentry id="guc-listen-addresses" xreflabel="listen_addresses">
      <term><varname>listen_addresses</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>listen_addresses</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
         Specifies the TCP/IP address(es) on which the server is
         to listen for connections from client applications.
         The value takes the form of a comma-separated list of host names
         and/or numeric IP addresses.  The special entry <literal>*</literal>
         corresponds to all available IP interfaces.  The entry
         <literal>0.0.0.0</literal> allows listening for all IPv4 addresses and
         <literal>::</literal> allows listening for all IPv6 addresses.
         If the list is empty, the server does not listen on any IP interface
         at all, in which case only Unix-domain sockets can be used to connect
         to it.
         The default value is <systemitem class="systemname">localhost</systemitem>,
         which allows only local TCP/IP <quote>loopback</quote> connections to be
         made.  While client authentication (<xref
         linkend="client-authentication"/>) allows fine-grained control
         over who can access the server, <varname>listen_addresses</varname>
         controls which interfaces accept connection attempts, which
         can help prevent repeated malicious connection requests on
         insecure network interfaces.  This parameter can only be set
         at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-port" xreflabel="port">
      <term><varname>port</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>port</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        The TCP port the server listens on; 5432 by default.  Note that the
        same port number is used for all IP addresses the server listens on.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-connections" xreflabel="max_connections">
      <term><varname>max_connections</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_connections</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Determines the maximum number of concurrent connections to the
        database server. The default is typically 100 connections, but
        might be less if your kernel settings will not support it (as
        determined during <application>initdb</application>).  This parameter can
        only be set at server start.
       </para>

       <para>
        When running a standby server, you must set this parameter to the
        same or higher value than on the master server. Otherwise, queries
        will not be allowed in the standby server.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-superuser-reserved-connections"
     xreflabel="superuser_reserved_connections">
      <term><varname>superuser_reserved_connections</varname>
      (<type>integer</type>)
      <indexterm>
       <primary><varname>superuser_reserved_connections</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Determines the number of connection <quote>slots</quote> that
        are reserved for connections by <productname>PostgreSQL</productname>
        superusers.  At most <xref linkend="guc-max-connections"/>
        connections can ever be active simultaneously.  Whenever the
        number of active concurrent connections is at least
        <varname>max_connections</varname> minus
        <varname>superuser_reserved_connections</varname>, new
        connections will be accepted only for superusers, and no
        new replication connections will be accepted.
       </para>

       <para>
        The default value is three connections. The value must be less
        than <varname>max_connections</varname> minus
        <xref linkend="guc-max-wal-senders"/>.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-unix-socket-directories" xreflabel="unix_socket_directories">
      <term><varname>unix_socket_directories</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>unix_socket_directories</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the directory of the Unix-domain socket(s) on which the
        server is to listen for connections from client applications.
        Multiple sockets can be created by listing multiple directories
        separated by commas.  Whitespace between entries is
        ignored; surround a directory name with double quotes if you need
        to include whitespace or commas in the name.
        An empty value
        specifies not listening on any Unix-domain sockets, in which case
        only TCP/IP sockets can be used to connect to the server.
        The default value is normally
        <filename>/tmp</filename>, but that can be changed at build time.
        This parameter can only be set at server start.
       </para>

       <para>
        In addition to the socket file itself, which is named
        <literal>.s.PGSQL.<replaceable>nnnn</replaceable></literal> where
        <replaceable>nnnn</replaceable> is the server's port number, an ordinary file
        named <literal>.s.PGSQL.<replaceable>nnnn</replaceable>.lock</literal> will be
        created in each of the <varname>unix_socket_directories</varname> directories.
        Neither file should ever be removed manually.
       </para>

       <para>
        This parameter is irrelevant on Windows, which does not have
        Unix-domain sockets.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-unix-socket-group" xreflabel="unix_socket_group">
      <term><varname>unix_socket_group</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>unix_socket_group</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the owning group of the Unix-domain socket(s).  (The owning
        user of the sockets is always the user that starts the
        server.)  In combination with the parameter
        <varname>unix_socket_permissions</varname> this can be used as
        an additional access control mechanism for Unix-domain connections.
        By default this is the empty string, which uses the default
        group of the server user.  This parameter can only be set at
        server start.
       </para>

       <para>
        This parameter is irrelevant on Windows, which does not have
        Unix-domain sockets.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-unix-socket-permissions" xreflabel="unix_socket_permissions">
      <term><varname>unix_socket_permissions</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>unix_socket_permissions</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the access permissions of the Unix-domain socket(s).  Unix-domain
        sockets use the usual Unix file system permission set.
        The parameter value is expected to be a numeric mode
        specified in the format accepted by the
        <function>chmod</function> and <function>umask</function>
        system calls.  (To use the customary octal format the number
        must start with a <literal>0</literal> (zero).)
       </para>

       <para>
        The default permissions are <literal>0777</literal>, meaning
        anyone can connect. Reasonable alternatives are
        <literal>0770</literal> (only user and group, see also
        <varname>unix_socket_group</varname>) and <literal>0700</literal>
        (only user). (Note that for a Unix-domain socket, only write
        permission matters, so there is no point in setting or revoking
        read or execute permissions.)
       </para>

       <para>
        This access control mechanism is independent of the one
        described in <xref linkend="client-authentication"/>.
       </para>

       <para>
        This parameter can only be set at server start.
       </para>

       <para>
        This parameter is irrelevant on systems, notably Solaris as of Solaris
        10, that ignore socket permissions entirely.  There, one can achieve a
        similar effect by pointing <varname>unix_socket_directories</varname> to a
        directory having search permission limited to the desired audience.
        This parameter is also irrelevant on Windows, which does not have
        Unix-domain sockets.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-bonjour" xreflabel="bonjour">
      <term><varname>bonjour</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>bonjour</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables advertising the server's existence via
        <productname>Bonjour</productname>.  The default is off.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-bonjour-name" xreflabel="bonjour_name">
      <term><varname>bonjour_name</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>bonjour_name</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the <productname>Bonjour</productname> service
        name.  The computer name is used if this parameter is set to the
        empty string <literal>''</literal> (which is the default).  This parameter is
        ignored if the server was not compiled with
        <productname>Bonjour</productname> support.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-tcp-keepalives-idle" xreflabel="tcp_keepalives_idle">
      <term><varname>tcp_keepalives_idle</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>tcp_keepalives_idle</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the number of seconds of inactivity after which TCP
        should send a keepalive message to the client.  A value of 0 uses
        the system default.
        This parameter is supported only on systems that support
        <symbol>TCP_KEEPIDLE</symbol> or an equivalent socket option, and on
        Windows; on other systems, it must be zero.
        In sessions connected via a Unix-domain socket, this parameter is
        ignored and always reads as zero.
       </para>
       <note>
        <para>
         On Windows, a value of 0 will set this parameter to 2 hours,
         since Windows does not provide a way to read the system default value.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-tcp-keepalives-interval" xreflabel="tcp_keepalives_interval">
      <term><varname>tcp_keepalives_interval</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>tcp_keepalives_interval</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the number of seconds after which a TCP keepalive message
        that is not acknowledged by the client should be retransmitted.
        A value of 0 uses the system default.
        This parameter is supported only on systems that support
        <symbol>TCP_KEEPINTVL</symbol> or an equivalent socket option, and on
        Windows; on other systems, it must be zero.
        In sessions connected via a Unix-domain socket, this parameter is
        ignored and always reads as zero.
       </para>
       <note>
        <para>
         On Windows, a value of 0 will set this parameter to 1 second,
         since Windows does not provide a way to read the system default value.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-tcp-keepalives-count" xreflabel="tcp_keepalives_count">
      <term><varname>tcp_keepalives_count</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>tcp_keepalives_count</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the number of TCP keepalives that can be lost before
        the server's connection to the client is considered dead.
        A value of 0 uses the system default.
        This parameter is supported only on systems that support
        <symbol>TCP_KEEPCNT</symbol> or an equivalent socket option;
        on other systems, it must be zero.
        In sessions connected via a Unix-domain socket, this parameter is
        ignored and always reads as zero.
       </para>
       <note>
        <para>
         This parameter is not supported on Windows, and must be zero.
        </para>
       </note>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>

     <sect2 id="runtime-config-connection-authentication">
     <title>Authentication</title>

     <variablelist>
     <varlistentry id="guc-authentication-timeout" xreflabel="authentication_timeout">
      <term><varname>authentication_timeout</varname> (<type>integer</type>)
      <indexterm><primary>timeout</primary><secondary>client authentication</secondary></indexterm>
      <indexterm><primary>client authentication</primary><secondary>timeout during</secondary></indexterm>
      <indexterm>
       <primary><varname>authentication_timeout</varname> configuration parameter</primary>
      </indexterm>
      </term>

      <listitem>
       <para>
        Maximum time to complete client authentication, in seconds. If a
        would-be client has not completed the authentication protocol in
        this much time, the server closes the connection. This prevents
        hung clients from occupying a connection indefinitely.
        The default is one minute (<literal>1m</literal>).
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-password-encryption" xreflabel="password_encryption">
      <term><varname>password_encryption</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>password_encryption</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When a password is specified in <xref linkend="sql-createrole"/> or
        <xref linkend="sql-alterrole"/>, this parameter determines the algorithm
        to use to encrypt the password. The default value is <literal>md5</literal>,
        which stores the password as an MD5 hash (<literal>on</literal> is also
        accepted, as alias for <literal>md5</literal>). Setting this parameter to
        <literal>scram-sha-256</literal> will encrypt the password with SCRAM-SHA-256.
       </para>
       <para>
        Note that older clients might lack support for the SCRAM authentication
        mechanism, and hence not work with passwords encrypted with
        SCRAM-SHA-256.  See <xref linkend="auth-password"/> for more details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
      <term><varname>krb_server_keyfile</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>krb_server_keyfile</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the location of the Kerberos server key file. See
        <xref linkend="gssapi-auth"/>
        for details. This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-krb-caseins-users" xreflabel="krb_caseins_users">
      <term><varname>krb_caseins_users</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>krb_caseins_users</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets whether GSSAPI user names should be treated
        case-insensitively.
        The default is <literal>off</literal> (case sensitive). This parameter can only be
        set in the <filename>postgresql.conf</filename> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-db-user-namespace" xreflabel="db_user_namespace">
      <term><varname>db_user_namespace</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>db_user_namespace</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This parameter enables per-database user names.  It is off by default.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>

       <para>
        If this is on, you should create users as <replaceable>username@dbname</replaceable>.
        When <replaceable>username</replaceable> is passed by a connecting client,
        <literal>@</literal> and the database name are appended to the user
        name and that database-specific user name is looked up by the
        server. Note that when you create users with names containing
        <literal>@</literal> within the SQL environment, you will need to
        quote the user name.
       </para>

       <para>
        With this parameter enabled, you can still create ordinary global
        users.  Simply append <literal>@</literal> when specifying the user
        name in the client, e.g., <literal>joe@</literal>.  The <literal>@</literal>
        will be stripped off before the user name is looked up by the
        server.
       </para>

       <para>
        <varname>db_user_namespace</varname> causes the client's and
        server's user name representation to differ.
        Authentication checks are always done with the server's user name
        so authentication methods must be configured for the
        server's user name, not the client's.  Because
        <literal>md5</literal> uses the user name as salt on both the
        client and server, <literal>md5</literal> cannot be used with
        <varname>db_user_namespace</varname>.
       </para>

       <note>
        <para>
         This feature is intended as a temporary measure until a
         complete solution is found.  At that time, this option will
         be removed.
        </para>
       </note>
      </listitem>
     </varlistentry>
     </variablelist>
     </sect2>

     <sect2 id="runtime-config-connection-ssl">
     <title>SSL</title>

     <para>
      See <xref linkend="ssl-tcp"/> for more information about setting up SSL.
     </para>

     <variablelist>
     <varlistentry id="guc-ssl" xreflabel="ssl">
      <term><varname>ssl</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>ssl</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables <acronym>SSL</acronym> connections.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        The default is <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-ca-file" xreflabel="ssl_ca_file">
      <term><varname>ssl_ca_file</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>ssl_ca_file</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the name of the file containing the SSL server certificate
        authority (CA).
        Relative paths are relative to the data directory.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        The default is empty, meaning no CA file is loaded,
        and client certificate verification is not performed.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-cert-file" xreflabel="ssl_cert_file">
      <term><varname>ssl_cert_file</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>ssl_cert_file</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the name of the file containing the SSL server certificate.
        Relative paths are relative to the data directory.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        The default is <filename>server.crt</filename>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-crl-file" xreflabel="ssl_crl_file">
      <term><varname>ssl_crl_file</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>ssl_crl_file</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the name of the file containing the SSL client certificate
        revocation list (CRL).
        Relative paths are relative to the data directory.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        The default is empty, meaning no CRL file is loaded.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-key-file" xreflabel="ssl_key_file">
      <term><varname>ssl_key_file</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>ssl_key_file</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the name of the file containing the SSL server private key.
        Relative paths are relative to the data directory.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        The default is <filename>server.key</filename>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-ciphers" xreflabel="ssl_ciphers">
      <term><varname>ssl_ciphers</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>ssl_ciphers</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies a list of <acronym>SSL</acronym> cipher suites that are
        allowed to be used by SSL connections.  See the
        <citerefentry><refentrytitle>ciphers</refentrytitle></citerefentry>
        manual page in the <application>OpenSSL</application> package for the
        syntax of this setting and a list of supported values.  Only
        connections using TLS version 1.2 and lower are affected.  There is
        currently no setting that controls the cipher choices used by TLS
        version 1.3 connections.  The default value is
        <literal>HIGH:MEDIUM:+3DES:!aNULL</literal>.  The default is usually a
        reasonable choice unless you have specific security requirements.
       </para>

       <para>
        This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command
        line.
       </para>

       <para>
        Explanation of the default value:
        <variablelist>
         <varlistentry>
          <term><literal>HIGH</literal></term>
          <listitem>
           <para>
            Cipher suites that use ciphers from <literal>HIGH</literal> group (e.g.,
            AES, Camellia, 3DES)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>MEDIUM</literal></term>
          <listitem>
           <para>
            Cipher suites that use ciphers from <literal>MEDIUM</literal> group
            (e.g., RC4, SEED)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>+3DES</literal></term>
          <listitem>
           <para>
            The OpenSSL default order for <literal>HIGH</literal> is problematic
            because it orders 3DES higher than AES128.  This is wrong because
            3DES offers less security than AES128, and it is also much
            slower.  <literal>+3DES</literal> reorders it after all other
            <literal>HIGH</literal> and <literal>MEDIUM</literal> ciphers.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>!aNULL</literal></term>
          <listitem>
           <para>
            Disables anonymous cipher suites that do no authentication.  Such
            cipher suites are vulnerable to man-in-the-middle attacks and
            therefore should not be used.
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>

       <para>
        Available cipher suite details will vary across OpenSSL versions.  Use
        the command
        <literal>openssl ciphers -v 'HIGH:MEDIUM:+3DES:!aNULL'</literal> to
        see actual details for the currently installed <application>OpenSSL</application>
        version.  Note that this list is filtered at run time based on the
        server key type.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-prefer-server-ciphers" xreflabel="ssl_prefer_server_ciphers">
      <term><varname>ssl_prefer_server_ciphers</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>ssl_prefer_server_ciphers</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies whether to use the server's SSL cipher preferences, rather
        than the client's.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        The default is <literal>true</literal>.
       </para>

       <para>
        Older PostgreSQL versions do not have this setting and always use the
        client's preferences.  This setting is mainly for backward
        compatibility with those versions.  Using the server's preferences is
        usually better because it is more likely that the server is appropriately
        configured.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-ecdh-curve" xreflabel="ssl_ecdh_curve">
      <term><varname>ssl_ecdh_curve</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>ssl_ecdh_curve</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the name of the curve to use in <acronym>ECDH</acronym> key
        exchange.  It needs to be supported by all clients that connect.
        It does not need to be the same curve used by the server's Elliptic
        Curve key.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        The default is <literal>prime256v1</literal>.
       </para>

       <para>
        OpenSSL names for the most common curves are:
        <literal>prime256v1</literal> (NIST P-256),
        <literal>secp384r1</literal> (NIST P-384),
        <literal>secp521r1</literal> (NIST P-521).
        The full list of available curves can be shown with the command
        <command>openssl ecparam -list_curves</command>.  Not all of them
        are usable in <acronym>TLS</acronym> though.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-dh-params-file" xreflabel="ssl_dh_params_file">
      <term><varname>ssl_dh_params_file</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>ssl_dh_params_file</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the name of the file containing Diffie-Hellman parameters
        used for so-called ephemeral DH family of SSL ciphers. The default is
        empty, in which case compiled-in default DH parameters used. Using
        custom DH parameters reduces the exposure if an attacker manages to
        crack the well-known compiled-in DH parameters. You can create your own
        DH parameters file with the command
        <command>openssl dhparam -out dhparams.pem 2048</command>.
       </para>

       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-passphrase-command" xreflabel="ssl_passphrase_command">
      <term><varname>ssl_passphrase_command</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>ssl_passphrase_command</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets an external command to be invoked when a passphrase for
        decrypting an SSL file such as a private key needs to be obtained.  By
        default, this parameter is empty, which means the built-in prompting
        mechanism is used.
       </para>
       <para>
        The command must print the passphrase to the standard output and exit
        with code 0.  In the parameter value, <literal>%p</literal> is
        replaced by a prompt string.  (Write <literal>%%</literal> for a
        literal <literal>%</literal>.)  Note that the prompt string will
        probably contain whitespace, so be sure to quote adequately.  A single
        newline is stripped from the end of the output if present.
       </para>
       <para>
        The command does not actually have to prompt the user for a
        passphrase.  It can read it from a file, obtain it from a keychain
        facility, or similar.  It is up to the user to make sure the chosen
        mechanism is adequately secure.
       </para>
       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-passphrase-command-supports-reload" xreflabel="ssl_passphrase_command_supports_reload">
      <term><varname>ssl_passphrase_command_supports_reload</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>ssl_passphrase_command_supports_reload</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This parameter determines whether the passphrase command set by
        <varname>ssl_passphrase_command</varname> will also be called during a
        configuration reload if a key file needs a passphrase.  If this
        parameter is false (the default), then
        <varname>ssl_passphrase_command</varname> will be ignored during a
        reload and the SSL configuration will not be reloaded if a passphrase
        is needed.  That setting is appropriate for a command that requires a
        TTY for prompting, which might not be available when the server is
        running.  Setting this parameter to true might be appropriate if the
        passphrase is obtained from a file, for example.
       </para>
       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-resource">
    <title>Resource Consumption</title>

    <sect2 id="runtime-config-resource-memory">
     <title>Memory</title>

     <variablelist>
     <varlistentry id="guc-shared-buffers" xreflabel="shared_buffers">
      <term><varname>shared_buffers</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>shared_buffers</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the amount of memory the database server uses for shared
        memory buffers.  The default is typically 128 megabytes
        (<literal>128MB</literal>), but might be less if your kernel settings will
        not support it (as determined during <application>initdb</application>).
        This setting must be at least 128 kilobytes.  (Non-default
        values of <symbol>BLCKSZ</symbol> change the minimum.)  However,
        settings significantly higher than the minimum are usually needed
        for good performance.  This parameter can only be set at server start.
       </para>

       <para>
        If you have a dedicated database server with 1GB or more of RAM, a
        reasonable starting value for <varname>shared_buffers</varname> is 25%
        of the memory in your system.  There are some workloads where even
        larger settings for <varname>shared_buffers</varname> are effective, but
        because <productname>PostgreSQL</productname> also relies on the
        operating system cache, it is unlikely that an allocation of more than
        40% of RAM to <varname>shared_buffers</varname> will work better than a
        smaller amount.  Larger settings for <varname>shared_buffers</varname>
        usually require a corresponding increase in
        <varname>max_wal_size</varname>, in order to spread out the
        process of writing large quantities of new or changed data over a
        longer period of time.
       </para>

       <para>
        On systems with less than 1GB of RAM, a smaller percentage of RAM is
        appropriate, so as to leave adequate space for the operating system.
       </para>

      </listitem>
     </varlistentry>

     <varlistentry id="guc-huge-pages" xreflabel="huge_pages">
      <term><varname>huge_pages</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>huge_pages</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls whether huge pages are requested for the main shared memory
        area. Valid values are <literal>try</literal> (the default),
        <literal>on</literal>, and <literal>off</literal>.  With
        <varname>huge_pages</varname> set to <literal>try</literal>, the
        server will try to request huge pages, but fall back to the default if
        that fails. With <literal>on</literal>, failure to request huge pages
        will prevent the server from starting up. With <literal>off</literal>,
        huge pages will not be requested.
       </para>

       <para>
        At present, this setting is supported only on Linux and Windows. The
        setting is ignored on other systems when set to
        <literal>try</literal>.
       </para>

       <para>
        The use of huge pages results in smaller page tables and less CPU time
        spent on memory management, increasing performance. For more details about
        using huge pages on Linux, see <xref linkend="linux-huge-pages"/>.
       </para>

       <para>
        Huge pages are known as large pages on Windows.  To use them, you need to
        assign the user right Lock Pages in Memory to the Windows user account
        that runs <productname>PostgreSQL</productname>.
        You can use Windows Group Policy tool (gpedit.msc) to assign the user right
        Lock Pages in Memory.
        To start the database server on the command prompt as a standalone process,
        not as a Windows service, the command prompt must be run as an administrator or
        User Access Control (UAC) must be disabled. When the UAC is enabled, the normal
        command prompt revokes the user right Lock Pages in Memory when started.
       </para>

       <para>
        Note that this setting only affects the main shared memory area.
        Operating systems such as Linux, FreeBSD, and Illumos can also use
        huge pages (also known as <quote>super</quote> pages or
        <quote>large</quote> pages) automatically for normal memory
        allocation, without an explicit request from
        <productname>PostgreSQL</productname>. On Linux, this is called
        <quote>transparent huge pages</quote><indexterm><primary>transparent
        huge pages</primary></indexterm> (THP). That feature has been known to
        cause performance degradation with
        <productname>PostgreSQL</productname> for some users on some Linux
        versions, so its use is currently discouraged (unlike explicit use of
        <varname>huge_pages</varname>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-temp-buffers" xreflabel="temp_buffers">
      <term><varname>temp_buffers</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>temp_buffers</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the maximum number of temporary buffers used by each database
        session.  These are session-local buffers used only for access to
        temporary tables.  The default is eight megabytes
        (<literal>8MB</literal>).  The setting can be changed within individual
        sessions, but only before the first use of temporary tables
        within the session; subsequent attempts to change the value will
        have no effect on that session.
       </para>

       <para>
        A session will allocate temporary buffers as needed up to the limit
        given by <varname>temp_buffers</varname>.  The cost of setting a large
        value in sessions that do not actually need many temporary
        buffers is only a buffer descriptor, or about 64 bytes, per
        increment in <varname>temp_buffers</varname>.  However if a buffer is
        actually used an additional 8192 bytes will be consumed for it
        (or in general, <symbol>BLCKSZ</symbol> bytes).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-prepared-transactions" xreflabel="max_prepared_transactions">
      <term><varname>max_prepared_transactions</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_prepared_transactions</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the maximum number of transactions that can be in the
        <quote>prepared</quote> state simultaneously (see <xref
        linkend="sql-prepare-transaction"/>).
        Setting this parameter to zero (which is the default)
        disables the prepared-transaction feature.
        This parameter can only be set at server start.
       </para>

       <para>
        If you are not planning to use prepared transactions, this parameter
        should be set to zero to prevent accidental creation of prepared
        transactions.  If you are using prepared transactions, you will
        probably want <varname>max_prepared_transactions</varname> to be at
        least as large as <xref linkend="guc-max-connections"/>, so that every
        session can have a prepared transaction pending.
       </para>

       <para>
        When running a standby server, you must set this parameter to the
        same or higher value than on the master server. Otherwise, queries
        will not be allowed in the standby server.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-work-mem" xreflabel="work_mem">
      <term><varname>work_mem</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>work_mem</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the amount of memory to be used by internal sort operations
        and hash tables before writing to temporary disk files. The value
        defaults to four megabytes (<literal>4MB</literal>).
        Note that for a complex query, several sort or hash operations might be
        running in parallel; each operation will be allowed to use as much memory
        as this value specifies before it starts to write data into temporary
        files. Also, several running sessions could be doing such operations
        concurrently.  Therefore, the total memory used could be many
        times the value of <varname>work_mem</varname>; it is necessary to
        keep this fact in mind when choosing the value. Sort operations are
        used for <literal>ORDER BY</literal>, <literal>DISTINCT</literal>, and
        merge joins.
        Hash tables are used in hash joins, hash-based aggregation, and
        hash-based processing of <literal>IN</literal> subqueries.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-maintenance-work-mem" xreflabel="maintenance_work_mem">
      <term><varname>maintenance_work_mem</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>maintenance_work_mem</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the maximum amount of memory to be used by maintenance
        operations, such as <command>VACUUM</command>, <command>CREATE
        INDEX</command>, and <command>ALTER TABLE ADD FOREIGN KEY</command>.  It defaults
        to 64 megabytes (<literal>64MB</literal>).  Since only one of these
        operations can be executed at a time by a database session, and
        an installation normally doesn't have many of them running
        concurrently, it's safe to set this value significantly larger
        than <varname>work_mem</varname>.  Larger settings might improve
        performance for vacuuming and for restoring database dumps.
       </para>
       <para>
        Note that when autovacuum runs, up to
        <xref linkend="guc-autovacuum-max-workers"/> times this memory
        may be allocated, so be careful not to set the default value
        too high.  It may be useful to control for this by separately
        setting <xref linkend="guc-autovacuum-work-mem"/>.
       </para>
       <para>
        Note that for the collection of dead tuple identifiers,
        <command>VACUUM</command> is only able to utilize up to a maximum of
        <literal>1GB</literal> of memory.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-work-mem" xreflabel="autovacuum_work_mem">
      <term><varname>autovacuum_work_mem</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>autovacuum_work_mem</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the maximum amount of memory to be used by each
        autovacuum worker process.  It defaults to -1, indicating that
        the value of <xref linkend="guc-maintenance-work-mem"/> should
        be used instead.  The setting has no effect on the behavior of
        <command>VACUUM</command> when run in other contexts.
        This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command
        line.
       </para>
       <para>
        For the collection of dead tuple identifiers, autovacuum is only able
        to utilize up to a maximum of <literal>1GB</literal> of memory, so
        setting <varname>autovacuum_work_mem</varname> to a value higher than
        that has no effect on the number of dead tuples that autovacuum can
        collect while scanning a table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-stack-depth" xreflabel="max_stack_depth">
      <term><varname>max_stack_depth</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_stack_depth</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the maximum safe depth of the server's execution stack.
        The ideal setting for this parameter is the actual stack size limit
        enforced by the kernel (as set by <literal>ulimit -s</literal> or local
        equivalent), less a safety margin of a megabyte or so.  The safety
        margin is needed because the stack depth is not checked in every
        routine in the server, but only in key potentially-recursive routines
        such as expression evaluation.  The default setting is two
        megabytes (<literal>2MB</literal>), which is conservatively small and
        unlikely to risk crashes.  However, it might be too small to allow
        execution of complex functions.  Only superusers can change this
        setting.
       </para>

       <para>
        Setting <varname>max_stack_depth</varname> higher than
        the actual kernel limit will mean that a runaway recursive function
        can crash an individual backend process.  On platforms where
        <productname>PostgreSQL</productname> can determine the kernel limit,
        the server will not allow this variable to be set to an unsafe
        value.  However, not all platforms provide the information,
        so caution is recommended in selecting a value.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-dynamic-shared-memory-type" xreflabel="dynamic_shared_memory_type">
      <term><varname>dynamic_shared_memory_type</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>dynamic_shared_memory_type</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the dynamic shared memory implementation that the server
        should use.  Possible values are <literal>posix</literal> (for POSIX shared
        memory allocated using <literal>shm_open</literal>), <literal>sysv</literal>
        (for System V shared memory allocated via <literal>shmget</literal>),
        <literal>windows</literal> (for Windows shared memory), <literal>mmap</literal>
        (to simulate shared memory using memory-mapped files stored in the
        data directory), and <literal>none</literal> (to disable this feature).
        Not all values are supported on all platforms; the first supported
        option is the default for that platform.  The use of the
        <literal>mmap</literal> option, which is not the default on any platform,
        is generally discouraged because the operating system may write
        modified pages back to disk repeatedly, increasing system I/O load;
        however, it may be useful for debugging, when the
        <literal>pg_dynshmem</literal> directory is stored on a RAM disk, or when
        other shared memory facilities are not available.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>

     <sect2 id="runtime-config-resource-disk">
     <title>Disk</title>

     <variablelist>
     <varlistentry id="guc-temp-file-limit" xreflabel="temp_file_limit">
      <term><varname>temp_file_limit</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>temp_file_limit</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the maximum amount of disk space that a process can use
        for temporary files, such as sort and hash temporary files, or the
        storage file for a held cursor.  A transaction attempting to exceed
        this limit will be canceled.
        The value is specified in kilobytes, and <literal>-1</literal> (the
        default) means no limit.
        Only superusers can change this setting.
       </para>
       <para>
        This setting constrains the total space used at any instant by all
        temporary files used by a given <productname>PostgreSQL</productname> process.
        It should be noted that disk space used for explicit temporary
        tables, as opposed to temporary files used behind-the-scenes in query
        execution, does <emphasis>not</emphasis> count against this limit.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>

     <sect2 id="runtime-config-resource-kernel">
     <title>Kernel Resource Usage</title>

     <variablelist>
     <varlistentry id="guc-max-files-per-process" xreflabel="max_files_per_process">
      <term><varname>max_files_per_process</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_files_per_process</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the maximum number of simultaneously open files allowed to each
        server subprocess. The default is one thousand files. If the kernel is enforcing
        a safe per-process limit, you don't need to worry about this setting.
        But on some platforms (notably, most BSD systems), the kernel will
        allow individual processes to open many more files than the system
        can actually support if many processes all try to open
        that many files. If you find yourself seeing <quote>Too many open
        files</quote> failures, try reducing this setting.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     </variablelist>
    </sect2>

    <sect2 id="runtime-config-resource-vacuum-cost">
     <title>Cost-based Vacuum Delay</title>

     <para>
      During the execution of <xref linkend="sql-vacuum"/>
      and <xref linkend="sql-analyze"/>
      commands, the system maintains an
      internal counter that keeps track of the estimated cost of the
      various I/O operations that are performed.  When the accumulated
      cost reaches a limit (specified by
      <varname>vacuum_cost_limit</varname>), the process performing
      the operation will sleep for a short period of time, as specified by
      <varname>vacuum_cost_delay</varname>. Then it will reset the
      counter and continue execution.
     </para>

     <para>
      The intent of this feature is to allow administrators to reduce
      the I/O impact of these commands on concurrent database
      activity. There are many situations where it is not
      important that maintenance commands like
      <command>VACUUM</command> and <command>ANALYZE</command> finish
      quickly; however, it is usually very important that these
      commands do not significantly interfere with the ability of the
      system to perform other database operations. Cost-based vacuum
      delay provides a way for administrators to achieve this.
     </para>

     <para>
      This feature is disabled by default for manually issued
      <command>VACUUM</command> commands. To enable it, set the
      <varname>vacuum_cost_delay</varname> variable to a nonzero
      value.
     </para>

     <variablelist>
      <varlistentry id="guc-vacuum-cost-delay" xreflabel="vacuum_cost_delay">
       <term><varname>vacuum_cost_delay</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>vacuum_cost_delay</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         The length of time, in milliseconds, that the process will sleep
         when the cost limit has been exceeded.
         The default value is zero, which disables the cost-based vacuum
         delay feature.  Positive values enable cost-based vacuuming.
         Note that on many systems, the effective resolution
         of sleep delays is 10 milliseconds; setting
         <varname>vacuum_cost_delay</varname> to a value that is
         not a multiple of 10 might have the same results as setting it
         to the next higher multiple of 10.
        </para>

        <para>
         When using cost-based vacuuming, appropriate values for
         <varname>vacuum_cost_delay</varname> are usually quite small, perhaps
         10 or 20 milliseconds.  Adjusting vacuum's resource consumption
         is best done by changing the other vacuum cost parameters.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-page-hit" xreflabel="vacuum_cost_page_hit">
       <term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>vacuum_cost_page_hit</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         The estimated cost for vacuuming a buffer found in the shared buffer
         cache. It represents the cost to lock the buffer pool, lookup
         the shared hash table and scan the content of the page. The
         default value is one.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-page-miss" xreflabel="vacuum_cost_page_miss">
       <term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>vacuum_cost_page_miss</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         The estimated cost for vacuuming a buffer that has to be read from
         disk.  This represents the effort to lock the buffer pool,
         lookup the shared hash table, read the desired block in from
         the disk and scan its content. The default value is 10.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-page-dirty" xreflabel="vacuum_cost_page_dirty">
       <term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>vacuum_cost_page_dirty</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         The estimated cost charged when vacuum modifies a block that was
         previously clean. It represents the extra I/O required to
         flush the dirty block out to disk again. The default value is
         20.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-limit" xreflabel="vacuum_cost_limit">
       <term><varname>vacuum_cost_limit</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>vacuum_cost_limit</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         The accumulated cost that will cause the vacuuming process to sleep.
         The default value is 200.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <note>
      <para>
       There are certain operations that hold critical locks and should
       therefore complete as quickly as possible.  Cost-based vacuum
       delays do not occur during such operations.  Therefore it is
       possible that the cost accumulates far higher than the specified
       limit.  To avoid uselessly long delays in such cases, the actual
       delay is calculated as <varname>vacuum_cost_delay</varname> *
       <varname>accumulated_balance</varname> /
       <varname>vacuum_cost_limit</varname> with a maximum of
       <varname>vacuum_cost_delay</varname> * 4.
      </para>
     </note>
    </sect2>

    <sect2 id="runtime-config-resource-background-writer">
     <title>Background Writer</title>

     <para>
      There is a separate server
      process called the <firstterm>background writer</firstterm>, whose function
      is to issue writes of <quote>dirty</quote> (new or modified) shared
      buffers.  When the number of clean shared buffers appears to be
      insufficient, the background writer writes some dirty buffers to the
      file system and marks them as clean.  This reduces the likelihood
      that server processes handling user queries will be unable to find
      clean buffers and have to write dirty buffers themselves.
      However, the background writer does cause a net overall
      increase in I/O load, because while a repeatedly-dirtied page might
      otherwise be written only once per checkpoint interval, the
      background writer might write it several times as it is dirtied
      in the same interval.  The parameters discussed in this subsection
      can be used to tune the behavior for local needs.
     </para>

     <variablelist>
      <varlistentry id="guc-bgwriter-delay" xreflabel="bgwriter_delay">
       <term><varname>bgwriter_delay</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>bgwriter_delay</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Specifies the delay between activity rounds for the
         background writer.  In each round the writer issues writes
         for some number of dirty buffers (controllable by the
         following parameters).  It then sleeps for <varname>bgwriter_delay</varname>
         milliseconds, and repeats.  When there are no dirty buffers in the
         buffer pool, though, it goes into a longer sleep regardless of
         <varname>bgwriter_delay</varname>.  The default value is 200
         milliseconds (<literal>200ms</literal>). Note that on many systems, the
         effective resolution of sleep delays is 10 milliseconds; setting
         <varname>bgwriter_delay</varname> to a value that is not a multiple of 10
         might have the same results as setting it to the next higher multiple
         of 10.  This parameter can only be set in the
         <filename>postgresql.conf</filename> file or on the server command line.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-bgwriter-lru-maxpages" xreflabel="bgwriter_lru_maxpages">
       <term><varname>bgwriter_lru_maxpages</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>bgwriter_lru_maxpages</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         In each round, no more than this many buffers will be written
         by the background writer.  Setting this to zero disables
         background writing.  (Note that checkpoints, which are managed by
         a separate, dedicated auxiliary process, are unaffected.)
         The default value is 100 buffers.
         This parameter can only be set in the <filename>postgresql.conf</filename>
         file or on the server command line.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-bgwriter-lru-multiplier" xreflabel="bgwriter_lru_multiplier">
       <term><varname>bgwriter_lru_multiplier</varname> (<type>floating point</type>)
       <indexterm>
        <primary><varname>bgwriter_lru_multiplier</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         The number of dirty buffers written in each round is based on the
         number of new buffers that have been needed by server processes
         during recent rounds.  The average recent need is multiplied by
         <varname>bgwriter_lru_multiplier</varname> to arrive at an estimate of the
         number of buffers that will be needed during the next round.  Dirty
         buffers are written until there are that many clean, reusable buffers
         available.  (However, no more than <varname>bgwriter_lru_maxpages</varname>
         buffers will be written per round.)
         Thus, a setting of 1.0 represents a <quote>just in time</quote> policy
         of writing exactly the number of buffers predicted to be needed.
         Larger values provide some cushion against spikes in demand,
         while smaller values intentionally leave writes to be done by
         server processes.
         The default is 2.0.
         This parameter can only be set in the <filename>postgresql.conf</filename>
         file or on the server command line.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-bgwriter-flush-after" xreflabel="bgwriter_flush_after">
       <term><varname>bgwriter_flush_after</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>bgwriter_flush_after</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Whenever more than <varname>bgwriter_flush_after</varname> bytes have
         been written by the background writer, attempt to force the OS to issue these
         writes to the underlying storage.  Doing so will limit the amount of
         dirty data in the kernel's page cache, reducing the likelihood of
         stalls when an <function>fsync</function> is issued at the end of a checkpoint, or when
         the OS writes data back in larger batches in the background.  Often
         that will result in greatly reduced transaction latency, but there
         also are some cases, especially with workloads that are bigger than
         <xref linkend="guc-shared-buffers"/>, but smaller than the OS's page
         cache, where performance might degrade.  This setting may have no
         effect on some platforms.  The valid range is between
         <literal>0</literal>, which disables forced writeback, and
         <literal>2MB</literal>.  The default is <literal>512kB</literal> on Linux,
         <literal>0</literal> elsewhere.  (If <symbol>BLCKSZ</symbol> is not 8kB,
         the default and maximum values scale proportionally to it.)
         This parameter can only be set in the <filename>postgresql.conf</filename>
         file or on the server command line.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Smaller values of <varname>bgwriter_lru_maxpages</varname> and
      <varname>bgwriter_lru_multiplier</varname> reduce the extra I/O load
      caused by the background writer, but make it more likely that server
      processes will have to issue writes for themselves, delaying interactive
      queries.
     </para>
    </sect2>

    <sect2 id="runtime-config-resource-async-behavior">
     <title>Asynchronous Behavior</title>

     <variablelist>
      <varlistentry id="guc-effective-io-concurrency" xreflabel="effective_io_concurrency">
       <term><varname>effective_io_concurrency</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>effective_io_concurrency</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Sets the number of concurrent disk I/O operations that
         <productname>PostgreSQL</productname> expects can be executed
         simultaneously.  Raising this value will increase the number of I/O
         operations that any individual <productname>PostgreSQL</productname> session
         attempts to initiate in parallel.  The allowed range is 1 to 1000,
         or zero to disable issuance of asynchronous I/O requests. Currently,
         this setting only affects bitmap heap scans.
        </para>

        <para>
         For magnetic drives, a good starting point for this setting is the
         number of separate
         drives comprising a RAID 0 stripe or RAID 1 mirror being used for the
         database.  (For RAID 5 the parity drive should not be counted.)
         However, if the database is often busy with multiple queries issued in
         concurrent sessions, lower values may be sufficient to keep the disk
         array busy.  A value higher than needed to keep the disks busy will
         only result in extra CPU overhead.
         SSDs and other memory-based storage can often process many
         concurrent requests, so the best value might be in the hundreds.
        </para>

        <para>
         Asynchronous I/O depends on an effective <function>posix_fadvise</function>
         function, which some operating systems lack.  If the function is not
         present then setting this parameter to anything but zero will result
         in an error.  On some operating systems (e.g., Solaris), the function
         is present but does not actually do anything.
        </para>

        <para>
         The default is 1 on supported systems, otherwise 0.  This value can
         be overridden for tables in a particular tablespace by setting the
         tablespace parameter of the same name (see
         <xref linkend="sql-altertablespace"/>).
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-max-worker-processes" xreflabel="max_worker_processes">
       <term><varname>max_worker_processes</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>max_worker_processes</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Sets the maximum number of background processes that the system
         can support.  This parameter can only be set at server start.  The
         default is 8.
        </para>

        <para>
         When running a standby server, you must set this parameter to the
         same or higher value than on the master server. Otherwise, queries
         will not be allowed in the standby server.
        </para>

        <para>
         When changing this value, consider also adjusting
         <xref linkend="guc-max-parallel-workers"/>,
         <xref linkend="guc-max-parallel-workers-maintenance"/>, and
         <xref linkend="guc-max-parallel-workers-per-gather"/>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-max-parallel-workers-per-gather" xreflabel="max_parallel_workers_per_gather">
       <term><varname>max_parallel_workers_per_gather</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>max_parallel_workers_per_gather</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Sets the maximum number of workers that can be started by a single
         <literal>Gather</literal> or <literal>Gather Merge</literal> node.
         Parallel workers are taken from the pool of processes established by
         <xref linkend="guc-max-worker-processes"/>, limited by
         <xref linkend="guc-max-parallel-workers"/>.  Note that the requested
         number of workers may not actually be available at run time.  If this
         occurs, the plan will run with fewer workers than expected, which may
         be inefficient.  The default value is 2.  Setting this value to 0
         disables parallel query execution.
        </para>

        <para>
         Note that parallel queries may consume very substantially more
         resources than non-parallel queries, because each worker process is
         a completely separate process which has roughly the same impact on the
         system as an additional user session.  This should be taken into
         account when choosing a value for this setting, as well as when
         configuring other settings that control resource utilization, such
         as <xref linkend="guc-work-mem"/>.  Resource limits such as
         <varname>work_mem</varname> are applied individually to each worker,
         which means the total utilization may be much higher across all
         processes than it would normally be for any single process.
         For example, a parallel query using 4 workers may use up to 5 times
         as much CPU time, memory, I/O bandwidth, and so forth as a query which
         uses no workers at all.
        </para>

        <para>
         For more information on parallel query, see
         <xref linkend="parallel-query"/>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-max-parallel-workers-maintenance" xreflabel="max_parallel_maintenance_workers">
       <term><varname>max_parallel_maintenance_workers</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>max_parallel_maintenance_workers</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Sets the maximum number of parallel workers that can be
         started by a single utility command.  Currently, the only
         parallel utility command that supports the use of parallel
         workers is <command>CREATE INDEX</command>, and only when
         building a B-tree index.  Parallel workers are taken from the
         pool of processes established by <xref
         linkend="guc-max-worker-processes"/>, limited by <xref
         linkend="guc-max-parallel-workers"/>.  Note that the requested
         number of workers may not actually be available at run time.
         If this occurs, the utility operation will run with fewer
         workers than expected.  The default value is 2.  Setting this
         value to 0 disables the use of parallel workers by utility
         commands.
        </para>

        <para>
         Note that parallel utility commands should not consume
         substantially more memory than equivalent non-parallel
         operations.  This strategy differs from that of parallel
         query, where resource limits generally apply per worker
         process.  Parallel utility commands treat the resource limit
         <varname>maintenance_work_mem</varname> as a limit to be applied to
         the entire utility command, regardless of the number of
         parallel worker processes.  However, parallel utility
         commands may still consume substantially more CPU resources
         and I/O bandwidth.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-max-parallel-workers" xreflabel="max_parallel_workers">
       <term><varname>max_parallel_workers</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>max_parallel_workers</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Sets the maximum number of workers that the system can support for
         parallel operations.  The default value is 8.  When increasing or
         decreasing this value, consider also adjusting
         <xref linkend="guc-max-parallel-workers-maintenance"/> and
         <xref linkend="guc-max-parallel-workers-per-gather"/>.
         Also, note that a setting for this value which is higher than
         <xref linkend="guc-max-worker-processes"/> will have no effect,
         since parallel workers are taken from the pool of worker processes
         established by that setting.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-backend-flush-after" xreflabel="backend_flush_after">
       <term><varname>backend_flush_after</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>backend_flush_after</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Whenever more than <varname>backend_flush_after</varname> bytes have
         been written by a single backend, attempt to force the OS to issue
         these writes to the underlying storage.  Doing so will limit the
         amount of dirty data in the kernel's page cache, reducing the
         likelihood of stalls when an <function>fsync</function> is issued at the end of a
         checkpoint, or when the OS writes data back in larger batches in the
         background.  Often that will result in greatly reduced transaction
         latency, but there also are some cases, especially with workloads
         that are bigger than <xref linkend="guc-shared-buffers"/>, but smaller
         than the OS's page cache, where performance might degrade.  This
         setting may have no effect on some platforms.  The valid range is
         between <literal>0</literal>, which disables forced writeback,
         and <literal>2MB</literal>.  The default is <literal>0</literal>, i.e., no
         forced writeback.  (If <symbol>BLCKSZ</symbol> is not 8kB,
         the maximum value scales proportionally to it.)
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-old-snapshot-threshold" xreflabel="old_snapshot_threshold">
       <term><varname>old_snapshot_threshold</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>old_snapshot_threshold</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Sets the minimum time that a snapshot can be used without risk of a
         <literal>snapshot too old</literal> error occurring when using the snapshot.
         This parameter can only be set at server start.
        </para>

        <para>
         Beyond the threshold, old data may be vacuumed away.  This can help
         prevent bloat in the face of snapshots which remain in use for a
         long time.  To prevent incorrect results due to cleanup of data which
         would otherwise be visible to the snapshot, an error is generated
         when the snapshot is older than this threshold and the snapshot is
         used to read a page which has been modified since the snapshot was
         built.
        </para>

        <para>
         A value of <literal>-1</literal> disables this feature, and is the default.
         Useful values for production work probably range from a small number
         of hours to a few days.  The setting will be coerced to a granularity
         of minutes, and small numbers (such as <literal>0</literal> or
         <literal>1min</literal>) are only allowed because they may sometimes be
         useful for testing.  While a setting as high as <literal>60d</literal> is
         allowed, please note that in many workloads extreme bloat or
         transaction ID wraparound may occur in much shorter time frames.
        </para>

        <para>
         When this feature is enabled, freed space at the end of a relation
         cannot be released to the operating system, since that could remove
         information needed to detect the <literal>snapshot too old</literal>
         condition.  All space allocated to a relation remains associated with
         that relation for reuse only within that relation unless explicitly
         freed (for example, with <command>VACUUM FULL</command>).
        </para>

        <para>
         This setting does not attempt to guarantee that an error will be
         generated under any particular circumstances.  In fact, if the
         correct results can be generated from (for example) a cursor which
         has materialized a result set, no error will be generated even if the
         underlying rows in the referenced table have been vacuumed away.
         Some tables cannot safely be vacuumed early, and so will not be
         affected by this setting, such as system catalogs.  For such tables
         this setting will neither reduce bloat nor create a possibility
         of a <literal>snapshot too old</literal> error on scanning.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-wal">
    <title>Write Ahead Log</title>

   <para>
    For additional information on tuning these settings,
    see <xref linkend="wal-configuration"/>.
   </para>

    <sect2 id="runtime-config-wal-settings">
     <title>Settings</title>
     <variablelist>

     <varlistentry id="guc-wal-level" xreflabel="wal_level">
      <term><varname>wal_level</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>wal_level</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        <varname>wal_level</varname> determines how much information is written to
        the WAL. The default value is <literal>replica</literal>, which writes enough
        data to support WAL archiving and replication, including running
        read-only queries on a standby server. <literal>minimal</literal> removes all
        logging except the information required to recover from a crash or
        immediate shutdown.  Finally,
        <literal>logical</literal> adds information necessary to support logical
        decoding.  Each level includes the information logged at all lower
        levels.  This parameter can only be set at server start.
       </para>
       <para>
        In <literal>minimal</literal> level, WAL-logging of some bulk
        operations can be safely skipped, which can make those
        operations much faster (see <xref linkend="populate-pitr"/>).
        Operations in which this optimization can be applied include:
        <simplelist>
         <member><command>CREATE TABLE AS</command></member>
         <member><command>CREATE INDEX</command></member>
         <member><command>CLUSTER</command></member>
         <member><command>COPY</command> into tables that were created or truncated in the same
         transaction</member>
        </simplelist>
        But minimal WAL does not contain enough information to reconstruct the
        data from a base backup and the WAL logs, so <literal>replica</literal> or
        higher must be used to enable WAL archiving
        (<xref linkend="guc-archive-mode"/>) and streaming replication.
       </para>
       <para>
        In <literal>logical</literal> level, the same information is logged as
        with <literal>replica</literal>, plus information needed to allow
        extracting logical change sets from the WAL. Using a level of
        <literal>logical</literal> will increase the WAL volume, particularly if many
        tables are configured for <literal>REPLICA IDENTITY FULL</literal> and
        many <command>UPDATE</command> and <command>DELETE</command> statements are
        executed.
       </para>
       <para>
        In releases prior to 9.6, this parameter also allowed the
        values <literal>archive</literal> and <literal>hot_standby</literal>.
        These are still accepted but mapped to <literal>replica</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-fsync" xreflabel="fsync">
      <term><varname>fsync</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>fsync</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If this parameter is on, the <productname>PostgreSQL</productname> server
        will try to make sure that updates are physically written to
        disk, by issuing <function>fsync()</function> system calls or various
        equivalent methods (see <xref linkend="guc-wal-sync-method"/>).
        This ensures that the database cluster can recover to a
        consistent state after an operating system or hardware crash.
       </para>

       <para>
        While turning off <varname>fsync</varname> is often a performance
        benefit, this can result in unrecoverable data corruption in
        the event of a power failure or system crash.  Thus it
        is only advisable to turn off <varname>fsync</varname> if
        you can easily recreate your entire database from external
        data.
       </para>

       <para>
        Examples of safe circumstances for turning off
        <varname>fsync</varname> include the initial loading of a new
        database cluster from a backup file, using a database cluster
        for processing a batch of data after which the database
        will be thrown away and recreated,
        or for a read-only database clone which
        gets recreated frequently and is not used for failover.  High
        quality hardware alone is not a sufficient justification for
        turning off <varname>fsync</varname>.
       </para>

       <para>
        For reliable recovery when changing <varname>fsync</varname>
        off to on, it is necessary to force all modified buffers in the
        kernel to durable storage.  This can be done while the cluster
        is shutdown or while <varname>fsync</varname> is on by running <command>initdb
        --sync-only</command>, running <command>sync</command>, unmounting the
        file system, or rebooting the server.
       </para>

       <para>
        In many situations, turning off <xref linkend="guc-synchronous-commit"/>
        for noncritical transactions can provide much of the potential
        performance benefit of turning off <varname>fsync</varname>, without
        the attendant risks of data corruption.
       </para>

       <para>
        <varname>fsync</varname> can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        If you turn this parameter off, also consider turning off
        <xref linkend="guc-full-page-writes"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-synchronous-commit" xreflabel="synchronous_commit">
      <term><varname>synchronous_commit</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>synchronous_commit</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies how much WAL processing must complete before
        the database server returns a <quote>success</quote>
        indication to the client.  Valid values are
        <literal>remote_apply</literal>, <literal>on</literal>
        (the default), <literal>remote_write</literal>,
        <literal>local</literal>, and <literal>off</literal>.
       </para>

       <para>
        If <varname>synchronous_standby_names</varname> is empty,
        the only meaningful settings are <literal>on</literal> and
        <literal>off</literal>;  <literal>remote_apply</literal>,
        <literal>remote_write</literal> and <literal>local</literal>
        all provide the same local synchronization level
        as <literal>on</literal>.  The local behavior of all
        non-<literal>off</literal> modes is to wait for local flush of WAL
        to disk.  In <literal>off</literal> mode, there is no waiting,
        so there can be a delay between when success is reported to the
        client and when the transaction is later guaranteed to be safe
        against a server crash.  (The maximum
        delay is three times <xref linkend="guc-wal-writer-delay"/>.)  Unlike
        <xref linkend="guc-fsync"/>, setting this parameter to <literal>off</literal>
        does not create any risk of database inconsistency: an operating
        system or database crash might
        result in some recent allegedly-committed transactions being lost, but
        the database state will be just the same as if those transactions had
        been aborted cleanly.  So, turning <varname>synchronous_commit</varname> off
        can be a useful alternative when performance is more important than
        exact certainty about the durability of a transaction.  For more
        discussion see <xref linkend="wal-async-commit"/>.
       </para>

       <para>
        If <xref linkend="guc-synchronous-standby-names"/> is non-empty,
        <varname>synchronous_commit</varname> also controls whether
        transaction commits will wait for their WAL records to be
        processed on the standby server(s).
       </para>

       <para>
        When set to <literal>remote_apply</literal>, commits will wait
        until replies from the current synchronous standby(s) indicate they
        have received the commit record of the transaction and applied
        it, so that it has become visible to queries on the standby(s),
        and also written to durable storage on the standbys.  This will
        cause much larger commit delays than previous settings since
        it waits for WAL replay.  When set to <literal>on</literal>,
        commits wait until replies
        from the current synchronous standby(s) indicate they have received
        the commit record of the transaction and flushed it to durable storage.  This
        ensures the transaction will not be lost unless both the primary and
        all synchronous standbys suffer corruption of their database storage.
        When set to <literal>remote_write</literal>, commits will wait until replies
        from the current synchronous standby(s) indicate they have
        received the commit record of the transaction and written it to
        their file systems. This setting ensures data preservation if a standby instance of
        <productname>PostgreSQL</productname> crashes, but not if the standby
        suffers an operating-system-level crash because the data has not
        necessarily reached durable storage on the standby.
        The setting <literal>local</literal> causes commits to wait for
        local flush to disk, but not for replication.  This is usually not
        desirable when synchronous replication is in use, but is provided for
        completeness.
       </para>

       <para>
        This parameter can be changed at any time; the behavior for any
        one transaction is determined by the setting in effect when it
        commits.  It is therefore possible, and useful, to have some
        transactions commit synchronously and others asynchronously.
        For example, to make a single multistatement transaction commit
        asynchronously when the default is the opposite, issue <command>SET
        LOCAL synchronous_commit TO OFF</command> within the transaction.
       </para>

       <para>
        <xref linkend="synchronous-commit-matrix"/> summarizes the
        capabilities of the <varname>synchronous_commit</varname> settings.
       </para>

       <table id="synchronous-commit-matrix">
        <title>synchronous_commit Modes</title>
        <tgroup cols="5">
         <colspec colname="col1" colwidth="1.5*"/>
         <colspec colname="col2" colwidth="1*"/>
         <colspec colname="col3" colwidth="1*"/>
         <colspec colname="col4" colwidth="1*"/>
         <colspec colname="col5" colwidth="1*"/>
         <thead>
          <row>
           <entry>synchronous_commit setting</entry>
           <entry>local durable commit</entry>
           <entry>standby durable commit after PG crash</entry>
           <entry>standby durable commit after OS crash</entry>
           <entry>standby query consistency</entry>
          </row>
         </thead>

         <tbody>

          <row>
           <entry>remote_apply</entry>
           <entry align="center">&bull;</entry>
           <entry align="center">&bull;</entry>
           <entry align="center">&bull;</entry>
           <entry align="center">&bull;</entry>
          </row>

          <row>
           <entry>on</entry>
           <entry align="center">&bull;</entry>
           <entry align="center">&bull;</entry>
           <entry align="center">&bull;</entry>
           <entry align="center"></entry>
          </row>

          <row>
           <entry>remote_write</entry>
           <entry align="center">&bull;</entry>
           <entry align="center">&bull;</entry>
           <entry align="center"></entry>
           <entry align="center"></entry>
          </row>

          <row>
           <entry>local</entry>
           <entry align="center">&bull;</entry>
           <entry align="center"></entry>
           <entry align="center"></entry>
           <entry align="center"></entry>
          </row>

          <row>
           <entry>off</entry>
           <entry align="center"></entry>
           <entry align="center"></entry>
           <entry align="center"></entry>
           <entry align="center"></entry>
          </row>

         </tbody>
        </tgroup>
       </table>

      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-sync-method" xreflabel="wal_sync_method">
      <term><varname>wal_sync_method</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>wal_sync_method</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Method used for forcing WAL updates out to disk.
        If <varname>fsync</varname> is off then this setting is irrelevant,
        since WAL file updates will not be forced out at all.
        Possible values are:
       </para>
       <itemizedlist>
        <listitem>
        <para>
         <literal>open_datasync</literal> (write WAL files with <function>open()</function> option <symbol>O_DSYNC</symbol>)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>fdatasync</literal> (call <function>fdatasync()</function> at each commit)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>fsync</literal> (call <function>fsync()</function> at each commit)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>fsync_writethrough</literal> (call <function>fsync()</function> at each commit, forcing write-through of any disk write cache)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>open_sync</literal> (write WAL files with <function>open()</function> option <symbol>O_SYNC</symbol>)
        </para>
        </listitem>
       </itemizedlist>
       <para>
        The <literal>open_</literal>* options also use <literal>O_DIRECT</literal> if available.
        Not all of these choices are available on all platforms.
        The default is the first method in the above list that is supported
        by the platform, except that <literal>fdatasync</literal> is the default on
        Linux and FreeBSD.  The default is not necessarily ideal; it might be
        necessary to change this setting or other aspects of your system
        configuration in order to create a crash-safe configuration or
        achieve optimal performance.
        These aspects are discussed in <xref linkend="wal-reliability"/>.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-full-page-writes" xreflabel="full_page_writes">
      <term><varname>full_page_writes</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>full_page_writes</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When this parameter is on, the <productname>PostgreSQL</productname> server
        writes the entire content of each disk page to WAL during the
        first modification of that page after a checkpoint.
        This is needed because
        a page write that is in process during an operating system crash might
        be only partially completed, leading to an on-disk page
        that contains a mix of old and new data.  The row-level change data
        normally stored in WAL will not be enough to completely restore
        such a page during post-crash recovery.  Storing the full page image
        guarantees that the page can be correctly restored, but at the price
        of increasing the amount of data that must be written to WAL.
        (Because WAL replay always starts from a checkpoint, it is sufficient
        to do this during the first change of each page after a checkpoint.
        Therefore, one way to reduce the cost of full-page writes is to
        increase the checkpoint interval parameters.)
       </para>

       <para>
        Turning this parameter off speeds normal operation, but
        might lead to either unrecoverable data corruption, or silent
        data corruption, after a system failure. The risks are similar to turning off
        <varname>fsync</varname>, though smaller, and it should be turned off
        only based on the same circumstances recommended for that parameter.
       </para>

       <para>
        Turning off this parameter does not affect use of
        WAL archiving for point-in-time recovery (PITR)
        (see <xref linkend="continuous-archiving"/>).
       </para>

       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-log-hints" xreflabel="wal_log_hints">
      <term><varname>wal_log_hints</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>wal_log_hints</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When this parameter is <literal>on</literal>, the <productname>PostgreSQL</productname>
        server writes the entire content of each disk page to WAL during the
        first modification of that page after a checkpoint, even for
        non-critical modifications of so-called hint bits.
       </para>

       <para>
        If data checksums are enabled, hint bit updates are always WAL-logged
        and this setting is ignored. You can use this setting to test how much
        extra WAL-logging would occur if your database had data checksums
        enabled.
       </para>

       <para>
        This parameter can only be set at server start. The default value is <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-compression" xreflabel="wal_compression">
      <term><varname>wal_compression</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>wal_compression</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When this parameter is <literal>on</literal>, the <productname>PostgreSQL</productname>
        server compresses a full page image written to WAL when
        <xref linkend="guc-full-page-writes"/> is on or during a base backup.
        A compressed page image will be decompressed during WAL replay.
        The default value is <literal>off</literal>.
        Only superusers can change this setting.
       </para>

       <para>
        Turning this parameter on can reduce the WAL volume without
        increasing the risk of unrecoverable data corruption,
        but at the cost of some extra CPU spent on the compression during
        WAL logging and on the decompression during WAL replay.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-buffers" xreflabel="wal_buffers">
      <term><varname>wal_buffers</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>wal_buffers</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        The amount of shared memory used for WAL data that has not yet been
        written to disk.  The default setting of -1 selects a size equal to
        1/32nd (about 3%) of <xref linkend="guc-shared-buffers"/>, but not less
        than <literal>64kB</literal> nor more than the size of one WAL
        segment, typically <literal>16MB</literal>.  This value can be set
        manually if the automatic choice is too large or too small,
        but any positive value less than <literal>32kB</literal> will be
        treated as <literal>32kB</literal>.
        This parameter can only be set at server start.
       </para>

       <para>
        The contents of the WAL buffers are written out to disk at every
        transaction commit, so extremely large values are unlikely to
        provide a significant benefit.  However, setting this value to at
        least a few megabytes can improve write performance on a busy
        server where many clients are committing at once.  The auto-tuning
        selected by the default setting of -1 should give reasonable
        results in most cases.
       </para>

      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-writer-delay" xreflabel="wal_writer_delay">
      <term><varname>wal_writer_delay</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>wal_writer_delay</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
      <para>
        Specifies how often the WAL writer flushes WAL. After flushing WAL it
        sleeps for <varname>wal_writer_delay</varname> milliseconds, unless woken up
        by an asynchronously committing transaction. If the last flush
        happened less than <varname>wal_writer_delay</varname> milliseconds ago and
        less than <varname>wal_writer_flush_after</varname> bytes of WAL have been
        produced since, then WAL is only written to the operating system, not
        flushed to disk.
        The default value is 200 milliseconds (<literal>200ms</literal>).  Note that
        on many systems, the effective resolution of sleep delays is 10
        milliseconds; setting <varname>wal_writer_delay</varname> to a value that is
        not a multiple of 10 might have the same results as setting it to the
        next higher multiple of 10. This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-writer-flush-after" xreflabel="wal_writer_flush_after">
      <term><varname>wal_writer_flush_after</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>wal_writer_flush_after</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
      <para>
        Specifies how often the WAL writer flushes WAL. If the last flush
        happened less than <varname>wal_writer_delay</varname> milliseconds ago and
        less than <varname>wal_writer_flush_after</varname> bytes of WAL have been
        produced since, then WAL is only written to the operating system, not
        flushed to disk.  If <varname>wal_writer_flush_after</varname> is set
        to <literal>0</literal> then WAL data is flushed immediately.  The default is
        <literal>1MB</literal>. This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-commit-delay" xreflabel="commit_delay">
      <term><varname>commit_delay</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>commit_delay</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        <varname>commit_delay</varname> adds a time delay, measured in
        microseconds, before a WAL flush is initiated.  This can improve
        group commit throughput by allowing a larger number of transactions
        to commit via a single WAL flush, if system load is high enough
        that additional transactions become ready to commit within the
        given interval.  However, it also increases latency by up to
        <varname>commit_delay</varname> microseconds for each WAL
        flush.  Because the delay is just wasted if no other transactions
        become ready to commit, a delay is only performed if at least
        <varname>commit_siblings</varname> other transactions are active
        when a flush is about to be initiated.  Also, no delays are
        performed if <varname>fsync</varname> is disabled.
        The default <varname>commit_delay</varname> is zero (no delay).
        Only superusers can change this setting.
       </para>
       <para>
        In <productname>PostgreSQL</productname> releases prior to 9.3,
        <varname>commit_delay</varname> behaved differently and was much
        less effective: it affected only commits, rather than all WAL flushes,
        and waited for the entire configured delay even if the WAL flush
        was completed sooner.  Beginning in <productname>PostgreSQL</productname> 9.3,
        the first process that becomes ready to flush waits for the configured
        interval, while subsequent processes wait only until the leader
        completes the flush operation.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-commit-siblings" xreflabel="commit_siblings">
      <term><varname>commit_siblings</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>commit_siblings</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Minimum number of concurrent open transactions to require
        before performing the <varname>commit_delay</varname> delay. A larger
        value makes it more probable that at least one other
        transaction will become ready to commit during the delay
        interval. The default is five transactions.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>
     <sect2 id="runtime-config-wal-checkpoints">
     <title>Checkpoints</title>

    <variablelist>
     <varlistentry id="guc-checkpoint-timeout" xreflabel="checkpoint_timeout">
      <term><varname>checkpoint_timeout</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>checkpoint_timeout</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Maximum time between automatic WAL checkpoints, in seconds.
        The valid range is between 30 seconds and one day.
        The default is five minutes (<literal>5min</literal>).
        Increasing this parameter can increase the amount of time needed
        for crash recovery.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-checkpoint-completion-target" xreflabel="checkpoint_completion_target">
      <term><varname>checkpoint_completion_target</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>checkpoint_completion_target</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the target of checkpoint completion, as a fraction of
        total time between checkpoints. The default is 0.5.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-checkpoint-flush-after" xreflabel="checkpoint_flush_after">
      <term><varname>checkpoint_flush_after</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>checkpoint_flush_after</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Whenever more than <varname>checkpoint_flush_after</varname> bytes
        have been written while performing a checkpoint, attempt to force the
        OS to issue these writes to the underlying storage.  Doing so will
        limit the amount of dirty data in the kernel's page cache, reducing
        the likelihood of stalls when an <function>fsync</function> is issued at the end of the
        checkpoint, or when the OS writes data back in larger batches in the
        background.  Often that will result in greatly reduced transaction
        latency, but there also are some cases, especially with workloads
        that are bigger than <xref linkend="guc-shared-buffers"/>, but smaller
        than the OS's page cache, where performance might degrade.  This
        setting may have no effect on some platforms.  The valid range is
        between <literal>0</literal>, which disables forced writeback,
        and <literal>2MB</literal>.  The default is <literal>256kB</literal> on
        Linux, <literal>0</literal> elsewhere.  (If <symbol>BLCKSZ</symbol> is not
        8kB, the default and maximum values scale proportionally to it.)
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-checkpoint-warning" xreflabel="checkpoint_warning">
      <term><varname>checkpoint_warning</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>checkpoint_warning</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Write a message to the server log if checkpoints caused by
        the filling of WAL segment files happen closer together
        than this many seconds (which suggests that
        <varname>max_wal_size</varname> ought to be raised).  The default is
        30 seconds (<literal>30s</literal>).  Zero disables the warning.
        No warnings will be generated if <varname>checkpoint_timeout</varname>
        is less than <varname>checkpoint_warning</varname>.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-wal-size" xreflabel="max_wal_size">
      <term><varname>max_wal_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_wal_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Maximum size to let the WAL grow during automatic
        checkpoints. This is a soft limit; WAL size can exceed
        <varname>max_wal_size</varname> under special circumstances, like
        under heavy load, a failing <varname>archive_command</varname>, or a high
        <varname>wal_keep_segments</varname> setting. The default is 1 GB.
        Increasing this parameter can increase the amount of time needed for
        crash recovery.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-min-wal-size" xreflabel="min_wal_size">
      <term><varname>min_wal_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>min_wal_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        As long as WAL disk usage stays below this setting, old WAL files are
        always recycled for future use at a checkpoint, rather than removed.
        This can be used to ensure that enough WAL space is reserved to
        handle spikes in WAL usage, for example when running large batch
        jobs. The default is 80 MB.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>
     <sect2 id="runtime-config-wal-archiving">
     <title>Archiving</title>

    <variablelist>
     <varlistentry id="guc-archive-mode" xreflabel="archive_mode">
      <term><varname>archive_mode</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>archive_mode</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When <varname>archive_mode</varname> is enabled, completed WAL segments
        are sent to archive storage by setting
        <xref linkend="guc-archive-command"/>. In addition to <literal>off</literal>,
        to disable, there are two modes: <literal>on</literal>, and
        <literal>always</literal>. During normal operation, there is no
        difference between the two modes, but when set to <literal>always</literal>
        the WAL archiver is enabled also during archive recovery or standby
        mode. In <literal>always</literal> mode, all files restored from the archive
        or streamed with streaming replication will be archived (again). See
        <xref linkend="continuous-archiving-in-standby"/> for details.
       </para>
       <para>
        <varname>archive_mode</varname> and <varname>archive_command</varname> are
        separate variables so that <varname>archive_command</varname> can be
        changed without leaving archiving mode.
        This parameter can only be set at server start.
        <varname>archive_mode</varname> cannot be enabled when
        <varname>wal_level</varname> is set to <literal>minimal</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-archive-command" xreflabel="archive_command">
      <term><varname>archive_command</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>archive_command</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        The local shell command to execute to archive a completed WAL file
        segment.  Any <literal>%p</literal> in the string is
        replaced by the path name of the file to archive, and any
        <literal>%f</literal> is replaced by only the file name.
        (The path name is relative to the working directory of the server,
        i.e., the cluster's data directory.)
        Use <literal>%%</literal> to embed an actual <literal>%</literal> character in the
        command.  It is important for the command to return a zero
        exit status only if it succeeds. For more information see
        <xref linkend="backup-archiving-wal"/>.
       </para>
       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.  It is ignored unless
        <varname>archive_mode</varname> was enabled at server start.
        If <varname>archive_command</varname> is an empty string (the default) while
        <varname>archive_mode</varname> is enabled, WAL archiving is temporarily
        disabled, but the server continues to accumulate WAL segment files in
        the expectation that a command will soon be provided.  Setting
        <varname>archive_command</varname> to a command that does nothing but
        return true, e.g., <literal>/bin/true</literal> (<literal>REM</literal> on
        Windows), effectively disables
        archiving, but also breaks the chain of WAL files needed for
        archive recovery, so it should only be used in unusual circumstances.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-archive-timeout" xreflabel="archive_timeout">
      <term><varname>archive_timeout</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>archive_timeout</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        The <xref linkend="guc-archive-command"/> is only invoked for
        completed WAL segments. Hence, if your server generates little WAL
        traffic (or has slack periods where it does so), there could be a
        long delay between the completion of a transaction and its safe
        recording in archive storage.  To limit how old unarchived
        data can be, you can set <varname>archive_timeout</varname> to force the
        server to switch to a new WAL segment file periodically.  When this
        parameter is greater than zero, the server will switch to a new
        segment file whenever this many seconds have elapsed since the last
        segment file switch, and there has been any database activity,
        including a single checkpoint (checkpoints are skipped if there is
        no database activity).  Note that archived files that are closed
        early due to a forced switch are still the same length as completely
        full files.  Therefore, it is unwise to use a very short
        <varname>archive_timeout</varname> &mdash; it will bloat your archive
        storage.  <varname>archive_timeout</varname> settings of a minute or so are
        usually reasonable.  You should consider using streaming replication,
        instead of archiving, if you want data to be copied off the master
        server more quickly than that.
        This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

   </sect1>

   <sect1 id="runtime-config-replication">
    <title>Replication</title>

    <para>
     These settings control the behavior of the built-in
     <firstterm>streaming replication</firstterm> feature (see
     <xref linkend="streaming-replication"/>).  Servers will be either a
     master or a standby server.  Masters can send data, while standbys
     are always receivers of replicated data.  When cascading replication
     (see <xref linkend="cascading-replication"/>) is used, standby servers
     can also be senders, as well as receivers.
     Parameters are mainly for sending and standby servers, though some
     parameters have meaning only on the master server.  Settings may vary
     across the cluster without problems if that is required.
    </para>

    <sect2 id="runtime-config-replication-sender">
     <title>Sending Servers</title>

     <para>
      These parameters can be set on any server that is
      to send replication data to one or more standby servers.
      The master is always a sending server, so these parameters must
      always be set on the master.
      The role and meaning of these parameters does not change after a
      standby becomes the master.
     </para>

     <variablelist>
      <varlistentry id="guc-max-wal-senders" xreflabel="max_wal_senders">
       <term><varname>max_wal_senders</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>max_wal_senders</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
       <para>
        Specifies the maximum number of concurrent connections from
        standby servers or streaming base backup clients (i.e., the
        maximum number of simultaneously running WAL sender
        processes). The default is 10. The value 0 means replication is
        disabled. WAL sender processes count towards the total number
        of connections, so this parameter's value must be less than
        <xref linkend="guc-max-connections"/> minus
        <xref linkend="guc-superuser-reserved-connections"/>.
        Abrupt streaming client disconnection might leave an orphaned
        connection slot behind until
        a timeout is reached, so this parameter should be set slightly
        higher than the maximum number of expected clients so disconnected
        clients can immediately reconnect.  This parameter can only
        be set at server start.
        Also, <varname>wal_level</varname> must be set to
        <literal>replica</literal> or higher to allow connections from standby
        servers.
       </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-max-replication-slots" xreflabel="max_replication_slots">
       <term><varname>max_replication_slots</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>max_replication_slots</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
         Specifies the maximum number of replication slots
         (see <xref linkend="streaming-replication-slots"/>) that the server
         can support. The default is 10.  This parameter can only be set at
         server start.
         Setting it to a lower value than the number of currently
         existing replication slots will prevent the server from starting.
         Also, <varname>wal_level</varname> must be set
         to <literal>replica</literal> or higher to allow replication slots to
         be used.
        </para>

        <para>
         On the subscriber side, specifies how many replication origins (see
         <xref linkend="replication-origins"/>) can be tracked simultaneously,
         effectively limiting how many logical replication subscriptions can
         be created on the server. Setting it a lower value than the current
         number of tracked replication origins (reflected in
         <link linkend="view-pg-replication-origin-status">pg_replication_origin_status</link>,
         not <link linkend="catalog-pg-replication-origin">pg_replication_origin</link>)
         will prevent the server from starting.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-wal-keep-segments" xreflabel="wal_keep_segments">
       <term><varname>wal_keep_segments</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>wal_keep_segments</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
       <para>
        Specifies the minimum number of past log file segments kept in the
        <filename>pg_wal</filename>
        directory, in case a standby server needs to fetch them for streaming
        replication. Each segment is normally 16 megabytes. If a standby
        server connected to the sending server falls behind by more than
        <varname>wal_keep_segments</varname> segments, the sending server might remove
        a WAL segment still needed by the standby, in which case the
        replication connection will be terminated.  Downstream connections
        will also eventually fail as a result.  (However, the standby
        server can recover by fetching the segment from archive, if WAL
        archiving is in use.)
       </para>

       <para>
        This sets only the minimum number of segments retained in
        <filename>pg_wal</filename>; the system might need to retain more segments
        for WAL archival or to recover from a checkpoint. If
        <varname>wal_keep_segments</varname> is zero (the default), the system
        doesn't keep any extra segments for standby purposes, so the number
        of old WAL segments available to standby servers is a function of
        the location of the previous checkpoint and status of WAL
        archiving.
        This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command line.
       </para>
       </listitem>
      </varlistentry>

     <varlistentry id="guc-wal-sender-timeout" xreflabel="wal_sender_timeout">
      <term><varname>wal_sender_timeout</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>wal_sender_timeout</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Terminate replication connections that are inactive longer
        than the specified number of milliseconds. This is useful for
        the sending server to detect a standby crash or network outage.
        A value of zero disables the timeout mechanism.  This parameter
        can only be set in
        the <filename>postgresql.conf</filename> file or on the server command line.
        The default value is 60 seconds.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-track-commit-timestamp" xreflabel="track_commit_timestamp">
      <term><varname>track_commit_timestamp</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>track_commit_timestamp</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Record commit time of transactions. This parameter
        can only be set in <filename>postgresql.conf</filename> file or on the server
        command line. The default value is <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

    <sect2 id="runtime-config-replication-master">
     <title>Master Server</title>

     <para>
      These parameters can be set on the master/primary server that is
      to send replication data to one or more standby servers.
      Note that in addition to these parameters,
      <xref linkend="guc-wal-level"/> must be set appropriately on the master
      server, and optionally WAL archiving can be enabled as
      well (see <xref linkend="runtime-config-wal-archiving"/>).
      The values of these parameters on standby servers are irrelevant,
      although you may wish to set them there in preparation for the
      possibility of a standby becoming the master.
     </para>

    <variablelist>

     <varlistentry id="guc-synchronous-standby-names" xreflabel="synchronous_standby_names">
      <term><varname>synchronous_standby_names</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>synchronous_standby_names</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies a list of standby servers that can support
        <firstterm>synchronous replication</firstterm>, as described in
        <xref linkend="synchronous-replication"/>.
        There will be one or more active synchronous standbys;
        transactions waiting for commit will be allowed to proceed after
        these standby servers confirm receipt of their data.
        The synchronous standbys will be those whose names appear
        in this list, and
        that are both currently connected and streaming data in real-time
        (as shown by a state of <literal>streaming</literal> in the <link
        linkend="pg-stat-replication-view"><literal>pg_stat_replication</literal></link>
        view).
        Specifying more than one synchronous standby can allow for very high
        availability and protection against data loss.
       </para>
       <para>
        The name of a standby server for this purpose is the
        <varname>application_name</varname> setting of the standby, as set in the
        standby's connection information.  In case of a physical replication
        standby, this should be set in the <varname>primary_conninfo</varname>
        setting in <filename>recovery.conf</filename>; the default
        is <literal>walreceiver</literal>.  For logical replication, this can
        be set in the connection information of the subscription, and it
        defaults to the subscription name.  For other replication stream
        consumers, consult their documentation.
       </para>
       <para>
        This parameter specifies a list of standby servers using
        either of the following syntaxes:
<synopsis>
[FIRST] <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="parameter">standby_name</replaceable> [, ...] )
ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="parameter">standby_name</replaceable> [, ...] )
<replaceable class="parameter">standby_name</replaceable> [, ...]
</synopsis>
        where <replaceable class="parameter">num_sync</replaceable> is
        the number of synchronous standbys that transactions need to
        wait for replies from,
        and <replaceable class="parameter">standby_name</replaceable>
        is the name of a standby server.
        <literal>FIRST</literal> and <literal>ANY</literal> specify the method to choose
        synchronous standbys from the listed servers.
       </para>
       <para>
        The keyword <literal>FIRST</literal>, coupled with
        <replaceable class="parameter">num_sync</replaceable>, specifies a
        priority-based synchronous replication and makes transaction commits
        wait until their WAL records are replicated to
        <replaceable class="parameter">num_sync</replaceable> synchronous
        standbys chosen based on their priorities. For example, a setting of
        <literal>FIRST 3 (s1, s2, s3, s4)</literal> will cause each commit to wait for
        replies from three higher-priority standbys chosen from standby servers
        <literal>s1</literal>, <literal>s2</literal>, <literal>s3</literal> and <literal>s4</literal>.
        The standbys whose names appear earlier in the list are given higher
        priority and will be considered as synchronous. Other standby servers
        appearing later in this list represent potential synchronous standbys.
        If any of the current synchronous standbys disconnects for whatever
        reason, it will be replaced immediately with the next-highest-priority
        standby. The keyword <literal>FIRST</literal> is optional.
       </para>
       <para>
        The keyword <literal>ANY</literal>, coupled with
        <replaceable class="parameter">num_sync</replaceable>, specifies a
        quorum-based synchronous replication and makes transaction commits
        wait until their WAL records are replicated to <emphasis>at least</emphasis>
        <replaceable class="parameter">num_sync</replaceable> listed standbys.
        For example, a setting of <literal>ANY 3 (s1, s2, s3, s4)</literal> will cause
        each commit to proceed as soon as at least any three standbys of
        <literal>s1</literal>, <literal>s2</literal>, <literal>s3</literal> and <literal>s4</literal>
        reply.
       </para>
       <para>
        <literal>FIRST</literal> and <literal>ANY</literal> are case-insensitive. If these
        keywords are used as the name of a standby server,
        its <replaceable class="parameter">standby_name</replaceable> must
        be double-quoted.
       </para>
       <para>
        The third syntax was used before <productname>PostgreSQL</productname>
        version 9.6 and is still supported. It's the same as the first syntax
        with <literal>FIRST</literal> and
        <replaceable class="parameter">num_sync</replaceable> equal to 1.
        For example, <literal>FIRST 1 (s1, s2)</literal> and <literal>s1, s2</literal> have
        the same meaning: either <literal>s1</literal> or <literal>s2</literal> is chosen
        as a synchronous standby.
       </para>
       <para>
        The special entry <literal>*</literal> matches any standby name.
       </para>
       <para>
        There is no mechanism to enforce uniqueness of standby names.  In case
        of duplicates one of the matching standbys will be considered as
        higher priority, though exactly which one is indeterminate.
       </para>
       <note>
        <para>
         Each <replaceable class="parameter">standby_name</replaceable>
         should have the form of a valid SQL identifier, unless it
         is <literal>*</literal>.  You can use double-quoting if necessary.  But note
         that <replaceable class="parameter">standby_name</replaceable>s are
         compared to standby application names case-insensitively, whether
         double-quoted or not.
        </para>
       </note>
       <para>
        If no synchronous standby names are specified here, then synchronous
        replication is not enabled and transaction commits will not wait for
        replication.  This is the default configuration.  Even when
        synchronous replication is enabled, individual transactions can be
        configured not to wait for replication by setting the
        <xref linkend="guc-synchronous-commit"/> parameter to
        <literal>local</literal> or <literal>off</literal>.
       </para>
       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-vacuum-defer-cleanup-age" xreflabel="vacuum_defer_cleanup_age">
      <term><varname>vacuum_defer_cleanup_age</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>vacuum_defer_cleanup_age</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the number of transactions by which <command>VACUUM</command> and
        <link linkend="storage-hot"><acronym>HOT</acronym> updates</link>
        will defer cleanup of dead row versions. The
        default is zero transactions, meaning that dead row versions can be
        removed as soon as possible, that is, as soon as they are no longer
        visible to any open transaction.  You may wish to set this to a
        non-zero value on a primary server that is supporting hot standby
        servers, as described in <xref linkend="hot-standby"/>.  This allows
        more time for queries on the standby to complete without incurring
        conflicts due to early cleanup of rows.  However, since the value
        is measured in terms of number of write transactions occurring on the
        primary server, it is difficult to predict just how much additional
        grace time will be made available to standby queries.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
       <para>
        You should also consider setting <varname>hot_standby_feedback</varname>
        on standby server(s) as an alternative to using this parameter.
       </para>
       <para>
        This does not prevent cleanup of dead rows which have reached the age
        specified by <varname>old_snapshot_threshold</varname>.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

    <sect2 id="runtime-config-replication-standby">
     <title>Standby Servers</title>

     <para>
      These settings control the behavior of a standby server that is
      to receive replication data.  Their values on the master server
      are irrelevant.
     </para>

    <variablelist>

     <varlistentry id="guc-hot-standby" xreflabel="hot_standby">
      <term><varname>hot_standby</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>hot_standby</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies whether or not you can connect and run queries during
        recovery, as described in <xref linkend="hot-standby"/>.
        The default value is <literal>on</literal>.
        This parameter can only be set at server start. It only has effect
        during archive recovery or in standby mode.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-standby-archive-delay" xreflabel="max_standby_archive_delay">
      <term><varname>max_standby_archive_delay</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_standby_archive_delay</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When Hot Standby is active, this parameter determines how long the
        standby server should wait before canceling standby queries that
        conflict with about-to-be-applied WAL entries, as described in
        <xref linkend="hot-standby-conflict"/>.
        <varname>max_standby_archive_delay</varname> applies when WAL data is
        being read from WAL archive (and is therefore not current).
        The default is 30 seconds. Units are milliseconds if not specified.
        A value of -1 allows the standby to wait forever for conflicting
        queries to complete.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
       <para>
        Note that <varname>max_standby_archive_delay</varname> is not the same as the
        maximum length of time a query can run before cancellation; rather it
        is the maximum total time allowed to apply any one WAL segment's data.
        Thus, if one query has resulted in significant delay earlier in the
        WAL segment, subsequent conflicting queries will have much less grace
        time.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-standby-streaming-delay" xreflabel="max_standby_streaming_delay">
      <term><varname>max_standby_streaming_delay</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_standby_streaming_delay</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When Hot Standby is active, this parameter determines how long the
        standby server should wait before canceling standby queries that
        conflict with about-to-be-applied WAL entries, as described in
        <xref linkend="hot-standby-conflict"/>.
        <varname>max_standby_streaming_delay</varname> applies when WAL data is
        being received via streaming replication.
        The default is 30 seconds. Units are milliseconds if not specified.
        A value of -1 allows the standby to wait forever for conflicting
        queries to complete.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
       <para>
        Note that <varname>max_standby_streaming_delay</varname> is not the same as
        the maximum length of time a query can run before cancellation; rather
        it is the maximum total time allowed to apply WAL data once it has
        been received from the primary server.  Thus, if one query has
        resulted in significant delay, subsequent conflicting queries will
        have much less grace time until the standby server has caught up
        again.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-receiver-status-interval" xreflabel="wal_receiver_status_interval">
      <term><varname>wal_receiver_status_interval</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>wal_receiver_status_interval</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
      <para>
       Specifies the minimum frequency for the WAL receiver
       process on the standby to send information about replication progress
       to the primary or upstream standby, where it can be seen using the
       <link
       linkend="pg-stat-replication-view"><literal>pg_stat_replication</literal></link>
       view.  The standby will report
       the last write-ahead log location it has written, the last position it
       has flushed to disk, and the last position it has applied.
       This parameter's
       value is the maximum interval, in seconds, between reports.  Updates are
       sent each time the write or flush positions change, or at least as
       often as specified by this parameter.  Thus, the apply position may
       lag slightly behind the true position.  Setting this parameter to zero
       disables status updates completely.  This parameter can only be set in
       the <filename>postgresql.conf</filename> file or on the server command line.
       The default value is 10 seconds.
      </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-hot-standby-feedback" xreflabel="hot_standby_feedback">
      <term><varname>hot_standby_feedback</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>hot_standby_feedback</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies whether or not a hot standby will send feedback to the primary
        or upstream standby
        about queries currently executing on the standby. This parameter can
        be used to eliminate query cancels caused by cleanup records, but
        can cause database bloat on the primary for some workloads.
        Feedback messages will not be sent more frequently than once per
        <varname>wal_receiver_status_interval</varname>. The default value is
        <literal>off</literal>. This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command line.
       </para>
       <para>
        If cascaded replication is in use the feedback is passed upstream
        until it eventually reaches the primary.  Standbys make no other use
        of feedback they receive other than to pass upstream.
       </para>
       <para>
        This setting does not override the behavior of
        <varname>old_snapshot_threshold</varname> on the primary; a snapshot on the
        standby which exceeds the primary's age threshold can become invalid,
        resulting in cancellation of transactions on the standby.  This is
        because <varname>old_snapshot_threshold</varname> is intended to provide an
        absolute limit on the time which dead rows can contribute to bloat,
        which would otherwise be violated because of the configuration of a
        standby.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-receiver-timeout" xreflabel="wal_receiver_timeout">
      <term><varname>wal_receiver_timeout</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>wal_receiver_timeout</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Terminate replication connections that are inactive longer
        than the specified number of milliseconds. This is useful for
        the receiving standby server to detect a primary node crash or network
        outage.
        A value of zero disables the timeout mechanism.  This parameter
        can only be set in
        the <filename>postgresql.conf</filename> file or on the server command line.
        The default value is 60 seconds.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-retrieve-retry-interval" xreflabel="wal_retrieve_retry_interval">
      <term><varname>wal_retrieve_retry_interval</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>wal_retrieve_retry_interval</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specify how long the standby server should wait when WAL data is not
        available from any sources (streaming replication,
        local <filename>pg_wal</filename> or WAL archive) before retrying to
        retrieve WAL data.  This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command line.
        The default value is 5 seconds. Units are milliseconds if not specified.
       </para>
       <para>
        This parameter is useful in configurations where a node in recovery
        needs to control the amount of time to wait for new WAL data to be
        available. For example, in archive recovery, it is possible to
        make the recovery more responsive in the detection of a new WAL
        log file by reducing the value of this parameter. On a system with
        low WAL activity, increasing it reduces the amount of requests necessary
        to access WAL archives, something useful for example in cloud
        environments where the amount of times an infrastructure is accessed
        is taken into account.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

    <sect2 id="runtime-config-replication-subscriber">
     <title>Subscribers</title>

     <para>
      These settings control the behavior of a logical replication subscriber.
      Their values on the publisher are irrelevant.
     </para>

     <para>
      Note that <varname>wal_receiver_timeout</varname>,
      <varname>wal_receiver_status_interval</varname> and
      <varname>wal_retrieve_retry_interval</varname> configuration parameters
      affect the logical replication workers as well.
     </para>

     <variablelist>

     <varlistentry id="guc-max-logical-replication-workers" xreflabel="max_logical_replication_workers">
      <term><varname>max_logical_replication_workers</varname> (<type>int</type>)
      <indexterm>
       <primary><varname>max_logical_replication_workers</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies maximum number of logical replication workers. This includes
        both apply workers and table synchronization workers.
       </para>
       <para>
        Logical replication workers are taken from the pool defined by
        <varname>max_worker_processes</varname>.
       </para>
       <para>
        The default value is 4. This parameter can only be set at server
        start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-sync-workers-per-subscription" xreflabel="max_sync_workers_per_subscription">
      <term><varname>max_sync_workers_per_subscription</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_sync_workers_per_subscription</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Maximum number of synchronization workers per subscription. This
        parameter controls the amount of parallelism of the initial data copy
        during the subscription initialization or when new tables are added.
       </para>
       <para>
        Currently, there can be only one synchronization worker per table.
       </para>
       <para>
        The synchronization workers are taken from the pool defined by
        <varname>max_logical_replication_workers</varname>.
       </para>
       <para>
        The default value is 2. This parameter can only be set in the
        <filename>postgresql.conf</filename> file or on the server command
        line.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

   </sect1>

   <sect1 id="runtime-config-query">
    <title>Query Planning</title>

    <sect2 id="runtime-config-query-enable">
     <title>Planner Method Configuration</title>

      <para>
       These configuration parameters provide a crude method of
       influencing the query plans chosen by the query optimizer. If
       the default plan chosen by the optimizer for a particular query
       is not optimal, a <emphasis>temporary</emphasis> solution is to use one
       of these configuration parameters to force the optimizer to
       choose a different plan.
       Better ways to improve the quality of the
       plans chosen by the optimizer include adjusting the planner cost
       constants (see <xref linkend="runtime-config-query-constants"/>),
       running <xref linkend="sql-analyze"/> manually, increasing
       the value of the <xref
       linkend="guc-default-statistics-target"/> configuration parameter,
       and increasing the amount of statistics collected for
       specific columns using <command>ALTER TABLE SET
       STATISTICS</command>.
      </para>

     <variablelist>
     <varlistentry id="guc-enable-bitmapscan" xreflabel="enable_bitmapscan">
      <term><varname>enable_bitmapscan</varname> (<type>boolean</type>)
      <indexterm>
       <primary>bitmap scan</primary>
      </indexterm>
      <indexterm>
       <primary><varname>enable_bitmapscan</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of bitmap-scan plan
        types. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-gathermerge" xreflabel="enable_gathermerge">
      <term><varname>enable_gathermerge</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_gathermerge</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of gather
        merge plan types. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-hashagg" xreflabel="enable_hashagg">
      <term><varname>enable_hashagg</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_hashagg</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of hashed
        aggregation plan types. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-hashjoin" xreflabel="enable_hashjoin">
      <term><varname>enable_hashjoin</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_hashjoin</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of hash-join plan
        types. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-indexscan" xreflabel="enable_indexscan">
      <term><varname>enable_indexscan</varname> (<type>boolean</type>)
      <indexterm>
       <primary>index scan</primary>
      </indexterm>
      <indexterm>
       <primary><varname>enable_indexscan</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of index-scan plan
        types. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-indexonlyscan" xreflabel="enable_indexonlyscan">
      <term><varname>enable_indexonlyscan</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_indexonlyscan</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of index-only-scan plan
        types (see <xref linkend="indexes-index-only-scans"/>).
        The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-material" xreflabel="enable_material">
      <term><varname>enable_material</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_material</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of materialization.
        It is impossible to suppress materialization entirely,
        but turning this variable off prevents the planner from inserting
        materialize nodes except in cases where it is required for correctness.
        The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-mergejoin" xreflabel="enable_mergejoin">
      <term><varname>enable_mergejoin</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_mergejoin</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of merge-join plan
        types. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-nestloop" xreflabel="enable_nestloop">
      <term><varname>enable_nestloop</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_nestloop</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of nested-loop join
        plans. It is impossible to suppress nested-loop joins entirely,
        but turning this variable off discourages the planner from using
        one if there are other methods available. The default is
        <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-parallel-append" xreflabel="enable_parallel_append">
      <term><varname>enable_parallel_append</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_parallel_append</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of parallel-aware
        append plan types. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-parallel-hash" xreflabel="enable_parallel_hash">
      <term><varname>enable_parallel_hash</varname> (<type>boolean</type>)
       <indexterm>
        <primary><varname>enable_parallel_hash</varname> configuration parameter</primary>
       </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of hash-join plan
        types with parallel hash. Has no effect if hash-join plans are not
        also enabled. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-partition-pruning" xreflabel="enable_partition_pruning">
      <term><varname>enable_partition_pruning</varname> (<type>boolean</type>)
       <indexterm>
        <primary><varname>enable_partition_pruning</varname> configuration parameter</primary>
       </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's ability to eliminate a
        partitioned table's partitions from query plans.  This also controls
        the planner's ability to generate query plans which allow the query
        executor to remove (ignore) partitions during query execution.  The
        default is <literal>on</literal>.
        See <xref linkend="ddl-partition-pruning"/> for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-partitionwise-join" xreflabel="enable_partitionwise_join">
      <term><varname>enable_partitionwise_join</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_partitionwise_join</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of partitionwise join,
        which allows a join between partitioned tables to be performed by
        joining the matching partitions.  Partitionwise join currently applies
        only when the join conditions include all the partition keys, which
        must be of the same data type and have exactly matching sets of child
        partitions.  Because partitionwise join planning can use significantly
        more CPU time and memory during planning, the default is
        <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-partitionwise-aggregate" xreflabel="enable_partitionwise_aggregate">
      <term><varname>enable_partitionwise_aggregate</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_partitionwise_aggregate</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of partitionwise grouping
        or aggregation, which allows grouping or aggregation on a partitioned
        tables performed separately for each partition.  If the <literal>GROUP
        BY</literal> clause does not include the partition keys, only partial
        aggregation can be performed on a per-partition basis, and
        finalization must be performed later.  Because partitionwise grouping
        or aggregation can use significantly more CPU time and memory during
        planning, the default is <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-seqscan" xreflabel="enable_seqscan">
      <term><varname>enable_seqscan</varname> (<type>boolean</type>)
      <indexterm>
       <primary>sequential scan</primary>
      </indexterm>
      <indexterm>
       <primary><varname>enable_seqscan</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of sequential scan
        plan types. It is impossible to suppress sequential scans
        entirely, but turning this variable off discourages the planner
        from using one if there are other methods available. The
        default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-sort" xreflabel="enable_sort">
      <term><varname>enable_sort</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_sort</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of explicit sort
        steps. It is impossible to suppress explicit sorts entirely,
        but turning this variable off discourages the planner from
        using one if there are other methods available. The default
        is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-tidscan" xreflabel="enable_tidscan">
      <term><varname>enable_tidscan</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>enable_tidscan</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables the query planner's use of <acronym>TID</acronym>
        scan plan types. The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>
     <sect2 id="runtime-config-query-constants">
     <title>Planner Cost Constants</title>

    <para>
     The <firstterm>cost</firstterm> variables described in this section are measured
     on an arbitrary scale.  Only their relative values matter, hence
     scaling them all up or down by the same factor will result in no change
     in the planner's choices.  By default, these cost variables are based on
     the cost of sequential page fetches; that is,
     <varname>seq_page_cost</varname> is conventionally set to <literal>1.0</literal>
     and the other cost variables are set with reference to that.  But
     you can use a different scale if you prefer, such as actual execution
     times in milliseconds on a particular machine.
    </para>

   <note>
    <para>
     Unfortunately, there is no well-defined method for determining ideal
     values for the cost variables.  They are best treated as averages over
     the entire mix of queries that a particular installation will receive.  This
     means that changing them on the basis of just a few experiments is very
     risky.
    </para>
   </note>

     <variablelist>

     <varlistentry id="guc-seq-page-cost" xreflabel="seq_page_cost">
      <term><varname>seq_page_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>seq_page_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of a disk page fetch
        that is part of a series of sequential fetches.  The default is 1.0.
        This value can be overridden for tables and indexes in a particular
        tablespace by setting the tablespace parameter of the same name
        (see <xref linkend="sql-altertablespace"/>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-random-page-cost" xreflabel="random_page_cost">
      <term><varname>random_page_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>random_page_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of a
        non-sequentially-fetched disk page.  The default is 4.0.
        This value can be overridden for tables and indexes in a particular
        tablespace by setting the tablespace parameter of the same name
        (see <xref linkend="sql-altertablespace"/>).
       </para>

       <para>
        Reducing this value relative to <varname>seq_page_cost</varname>
        will cause the system to prefer index scans; raising it will
        make index scans look relatively more expensive.  You can raise
        or lower both values together to change the importance of disk I/O
        costs relative to CPU costs, which are described by the following
        parameters.
       </para>

       <para>
        Random access to mechanical disk storage is normally much more expensive
        than four times sequential access.  However, a lower default is used
        (4.0) because the majority of random accesses to disk, such as indexed
        reads, are assumed to be in cache.  The default value can be thought of
        as modeling random access as 40 times slower than sequential, while
        expecting 90% of random reads to be cached.
       </para>

       <para>
        If you believe a 90% cache rate is an incorrect assumption
        for your workload, you can increase random_page_cost to better
        reflect the true cost of random storage reads. Correspondingly,
        if your data is likely to be completely in cache, such as when
        the database is smaller than the total server memory, decreasing
        random_page_cost can be appropriate.  Storage that has a low random
        read cost relative to sequential, e.g., solid-state drives, might
        also be better modeled with a lower value for random_page_cost,
        e.g., <literal>1.1</literal>.
       </para>

       <tip>
        <para>
         Although the system will let you set <varname>random_page_cost</varname> to
         less than <varname>seq_page_cost</varname>, it is not physically sensible
         to do so.  However, setting them equal makes sense if the database
         is entirely cached in RAM, since in that case there is no penalty
         for touching pages out of sequence.  Also, in a heavily-cached
         database you should lower both values relative to the CPU parameters,
         since the cost of fetching a page already in RAM is much smaller
         than it would normally be.
        </para>
       </tip>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-cpu-tuple-cost" xreflabel="cpu_tuple_cost">
      <term><varname>cpu_tuple_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>cpu_tuple_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing
        each row during a query.
        The default is 0.01.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-cpu-index-tuple-cost" xreflabel="cpu_index_tuple_cost">
      <term><varname>cpu_index_tuple_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>cpu_index_tuple_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing
        each index entry during an index scan.
        The default is 0.005.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-cpu-operator-cost" xreflabel="cpu_operator_cost">
      <term><varname>cpu_operator_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>cpu_operator_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing each
        operator or function executed during a query.
        The default is 0.0025.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-parallel-setup-cost" xreflabel="parallel_setup_cost">
      <term><varname>parallel_setup_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>parallel_setup_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of launching parallel worker
        processes.
        The default is 1000.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-parallel-tuple-cost" xreflabel="parallel_tuple_cost">
      <term><varname>parallel_tuple_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>parallel_tuple_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of transferring one tuple
        from a parallel worker process to another process.
        The default is 0.1.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-min-parallel-table-scan-size" xreflabel="min_parallel_table_scan_size">
      <term><varname>min_parallel_table_scan_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>min_parallel_table_scan_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the minimum amount of table data that must be scanned in order
        for a parallel scan to be considered.  For a parallel sequential scan,
        the amount of table data scanned is always equal to the size of the
        table, but when indexes are used the amount of table data
        scanned will normally be less.  The default is 8
        megabytes (<literal>8MB</literal>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-min-parallel-index-scan-size" xreflabel="min_parallel_index_scan_size">
      <term><varname>min_parallel_index_scan_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>min_parallel_index_scan_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the minimum amount of index data that must be scanned in order
        for a parallel scan to be considered.  Note that a parallel index scan
        typically won't touch the entire index; it is the number of pages
        which the planner believes will actually be touched by the scan which
        is relevant.  The default is 512 kilobytes (<literal>512kB</literal>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-effective-cache-size" xreflabel="effective_cache_size">
      <term><varname>effective_cache_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>effective_cache_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's assumption about the effective size of the
        disk cache that is available to a single query.  This is
        factored into estimates of the cost of using an index; a
        higher value makes it more likely index scans will be used, a
        lower value makes it more likely sequential scans will be
        used. When setting this parameter you should consider both
        <productname>PostgreSQL</productname>'s shared buffers and the
        portion of the kernel's disk cache that will be used for
        <productname>PostgreSQL</productname> data files, though some
        data might exist in both places. Also, take
        into account the expected number of concurrent queries on different
        tables, since they will have to share the available
        space.  This parameter has no effect on the size of shared
        memory allocated by <productname>PostgreSQL</productname>, nor
        does it reserve kernel disk cache; it is used only for estimation
        purposes.  The system also does not assume data remains in
        the disk cache between queries.  The default is 4 gigabytes
        (<literal>4GB</literal>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit-above-cost" xreflabel="jit_above_cost">
      <term><varname>jit_above_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>jit_above_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the query cost above which JIT compilation is activated, if
        enabled (see <xref linkend="jit"/>).
        Performing <acronym>JIT</acronym> costs planning time but can
        accelerate query execution.
        Setting this to <literal>-1</literal> disables JIT compilation.
        The default is <literal>100000</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit-inline-above-cost" xreflabel="jit_inline_above_cost">
      <term><varname>jit_inline_above_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>jit_inline_above_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the query cost above which JIT compilation attempts to inline
        functions and operators.  Inlining adds planning time, but can
        improve execution speed.  It is not meaningful to set this to less
        than <varname>jit_above_cost</varname>.
        Setting this to <literal>-1</literal> disables inlining.
        The default is <literal>500000</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit-optimize-above-cost" xreflabel="jit_optimize_above_cost">
      <term><varname>jit_optimize_above_cost</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>jit_optimize_above_cost</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the query cost above which JIT compilation applies expensive
        optimizations.  Such optimization adds planning time, but can improve
        execution speed.  It is not meaningful to set this to less
        than <varname>jit_above_cost</varname>, and it is unlikely to be
        beneficial to set it to more
        than <varname>jit_inline_above_cost</varname>.
        Setting this to <literal>-1</literal> disables expensive optimizations.
        The default is <literal>500000</literal>.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect2>
     <sect2 id="runtime-config-query-geqo">
     <title>Genetic Query Optimizer</title>

     <para>
      The genetic query optimizer (GEQO) is an algorithm that does query
      planning using heuristic searching.  This reduces planning time for
      complex queries (those joining many relations), at the cost of producing
      plans that are sometimes inferior to those found by the normal
      exhaustive-search algorithm.
      For more information see <xref linkend="geqo"/>.
     </para>

     <variablelist>

     <varlistentry id="guc-geqo" xreflabel="geqo">
      <term><varname>geqo</varname> (<type>boolean</type>)
      <indexterm>
       <primary>genetic query optimization</primary>
      </indexterm>
      <indexterm>
       <primary>GEQO</primary>
       <see>genetic query optimization</see>
      </indexterm>
      <indexterm>
       <primary><varname>geqo</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables or disables genetic query optimization.
        This is on by default.  It is usually best not to turn it off in
        production; the <varname>geqo_threshold</varname> variable provides
        more granular control of GEQO.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-threshold" xreflabel="geqo_threshold">
      <term><varname>geqo_threshold</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>geqo_threshold</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Use genetic query optimization to plan queries with at least
        this many <literal>FROM</literal> items involved. (Note that a
        <literal>FULL OUTER JOIN</literal> construct counts as only one <literal>FROM</literal>
        item.) The default is 12. For simpler queries it is usually best
        to use the regular, exhaustive-search planner, but for queries with
        many tables the exhaustive search takes too long, often
        longer than the penalty of executing a suboptimal plan.  Thus,
        a threshold on the size of the query is a convenient way to manage
        use of GEQO.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-effort" xreflabel="geqo_effort">
      <term><varname>geqo_effort</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>geqo_effort</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls the trade-off between planning time and query plan
        quality in GEQO. This variable must be an integer in the
        range from 1 to 10. The default value is five. Larger values
        increase the time spent doing query planning, but also
        increase the likelihood that an efficient query plan will be
        chosen.
       </para>

       <para>
        <varname>geqo_effort</varname> doesn't actually do anything
        directly; it is only used to compute the default values for
        the other variables that influence GEQO behavior (described
        below). If you prefer, you can set the other parameters by
        hand instead.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-pool-size" xreflabel="geqo_pool_size">
      <term><varname>geqo_pool_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>geqo_pool_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls the pool size used by GEQO, that is the
        number of individuals in the genetic population.  It must be
        at least two, and useful values are typically 100 to 1000.  If
        it is set to zero (the default setting) then a suitable
        value is chosen based on <varname>geqo_effort</varname> and
        the number of tables in the query.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-generations" xreflabel="geqo_generations">
      <term><varname>geqo_generations</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>geqo_generations</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls the number of generations used by GEQO, that is
        the number of iterations of the algorithm.  It must
        be at least one, and useful values are in the same range as
        the pool size.  If it is set to zero (the default setting)
        then a suitable value is chosen based on
        <varname>geqo_pool_size</varname>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-selection-bias" xreflabel="geqo_selection_bias">
      <term><varname>geqo_selection_bias</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>geqo_selection_bias</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls the selection bias used by GEQO. The selection bias
        is the selective pressure within the population. Values can be
        from 1.50 to 2.00; the latter is the default.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-seed" xreflabel="geqo_seed">
      <term><varname>geqo_seed</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>geqo_seed</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls the initial value of the random number generator used
        by GEQO to select random paths through the join order search space.
        The value can range from zero (the default) to one.  Varying the
        value changes the set of join paths explored, and may result in a
        better or worse best path being found.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
     <sect2 id="runtime-config-query-other">
     <title>Other Planner Options</title>

     <variablelist>

     <varlistentry id="guc-default-statistics-target" xreflabel="default_statistics_target">
      <term><varname>default_statistics_target</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>default_statistics_target</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the default statistics target for table columns without
        a column-specific target set via <command>ALTER TABLE
        SET STATISTICS</command>.  Larger values increase the time needed to
        do <command>ANALYZE</command>, but might improve the quality of the
        planner's estimates. The default is 100. For more information
        on the use of statistics by the <productname>PostgreSQL</productname>
        query planner, refer to <xref linkend="planner-stats"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-constraint-exclusion" xreflabel="constraint_exclusion">
      <term><varname>constraint_exclusion</varname> (<type>enum</type>)
      <indexterm>
       <primary>constraint exclusion</primary>
      </indexterm>
      <indexterm>
       <primary><varname>constraint_exclusion</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls the query planner's use of table constraints to
        optimize queries.
        The allowed values of <varname>constraint_exclusion</varname> are
        <literal>on</literal> (examine constraints for all tables),
        <literal>off</literal> (never examine constraints), and
        <literal>partition</literal> (examine constraints only for inheritance
        child tables and <literal>UNION ALL</literal> subqueries).
        <literal>partition</literal> is the default setting.
        It is often used with traditional inheritance trees to improve
        performance.
      </para>

       <para>
        When this parameter allows it for a particular table, the planner
        compares query conditions with the table's <literal>CHECK</literal>
        constraints, and omits scanning tables for which the conditions
        contradict the constraints.  For example:

<programlisting>
CREATE TABLE parent(key integer, ...);
CREATE TABLE child1000(check (key between 1000 and 1999)) INHERITS(parent);
CREATE TABLE child2000(check (key between 2000 and 2999)) INHERITS(parent);
...
SELECT * FROM parent WHERE key = 2400;
</programlisting>

        With constraint exclusion enabled, this <command>SELECT</command>
        will not scan <structname>child1000</structname> at all, improving performance.
       </para>

       <para>
        Currently, constraint exclusion is enabled by default
        only for cases that are often used to implement table partitioning via
        inheritance trees.  Turning it on for all tables imposes extra
        planning overhead that is quite noticeable on simple queries, and most
        often will yield no benefit for simple queries.  If you have no
        tables that are partitioned using traditional inheritance, you might
        prefer to turn it off entirely.  (Note that the equivalent feature for
        partitioned tables is controlled by a separate parameter,
        <xref linkend="guc-enable-partition-pruning"/>.)
       </para>

       <para>
        Refer to <xref linkend="ddl-partitioning-constraint-exclusion"/> for
        more information on using constraint exclusion to implement
        partitioning.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-cursor-tuple-fraction" xreflabel="cursor_tuple_fraction">
      <term><varname>cursor_tuple_fraction</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>cursor_tuple_fraction</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the planner's estimate of the fraction of a cursor's rows that
        will be retrieved.  The default is 0.1.  Smaller values of this
        setting bias the planner towards using <quote>fast start</quote> plans
        for cursors, which will retrieve the first few rows quickly while
        perhaps taking a long time to fetch all rows.  Larger values
        put more emphasis on the total estimated time.  At the maximum
        setting of 1.0, cursors are planned exactly like regular queries,
        considering only the total estimated time and not how soon the
        first rows might be delivered.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-from-collapse-limit" xreflabel="from_collapse_limit">
      <term><varname>from_collapse_limit</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>from_collapse_limit</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        The planner will merge sub-queries into upper queries if the
        resulting <literal>FROM</literal> list would have no more than
        this many items.  Smaller values reduce planning time but might
        yield inferior query plans.  The default is eight.
        For more information see <xref linkend="explicit-joins"/>.
       </para>

       <para>
        Setting this value to <xref linkend="guc-geqo-threshold"/> or more
        may trigger use of the GEQO planner, resulting in non-optimal
        plans.  See <xref linkend="runtime-config-query-geqo"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit" xreflabel="jit">
      <term><varname>jit</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>jit</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Determines whether <acronym>JIT</acronym> compilation may be used by
        <productname>PostgreSQL</productname>, if available (see <xref
        linkend="jit"/>).
        The default is <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-join-collapse-limit" xreflabel="join_collapse_limit">
      <term><varname>join_collapse_limit</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>join_collapse_limit</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        The planner will rewrite explicit <literal>JOIN</literal>
        constructs (except <literal>FULL JOIN</literal>s) into lists of
        <literal>FROM</literal> items whenever a list of no more than this many items
        would result.  Smaller values reduce planning time but might
        yield inferior query plans.
       </para>

       <para>
        By default, this variable is set the same as
        <varname>from_collapse_limit</varname>, which is appropriate
        for most uses. Setting it to 1 prevents any reordering of
        explicit <literal>JOIN</literal>s. Thus, the explicit join order
        specified in the query will be the actual order in which the
        relations are joined. Because the query planner does not always choose
        the optimal join order, advanced users can elect to
        temporarily set this variable to 1, and then specify the join
        order they desire explicitly.
        For more information see <xref linkend="explicit-joins"/>.
       </para>

       <para>
        Setting this value to <xref linkend="guc-geqo-threshold"/> or more
        may trigger use of the GEQO planner, resulting in non-optimal
        plans.  See <xref linkend="runtime-config-query-geqo"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-parallel-leader-participation" xreflabel="parallel_leader_participation">
      <term>
       <varname>parallel_leader_participation</varname> (<type>boolean</type>)
       <indexterm>
        <primary>
         <varname>parallel_leader_participation</varname> configuration
         parameter
        </primary>
       </indexterm>
      </term>
      <listitem>
       <para>
        Allows the leader process to execute the query plan under
        <literal>Gather</literal> and <literal>Gather Merge</literal> nodes
        instead of waiting for worker processes.  The default is
        <literal>on</literal>.  Setting this value to <literal>off</literal>
        reduces the likelihood that workers will become blocked because the
        leader is not reading tuples fast enough, but requires the leader
        process to wait for worker processes to start up before the first
        tuples can be produced.  The degree to which the leader can help or
        hinder performance depends on the plan type, number of workers and
        query duration.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-force-parallel-mode" xreflabel="force_parallel_mode">
      <term><varname>force_parallel_mode</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>force_parallel_mode</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Allows the use of parallel queries for testing purposes even in cases
        where no performance benefit is expected.
        The allowed values of <varname>force_parallel_mode</varname> are
        <literal>off</literal> (use parallel mode only when it is expected to improve
        performance), <literal>on</literal> (force parallel query for all queries
        for which it is thought to be safe), and <literal>regress</literal> (like
        <literal>on</literal>, but with additional behavior changes as explained
        below).
       </para>

       <para>
        More specifically, setting this value to <literal>on</literal> will add
        a <literal>Gather</literal> node to the top of any query plan for which this
        appears to be safe, so that the query runs inside of a parallel worker.
        Even when a parallel worker is not available or cannot be used,
        operations such as starting a subtransaction that would be prohibited
        in a parallel query context will be prohibited unless the planner
        believes that this will cause the query to fail.  If failures or
        unexpected results occur when this option is set, some functions used
        by the query may need to be marked <literal>PARALLEL UNSAFE</literal>
        (or, possibly, <literal>PARALLEL RESTRICTED</literal>).
       </para>

       <para>
        Setting this value to <literal>regress</literal> has all of the same effects
        as setting it to <literal>on</literal> plus some additional effects that are
        intended to facilitate automated regression testing.  Normally,
        messages from a parallel worker include a context line indicating that,
        but a setting of <literal>regress</literal> suppresses this line so that the
        output is the same as in non-parallel execution.  Also,
        the <literal>Gather</literal> nodes added to plans by this setting are hidden
        in <literal>EXPLAIN</literal> output so that the output matches what
        would be obtained if this setting were turned <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-logging">
    <title>Error Reporting and Logging</title>

    <indexterm zone="runtime-config-logging">
     <primary>server log</primary>
    </indexterm>

    <sect2 id="runtime-config-logging-where">
     <title>Where To Log</title>

     <indexterm zone="runtime-config-logging-where">
      <primary>where to log</primary>
     </indexterm>

     <indexterm>
       <primary>current_logfiles</primary>
       <secondary>and the log_destination configuration parameter</secondary>
     </indexterm>

     <variablelist>

     <varlistentry id="guc-log-destination" xreflabel="log_destination">
      <term><varname>log_destination</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>log_destination</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        <productname>PostgreSQL</productname> supports several methods
         for logging server messages, including
         <systemitem>stderr</systemitem>, <systemitem>csvlog</systemitem> and
         <systemitem>syslog</systemitem>. On Windows,
         <systemitem>eventlog</systemitem> is also supported. Set this
         parameter to a list of desired log destinations separated by
         commas. The default is to log to <systemitem>stderr</systemitem>
         only.
         This parameter can only be set in the <filename>postgresql.conf</filename>
         file or on the server command line.
       </para>
       <para>
        If <systemitem>csvlog</systemitem> is included in <varname>log_destination</varname>,
        log entries are output in <quote>comma separated
        value</quote> (<acronym>CSV</acronym>) format, which is convenient for
        loading logs into programs.
        See <xref linkend="runtime-config-logging-csvlog"/> for details.
        <xref linkend="guc-logging-collector"/> must be enabled to generate
        CSV-format log output.
       </para>
       <para>
        When either <systemitem>stderr</systemitem> or
        <systemitem>csvlog</systemitem> are included, the file
        <filename>current_logfiles</filename> is created to record the location
        of the log file(s) currently in use by the logging collector and the
        associated logging destination. This provides a convenient way to
        find the logs currently in use by the instance. Here is an example of
        this file's content:
<programlisting>
stderr log/postgresql.log
csvlog log/postgresql.csv
</programlisting>

        <filename>current_logfiles</filename> is recreated when a new log file
        is created as an effect of rotation, and
        when <varname>log_destination</varname> is reloaded.  It is removed when
        neither <systemitem>stderr</systemitem>
        nor <systemitem>csvlog</systemitem> are included
        in <varname>log_destination</varname>, and when the logging collector is
        disabled.
       </para>

       <note>
        <para>
         On most Unix systems, you will need to alter the configuration of
         your system's <application>syslog</application> daemon in order
         to make use of the <systemitem>syslog</systemitem> option for
         <varname>log_destination</varname>.  <productname>PostgreSQL</productname>
         can log to <application>syslog</application> facilities
         <literal>LOCAL0</literal> through <literal>LOCAL7</literal> (see <xref
         linkend="guc-syslog-facility"/>), but the default
         <application>syslog</application> configuration on most platforms
         will discard all such messages.  You will need to add something like:
<programlisting>
local0.*    /var/log/postgresql
</programlisting>
         to the  <application>syslog</application> daemon's configuration file
         to make it work.
        </para>
        <para>
         On Windows, when you use the <literal>eventlog</literal>
         option for <varname>log_destination</varname>, you should
         register an event source and its library with the operating
         system so that the Windows Event Viewer can display event
         log messages cleanly.
         See <xref linkend="event-log-registration"/> for details.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-logging-collector" xreflabel="logging_collector">
      <term><varname>logging_collector</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>logging_collector</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
         This parameter enables the <firstterm>logging collector</firstterm>, which
         is a background process that captures log messages
         sent to <systemitem>stderr</systemitem> and redirects them into log files.
         This approach is often more useful than
         logging to <application>syslog</application>, since some types of messages
         might not appear in <application>syslog</application> output.  (One common
         example is dynamic-linker failure messages; another is error messages
         produced by scripts such as <varname>archive_command</varname>.)
         This parameter can only be set at server start.
       </para>

       <note>
        <para>
         It is possible to log to <systemitem>stderr</systemitem> without using the
         logging collector; the log messages will just go to wherever the
         server's <systemitem>stderr</systemitem> is directed.  However, that method is
         only suitable for low log volumes, since it provides no convenient
         way to rotate log files.  Also, on some platforms not using the
         logging collector can result in lost or garbled log output, because
         multiple processes writing concurrently to the same log file can
         overwrite each other's output.
        </para>
       </note>

       <note>
        <para>
          The logging collector is designed to never lose messages.  This means
          that in case of extremely high load, server processes could be
          blocked while trying to send additional log messages when the
          collector has fallen behind.  In contrast, <application>syslog</application>
          prefers to drop messages if it cannot write them, which means it
          may fail to log some messages in such cases but it will not block
          the rest of the system.
        </para>
       </note>

      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-directory" xreflabel="log_directory">
      <term><varname>log_directory</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>log_directory</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When <varname>logging_collector</varname> is enabled,
        this parameter determines the directory in which log files will be created.
        It can be specified as an absolute path, or relative to the
        cluster data directory.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
        The default is <literal>log</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-filename" xreflabel="log_filename">
      <term><varname>log_filename</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>log_filename</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When <varname>logging_collector</varname> is enabled,
        this parameter sets the file names of the created log files.  The value
        is treated as a <function>strftime</function> pattern,
        so <literal>%</literal>-escapes can be used to specify time-varying
        file names.  (Note that if there are
        any time-zone-dependent <literal>%</literal>-escapes, the computation
        is done in the zone specified
        by <xref linkend="guc-log-timezone"/>.)
        The supported <literal>%</literal>-escapes are similar to those
        listed in the Open Group's <ulink
        url="https://pubs.opengroup.org/onlinepubs/009695399/functions/strftime.html">strftime
        </ulink> specification.
        Note that the system's <function>strftime</function> is not used
        directly, so platform-specific (nonstandard) extensions do not work.
        The default is <literal>postgresql-%Y-%m-%d_%H%M%S.log</literal>.
       </para>
       <para>
        If you specify a file name without escapes, you should plan to
        use a log rotation utility to avoid eventually filling the
        entire disk.  In releases prior to 8.4, if
        no <literal>%</literal> escapes were
        present, <productname>PostgreSQL</productname> would append
        the epoch of the new log file's creation time, but this is no
        longer the case.
       </para>
       <para>
        If CSV-format output is enabled in <varname>log_destination</varname>,
        <literal>.csv</literal> will be appended to the timestamped
        log file name to create the file name for CSV-format output.
        (If <varname>log_filename</varname> ends in <literal>.log</literal>, the suffix is
        replaced instead.)
       </para>
       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-file-mode" xreflabel="log_file_mode">
      <term><varname>log_file_mode</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>log_file_mode</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        On Unix systems this parameter sets the permissions for log files
        when <varname>logging_collector</varname> is enabled. (On Microsoft
        Windows this parameter is ignored.)
        The parameter value is expected to be a numeric mode
        specified in the format accepted by the
        <function>chmod</function> and <function>umask</function>
        system calls.  (To use the customary octal format the number
        must start with a <literal>0</literal> (zero).)
       </para>
       <para>
        The default permissions are <literal>0600</literal>, meaning only the
        server owner can read or write the log files.  The other commonly
        useful setting is <literal>0640</literal>, allowing members of the owner's
        group to read the files.  Note however that to make use of such a
        setting, you'll need to alter <xref linkend="guc-log-directory"/> to
        store the files somewhere outside the cluster data directory.  In
        any case, it's unwise to make the log files world-readable, since
        they might contain sensitive data.
       </para>
       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-rotation-age" xreflabel="log_rotation_age">
      <term><varname>log_rotation_age</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>log_rotation_age</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When <varname>logging_collector</varname> is enabled,
        this parameter determines the maximum lifetime of an individual log file.
        After this many minutes have elapsed, a new log file will
        be created.  Set to zero to disable time-based creation of
        new log files.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-rotation-size" xreflabel="log_rotation_size">
      <term><varname>log_rotation_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>log_rotation_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When <varname>logging_collector</varname> is enabled,
        this parameter determines the maximum size of an individual log file.
        After this many kilobytes have been emitted into a log file,
        a new log file will be created.  Set to zero to disable size-based
        creation of new log files.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-truncate-on-rotation" xreflabel="log_truncate_on_rotation">
      <term><varname>log_truncate_on_rotation</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_truncate_on_rotation</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When <varname>logging_collector</varname> is enabled,
        this parameter will cause <productname>PostgreSQL</productname> to truncate (overwrite),
        rather than append to, any existing log file of the same name.
        However, truncation will occur only when a new file is being opened
        due to time-based rotation, not during server startup or size-based
        rotation.  When off, pre-existing files will be appended to in
        all cases.  For example, using this setting in combination with
        a <varname>log_filename</varname> like <literal>postgresql-%H.log</literal>
        would result in generating twenty-four hourly log files and then
        cyclically overwriting them.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
       <para>
        Example:  To keep 7 days of logs, one log file per day named
        <literal>server_log.Mon</literal>, <literal>server_log.Tue</literal>,
        etc, and automatically overwrite last week's log with this week's log,
        set <varname>log_filename</varname> to <literal>server_log.%a</literal>,
        <varname>log_truncate_on_rotation</varname> to <literal>on</literal>, and
        <varname>log_rotation_age</varname> to <literal>1440</literal>.
       </para>
       <para>
        Example: To keep 24 hours of logs, one log file per hour, but
        also rotate sooner if the log file size exceeds 1GB, set
        <varname>log_filename</varname> to <literal>server_log.%H%M</literal>,
        <varname>log_truncate_on_rotation</varname> to <literal>on</literal>,
        <varname>log_rotation_age</varname> to <literal>60</literal>, and
        <varname>log_rotation_size</varname> to <literal>1000000</literal>.
        Including <literal>%M</literal> in <varname>log_filename</varname> allows
        any size-driven rotations that might occur to select a file name
        different from the hour's initial file name.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-syslog-facility" xreflabel="syslog_facility">
      <term><varname>syslog_facility</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>syslog_facility</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When logging to <application>syslog</application> is enabled, this parameter
        determines the <application>syslog</application>
        <quote>facility</quote> to be used.  You can choose
        from <literal>LOCAL0</literal>, <literal>LOCAL1</literal>,
        <literal>LOCAL2</literal>, <literal>LOCAL3</literal>, <literal>LOCAL4</literal>,
        <literal>LOCAL5</literal>, <literal>LOCAL6</literal>, <literal>LOCAL7</literal>;
        the default is <literal>LOCAL0</literal>. See also the
        documentation of your system's
        <application>syslog</application> daemon.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-syslog-ident" xreflabel="syslog_ident">
      <term><varname>syslog_ident</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>syslog_ident</varname> configuration parameter</primary>
      </indexterm>
      </term>
       <listitem>
        <para>
         When logging to <application>syslog</application> is enabled, this parameter
         determines the program name used to identify
         <productname>PostgreSQL</productname> messages in
         <application>syslog</application> logs. The default is
         <literal>postgres</literal>.
         This parameter can only be set in the <filename>postgresql.conf</filename>
         file or on the server command line.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-syslog-sequence-numbers" xreflabel="syslog_sequence_numbers">
       <term><varname>syslog_sequence_numbers</varname> (<type>boolean</type>)
        <indexterm>
         <primary><varname>syslog_sequence_numbers</varname> configuration parameter</primary>
        </indexterm>
       </term>

       <listitem>
        <para>
         When logging to <application>syslog</application> and this is on (the
         default), then each message will be prefixed by an increasing
         sequence number (such as <literal>[2]</literal>).  This circumvents
         the <quote>--- last message repeated N times ---</quote> suppression
         that many syslog implementations perform by default.  In more modern
         syslog implementations, repeated message suppression can be configured
         (for example, <literal>$RepeatedMsgReduction</literal>
         in <productname>rsyslog</productname>), so this might not be
         necessary.  Also, you could turn this off if you actually want to
         suppress repeated messages.
        </para>

        <para>
         This parameter can only be set in the <filename>postgresql.conf</filename>
         file or on the server command line.
        </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-syslog-split-messages" xreflabel="syslog_split_messages">
      <term><varname>syslog_split_messages</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>syslog_split_messages</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When logging to <application>syslog</application> is enabled, this parameter
        determines how messages are delivered to syslog.  When on (the
        default), messages are split by lines, and long lines are split so
        that they will fit into 1024 bytes, which is a typical size limit for
        traditional syslog implementations.  When off, PostgreSQL server log
        messages are delivered to the syslog service as is, and it is up to
        the syslog service to cope with the potentially bulky messages.
       </para>

       <para>
        If syslog is ultimately logging to a text file, then the effect will
        be the same either way, and it is best to leave the setting on, since
        most syslog implementations either cannot handle large messages or
        would need to be specially configured to handle them.  But if syslog
        is ultimately writing into some other medium, it might be necessary or
        more useful to keep messages logically together.
       </para>

       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-event-source" xreflabel="event_source">
      <term><varname>event_source</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>event_source</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When logging to <application>event log</application> is enabled, this parameter
        determines the program name used to identify
        <productname>PostgreSQL</productname> messages in
        the log. The default is <literal>PostgreSQL</literal>.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

      </variablelist>
    </sect2>
     <sect2 id="runtime-config-logging-when">
     <title>When To Log</title>

     <variablelist>

     <varlistentry id="guc-log-min-messages" xreflabel="log_min_messages">
      <term><varname>log_min_messages</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>log_min_messages</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls which <link linkend="runtime-config-severity-levels">message
        levels</link> are written to the server log.
        Valid values are <literal>DEBUG5</literal>, <literal>DEBUG4</literal>,
        <literal>DEBUG3</literal>, <literal>DEBUG2</literal>, <literal>DEBUG1</literal>,
        <literal>INFO</literal>, <literal>NOTICE</literal>, <literal>WARNING</literal>,
        <literal>ERROR</literal>, <literal>LOG</literal>, <literal>FATAL</literal>, and
        <literal>PANIC</literal>.  Each level includes all the levels that
        follow it.  The later the level, the fewer messages are sent
        to the log.  The default is <literal>WARNING</literal>.  Note that
        <literal>LOG</literal> has a different rank here than in
        <xref linkend="guc-client-min-messages"/>.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-min-error-statement" xreflabel="log_min_error_statement">
      <term><varname>log_min_error_statement</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>log_min_error_statement</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls which SQL statements that cause an error
        condition are recorded in the server log.  The current
        SQL statement is included in the log entry for any message of
        the specified
        <link linkend="runtime-config-severity-levels">severity</link>
        or higher.
        Valid values are <literal>DEBUG5</literal>,
        <literal>DEBUG4</literal>, <literal>DEBUG3</literal>,
        <literal>DEBUG2</literal>, <literal>DEBUG1</literal>,
        <literal>INFO</literal>, <literal>NOTICE</literal>,
        <literal>WARNING</literal>, <literal>ERROR</literal>,
        <literal>LOG</literal>,
        <literal>FATAL</literal>, and <literal>PANIC</literal>.
        The default is <literal>ERROR</literal>, which means statements
        causing errors, log messages, fatal errors, or panics will be logged.
        To effectively turn off logging of failing statements,
        set this parameter to <literal>PANIC</literal>.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-min-duration-statement" xreflabel="log_min_duration_statement">
      <term><varname>log_min_duration_statement</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>log_min_duration_statement</varname> configuration parameter</primary>
      </indexterm>
      </term>
       <listitem>
        <para>
         Causes the duration of each completed statement to be logged
         if the statement ran for at least the specified number of
         milliseconds.  Setting this to zero prints all statement durations.
         Minus-one (the default) disables logging statement durations.
         For example, if you set it to <literal>250ms</literal>
         then all SQL statements that run 250ms or longer will be
         logged.  Enabling this parameter can be helpful in tracking down
         unoptimized queries in your applications.
         Only superusers can change this setting.
        </para>

        <para>
         For clients using extended query protocol, durations of the Parse,
         Bind, and Execute steps are logged independently.
        </para>

       <note>
        <para>
         When using this option together with
         <xref linkend="guc-log-statement"/>,
         the text of statements that are logged because of
         <varname>log_statement</varname> will not be repeated in the
         duration log message.
         If you are not using <application>syslog</application>, it is recommended
         that you log the PID or session ID using
         <xref linkend="guc-log-line-prefix"/>
         so that you can link the statement message to the later
         duration message using the process ID or session ID.
        </para>
       </note>
       </listitem>
      </varlistentry>

     </variablelist>

    <para>
     <xref linkend="runtime-config-severity-levels"/> explains the message
     severity levels used by <productname>PostgreSQL</productname>.  If logging output
     is sent to <systemitem>syslog</systemitem> or Windows'
     <systemitem>eventlog</systemitem>, the severity levels are translated
     as shown in the table.
    </para>

    <table id="runtime-config-severity-levels">
     <title>Message Severity Levels</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Severity</entry>
        <entry>Usage</entry>
        <entry><systemitem>syslog</systemitem></entry>
        <entry><systemitem>eventlog</systemitem></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><literal>DEBUG1..DEBUG5</literal></entry>
        <entry>Provides successively-more-detailed information for use by
         developers.</entry>
        <entry><literal>DEBUG</literal></entry>
        <entry><literal>INFORMATION</literal></entry>
       </row>

       <row>
        <entry><literal>INFO</literal></entry>
        <entry>Provides information implicitly requested by the user,
         e.g., output from <command>VACUUM VERBOSE</command>.</entry>
        <entry><literal>INFO</literal></entry>
        <entry><literal>INFORMATION</literal></entry>
       </row>

       <row>
        <entry><literal>NOTICE</literal></entry>
        <entry>Provides information that might be helpful to users, e.g.,
         notice of truncation of long identifiers.</entry>
        <entry><literal>NOTICE</literal></entry>
        <entry><literal>INFORMATION</literal></entry>
       </row>

       <row>
        <entry><literal>WARNING</literal></entry>
        <entry>Provides warnings of likely problems, e.g., <command>COMMIT</command>
         outside a transaction block.</entry>
        <entry><literal>NOTICE</literal></entry>
        <entry><literal>WARNING</literal></entry>
       </row>

       <row>
        <entry><literal>ERROR</literal></entry>
        <entry>Reports an error that caused the current command to
         abort.</entry>
        <entry><literal>WARNING</literal></entry>
        <entry><literal>ERROR</literal></entry>
       </row>

       <row>
        <entry><literal>LOG</literal></entry>
        <entry>Reports information of interest to administrators, e.g.,
         checkpoint activity.</entry>
        <entry><literal>INFO</literal></entry>
        <entry><literal>INFORMATION</literal></entry>
       </row>

       <row>
        <entry><literal>FATAL</literal></entry>
        <entry>Reports an error that caused the current session to
         abort.</entry>
        <entry><literal>ERR</literal></entry>
        <entry><literal>ERROR</literal></entry>
       </row>

       <row>
        <entry><literal>PANIC</literal></entry>
        <entry>Reports an error that caused all database sessions to abort.</entry>
        <entry><literal>CRIT</literal></entry>
        <entry><literal>ERROR</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    </sect2>
     <sect2 id="runtime-config-logging-what">
     <title>What To Log</title>

     <note>
      <para>
       What you choose to log can have security implications;  see
       <xref linkend="logfile-maintenance"/>.
      </para>
     </note>

     <variablelist>

     <varlistentry id="guc-application-name" xreflabel="application_name">
      <term><varname>application_name</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>application_name</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        The <varname>application_name</varname> can be any string of less than
        <symbol>NAMEDATALEN</symbol> characters (64 characters in a standard build).
        It is typically set by an application upon connection to the server.
        The name will be displayed in the <structname>pg_stat_activity</structname> view
        and included in CSV log entries.  It can also be included in regular
        log entries via the <xref linkend="guc-log-line-prefix"/> parameter.
        Only printable ASCII characters may be used in the
        <varname>application_name</varname> value. Other characters will be
        replaced with question marks (<literal>?</literal>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>debug_print_parse</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>debug_print_parse</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <term><varname>debug_print_rewritten</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>debug_print_rewritten</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <term><varname>debug_print_plan</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>debug_print_plan</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        These parameters enable various debugging output to be emitted.
        When set, they print the resulting parse tree, the query rewriter
        output, or the execution plan for each executed query.
        These messages are emitted at <literal>LOG</literal> message level, so by
        default they will appear in the server log but will not be sent to the
        client.  You can change that by adjusting
        <xref linkend="guc-client-min-messages"/> and/or
        <xref linkend="guc-log-min-messages"/>.
        These parameters are off by default.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>debug_pretty_print</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>debug_pretty_print</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When set, <varname>debug_pretty_print</varname> indents the messages
        produced by <varname>debug_print_parse</varname>,
        <varname>debug_print_rewritten</varname>, or
        <varname>debug_print_plan</varname>.  This results in more readable
        but much longer output than the <quote>compact</quote> format used when
        it is off.  It is on by default.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-checkpoints" xreflabel="log_checkpoints">
      <term><varname>log_checkpoints</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_checkpoints</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Causes checkpoints and restartpoints to be logged in the server log.
        Some statistics are included in the log messages, including the number
        of buffers written and the time spent writing them.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line. The default is off.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-connections" xreflabel="log_connections">
      <term><varname>log_connections</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_connections</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Causes each attempted connection to the server to be logged,
        as well as successful completion of client authentication.
        Only superusers can change this parameter at session start,
        and it cannot be changed at all within a session.
        The default is <literal>off</literal>.
       </para>

       <note>
        <para>
         Some client programs, like <application>psql</application>, attempt
         to connect twice while determining if a password is required, so
         duplicate <quote>connection received</quote> messages do not
         necessarily indicate a problem.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-disconnections" xreflabel="log_disconnections">
      <term><varname>log_disconnections</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_disconnections</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Causes session terminations to be logged.  The log output
        provides information similar to <varname>log_connections</varname>,
        plus the duration of the session.
        Only superusers can change this parameter at session start,
        and it cannot be changed at all within a session.
        The default is <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry id="guc-log-duration" xreflabel="log_duration">
      <term><varname>log_duration</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_duration</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Causes the duration of every completed statement to be logged.
        The default is <literal>off</literal>.
        Only superusers can change this setting.
       </para>

       <para>
        For clients using extended query protocol, durations of the Parse,
        Bind, and Execute steps are logged independently.
       </para>

       <note>
        <para>
         The difference between setting this option and setting
         <xref linkend="guc-log-min-duration-statement"/> to zero is that
         exceeding <varname>log_min_duration_statement</varname> forces the text of
         the query to be logged, but this option doesn't.  Thus, if
         <varname>log_duration</varname> is <literal>on</literal> and
         <varname>log_min_duration_statement</varname> has a positive value, all
         durations are logged but the query text is included only for
         statements exceeding the threshold.  This behavior can be useful for
         gathering statistics in high-load installations.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-error-verbosity" xreflabel="log_error_verbosity">
      <term><varname>log_error_verbosity</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>log_error_verbosity</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls the amount of detail written in the server log for each
        message that is logged.  Valid values are <literal>TERSE</literal>,
        <literal>DEFAULT</literal>, and <literal>VERBOSE</literal>, each adding more
        fields to displayed messages.  <literal>TERSE</literal> excludes
        the logging of <literal>DETAIL</literal>, <literal>HINT</literal>,
        <literal>QUERY</literal>, and <literal>CONTEXT</literal> error information.
        <literal>VERBOSE</literal> output includes the <symbol>SQLSTATE</symbol> error
        code (see also <xref linkend="errcodes-appendix"/>) and the source code file name, function name,
        and line number that generated the error.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-hostname" xreflabel="log_hostname">
      <term><varname>log_hostname</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_hostname</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        By default, connection log messages only show the IP address of the
        connecting host. Turning this parameter on causes logging of the
        host name as well.  Note that depending on your host name resolution
        setup this might impose a non-negligible performance penalty.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-line-prefix" xreflabel="log_line_prefix">
      <term><varname>log_line_prefix</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>log_line_prefix</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
         This is a <function>printf</function>-style string that is output at the
         beginning of each log line.
         <literal>%</literal> characters begin <quote>escape sequences</quote>
         that are replaced with status information as outlined below.
         Unrecognized escapes are ignored. Other
         characters are copied straight to the log line. Some escapes are
         only recognized by session processes, and will be treated as empty by
         background processes such as the main server process. Status
         information may be aligned either left or right by specifying a
         numeric literal after the % and before the option. A negative
         value will cause the status information to be padded on the
         right with spaces to give it a minimum width, whereas a positive
         value will pad on the left. Padding can be useful to aid human
         readability in log files.
         This parameter can only be set in the <filename>postgresql.conf</filename>
         file or on the server command line. The default is
         <literal>'%m [%p] '</literal> which logs a time stamp and the process ID.

         <informaltable>
          <tgroup cols="3">
           <thead>
            <row>
             <entry>Escape</entry>
             <entry>Effect</entry>
             <entry>Session only</entry>
             </row>
            </thead>
           <tbody>
            <row>
             <entry><literal>%a</literal></entry>
             <entry>Application name</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%u</literal></entry>
             <entry>User name</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%d</literal></entry>
             <entry>Database name</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%r</literal></entry>
             <entry>Remote host name or IP address, and remote port</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%h</literal></entry>
             <entry>Remote host name or IP address</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%p</literal></entry>
             <entry>Process ID</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%t</literal></entry>
             <entry>Time stamp without milliseconds</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%m</literal></entry>
             <entry>Time stamp with milliseconds</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%n</literal></entry>
             <entry>Time stamp with milliseconds (as a Unix epoch)</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%i</literal></entry>
             <entry>Command tag: type of session's current command</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%e</literal></entry>
             <entry>SQLSTATE error code</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%c</literal></entry>
             <entry>Session ID: see below</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%l</literal></entry>
             <entry>Number of the log line for each session or process, starting at 1</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%s</literal></entry>
             <entry>Process start time stamp</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%v</literal></entry>
             <entry>Virtual transaction ID (backendID/localXID)</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%x</literal></entry>
             <entry>Transaction ID (0 if none is assigned)</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%q</literal></entry>
             <entry>Produces no output, but tells non-session
             processes to stop at this point in the string; ignored by
             session processes</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%%</literal></entry>
             <entry>Literal <literal>%</literal></entry>
             <entry>no</entry>
            </row>
           </tbody>
          </tgroup>
         </informaltable>

         The <literal>%c</literal> escape prints a quasi-unique session identifier,
         consisting of two 4-byte hexadecimal numbers (without leading zeros)
         separated by a dot.  The numbers are the process start time and the
         process ID, so <literal>%c</literal> can also be used as a space saving way
         of printing those items.  For example, to generate the session
         identifier from <literal>pg_stat_activity</literal>, use this query:
<programlisting>
SELECT to_hex(trunc(EXTRACT(EPOCH FROM backend_start))::integer) || '.' ||
       to_hex(pid)
FROM pg_stat_activity;
</programlisting>

       </para>

       <tip>
        <para>
         If you set a nonempty value for <varname>log_line_prefix</varname>,
         you should usually make its last character be a space, to provide
         visual separation from the rest of the log line.  A punctuation
         character can be used too.
        </para>
       </tip>

       <tip>
        <para>
         <application>Syslog</application> produces its own
         time stamp and process ID information, so you probably do not want to
         include those escapes if you are logging to <application>syslog</application>.
        </para>
       </tip>

       <tip>
        <para>
         The <literal>%q</literal> escape is useful when including information that is
         only available in session (backend) context like user or database
         name.  For example:
<programlisting>
log_line_prefix = '%m [%p] %q%u@%d/%a '
</programlisting>
        </para>
       </tip>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-lock-waits" xreflabel="log_lock_waits">
      <term><varname>log_lock_waits</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_lock_waits</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls whether a log message is produced when a session waits
        longer than <xref linkend="guc-deadlock-timeout"/> to acquire a
        lock.  This is useful in determining if lock waits are causing
        poor performance.  The default is <literal>off</literal>.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-statement" xreflabel="log_statement">
      <term><varname>log_statement</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>log_statement</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls which SQL statements are logged. Valid values are
        <literal>none</literal> (off), <literal>ddl</literal>, <literal>mod</literal>, and
        <literal>all</literal> (all statements). <literal>ddl</literal> logs all data definition
        statements, such as <command>CREATE</command>, <command>ALTER</command>, and
        <command>DROP</command> statements. <literal>mod</literal> logs all
        <literal>ddl</literal> statements, plus data-modifying statements
        such as <command>INSERT</command>,
        <command>UPDATE</command>, <command>DELETE</command>, <command>TRUNCATE</command>,
        and <command>COPY FROM</command>.
        <command>PREPARE</command>, <command>EXECUTE</command>, and
        <command>EXPLAIN ANALYZE</command> statements are also logged if their
        contained command is of an appropriate type.  For clients using
        extended query protocol, logging occurs when an Execute message
        is received, and values of the Bind parameters are included
        (with any embedded single-quote marks doubled).
       </para>

       <para>
        The default is <literal>none</literal>. Only superusers can change this
        setting.
       </para>

       <note>
        <para>
         Statements that contain simple syntax errors are not logged
         even by the <varname>log_statement</varname> = <literal>all</literal> setting,
         because the log message is emitted only after basic parsing has
         been done to determine the statement type.  In the case of extended
         query protocol, this setting likewise does not log statements that
         fail before the Execute phase (i.e., during parse analysis or
         planning).  Set <varname>log_min_error_statement</varname> to
         <literal>ERROR</literal> (or lower) to log such statements.
        </para>
        <para>
         Logged statements might reveal sensitive data and even contain
         plaintext passwords.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-replication-commands" xreflabel="log_replication_commands">
      <term><varname>log_replication_commands</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_replication_commands</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Causes each replication command to be logged in the server log.
        See <xref linkend="protocol-replication"/> for more information about
        replication command. The default value is <literal>off</literal>.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-temp-files" xreflabel="log_temp_files">
      <term><varname>log_temp_files</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>log_temp_files</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls logging of temporary file names and sizes.
        Temporary files can be
        created for sorts, hashes, and temporary query results.
        A log entry is made for each temporary file when it is deleted.
        A value of zero logs all temporary file information, while positive
        values log only files whose size is greater than or equal to
        the specified number of kilobytes.  The
        default setting is -1, which disables such logging.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-timezone" xreflabel="log_timezone">
      <term><varname>log_timezone</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>log_timezone</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the time zone used for timestamps written in the server log.
        Unlike <xref linkend="guc-timezone"/>, this value is cluster-wide,
        so that all sessions will report timestamps consistently.
        The built-in default is <literal>GMT</literal>, but that is typically
        overridden in <filename>postgresql.conf</filename>; <application>initdb</application>
        will install a setting there corresponding to its system environment.
        See <xref linkend="datatype-timezones"/> for more information.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
     <sect2 id="runtime-config-logging-csvlog">
     <title>Using CSV-Format Log Output</title>

       <para>
        Including <literal>csvlog</literal> in the <varname>log_destination</varname> list
        provides a convenient way to import log files into a database table.
        This option emits log lines in comma-separated-values
        (<acronym>CSV</acronym>) format,
        with these columns:
        time stamp with milliseconds,
        user name,
        database name,
        process ID,
        client host:port number,
        session ID,
        per-session line number,
        command tag,
        session start time,
        virtual transaction ID,
        regular transaction ID,
        error severity,
        SQLSTATE code,
        error message,
        error message detail,
        hint,
        internal query that led to the error (if any),
        character count of the error position therein,
        error context,
        user query that led to the error (if any and enabled by
        <varname>log_min_error_statement</varname>),
        character count of the error position therein,
        location of the error in the PostgreSQL source code
        (if <varname>log_error_verbosity</varname> is set to <literal>verbose</literal>),
        and application name.
        Here is a sample table definition for storing CSV-format log output:

<programlisting>
CREATE TABLE postgres_log
(
  log_time timestamp(3) with time zone,
  user_name text,
  database_name text,
  process_id integer,
  connection_from text,
  session_id text,
  session_line_num bigint,
  command_tag text,
  session_start_time timestamp with time zone,
  virtual_transaction_id text,
  transaction_id bigint,
  error_severity text,
  sql_state_code text,
  message text,
  detail text,
  hint text,
  internal_query text,
  internal_query_pos integer,
  context text,
  query text,
  query_pos integer,
  location text,
  application_name text,
  PRIMARY KEY (session_id, session_line_num)
);
</programlisting>
       </para>

       <para>
        To import a log file into this table, use the <command>COPY FROM</command>
        command:

<programlisting>
COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</programlisting>
        It is also possible to access the file as a foreign table, using
        the supplied <xref linkend="file-fdw"/> module.
       </para>

       <para>
       There are a few things you need to do to simplify importing CSV log
       files:

       <orderedlist>
         <listitem>
           <para>
            Set <varname>log_filename</varname> and
            <varname>log_rotation_age</varname> to provide a consistent,
            predictable naming scheme for your log files.  This lets you
            predict what the file name will be and know when an individual log
            file is complete and therefore ready to be imported.
         </para>
        </listitem>

        <listitem>
           <para>
            Set <varname>log_rotation_size</varname> to 0 to disable
            size-based log rotation, as it makes the log file name difficult
            to predict.
           </para>
        </listitem>

        <listitem>
          <para>
           Set <varname>log_truncate_on_rotation</varname> to <literal>on</literal> so
           that old log data isn't mixed with the new in the same file.
          </para>
        </listitem>

        <listitem>
          <para>
           The table definition above includes a primary key specification.
           This is useful to protect against accidentally importing the same
           information twice.  The <command>COPY</command> command commits all of the
           data it imports at one time, so any error will cause the entire
           import to fail.  If you import a partial log file and later import
           the file again when it is complete, the primary key violation will
           cause the import to fail.  Wait until the log is complete and
           closed before importing.  This procedure will also protect against
           accidentally importing a partial line that hasn't been completely
           written, which would also cause <command>COPY</command> to fail.
          </para>
        </listitem>
        </orderedlist>
      </para>
    </sect2>

   <sect2>
    <title>Process Title</title>

    <para>
     These settings control how process titles of server processes are
     modified.  Process titles are typically viewed using programs like
     <application>ps</application> or, on Windows, <application>Process Explorer</application>.
     See <xref linkend="monitoring-ps"/> for details.
    </para>

    <variablelist>
     <varlistentry id="guc-cluster-name" xreflabel="cluster_name">
      <term><varname>cluster_name</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>cluster_name</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the cluster name that appears in the process title for all
        server processes in this cluster. The name can be any string of less
        than <symbol>NAMEDATALEN</symbol> characters (64 characters in a standard
        build). Only printable ASCII characters may be used in the
        <varname>cluster_name</varname> value. Other characters will be
        replaced with question marks (<literal>?</literal>).  No name is shown
        if this parameter is set to the empty string <literal>''</literal> (which is
        the default). This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-update-process-title" xreflabel="update_process_title">
      <term><varname>update_process_title</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>update_process_title</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables updating of the process title every time a new SQL command
        is received by the server.
        This setting defaults to <literal>on</literal> on most platforms, but it
        defaults to <literal>off</literal> on Windows due to that platform's larger
        overhead for updating the process title.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </sect2>
  </sect1>

   <sect1 id="runtime-config-statistics">
    <title>Run-time Statistics</title>

    <sect2 id="runtime-config-statistics-collector">
     <title>Query and Index Statistics Collector</title>

     <para>
      These parameters control server-wide statistics collection features.
      When statistics collection is enabled, the data that is produced can be
      accessed via the <structname>pg_stat</structname> and
      <structname>pg_statio</structname> family of system views.
      Refer to <xref linkend="monitoring"/> for more information.
     </para>

     <variablelist>

     <varlistentry id="guc-track-activities" xreflabel="track_activities">
      <term><varname>track_activities</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>track_activities</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables the collection of information on the currently
        executing command of each session, along with the time when
        that command began execution. This parameter is on by
        default. Note that even when enabled, this information is not
        visible to all users, only to superusers, roles with privileges of the
        <literal>pg_read_all_stats</literal> role and the user owning the
        sessions being reported on (including sessions belonging to a role they
        have the privileges of), so it should not represent a security risk.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-track-activity-query-size" xreflabel="track_activity_query_size">
      <term><varname>track_activity_query_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>track_activity_query_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
       Specifies the number of bytes reserved to track the currently
       executing command for each active session, for the
       <structname>pg_stat_activity</structname>.<structfield>query</structfield> field.
       The default value is 1024. This parameter can only be set at server
       start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-track-counts" xreflabel="track_counts">
      <term><varname>track_counts</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>track_counts</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables collection of statistics on database activity.
        This parameter is on by default, because the autovacuum
        daemon needs the collected information.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-track-io-timing" xreflabel="track_io_timing">
      <term><varname>track_io_timing</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>track_io_timing</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables timing of database I/O calls.  This parameter is off by
        default, because it will repeatedly query the operating system for
        the current time, which may cause significant overhead on some
        platforms.  You can use the <xref linkend="pgtesttiming"/> tool to
        measure the overhead of timing on your system.
        I/O timing information is
        displayed in <xref linkend="pg-stat-database-view"/>, in the output of
        <xref linkend="sql-explain"/> when the <literal>BUFFERS</literal> option is
        used, and by <xref linkend="pgstatstatements"/>.  Only superusers can
        change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-track-functions" xreflabel="track_functions">
      <term><varname>track_functions</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>track_functions</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables tracking of function call counts and time used. Specify
        <literal>pl</literal> to track only procedural-language functions,
        <literal>all</literal> to also track SQL and C language functions.
        The default is <literal>none</literal>, which disables function
        statistics tracking.  Only superusers can change this setting.
       </para>

       <note>
        <para>
         SQL-language functions that are simple enough to be <quote>inlined</quote>
         into the calling query will not be tracked, regardless of this
         setting.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-stats-temp-directory" xreflabel="stats_temp_directory">
      <term><varname>stats_temp_directory</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>stats_temp_directory</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the directory to store temporary statistics data in. This can be
        a path relative to the data directory or an absolute path. The default
        is <filename>pg_stat_tmp</filename>. Pointing this at a RAM-based
        file system will decrease physical I/O requirements and can lead to
        improved performance.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

    <sect2 id="runtime-config-statistics-monitor">
     <title>Statistics Monitoring</title>
     <variablelist>

     <varlistentry>
      <term><varname>log_statement_stats</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_statement_stats</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <term><varname>log_parser_stats</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_parser_stats</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <term><varname>log_planner_stats</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_planner_stats</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <term><varname>log_executor_stats</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_executor_stats</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        For each query, output performance statistics of the respective
        module to the server log. This is a crude profiling
        instrument, similar to the Unix <function>getrusage()</function> operating
        system facility.  <varname>log_statement_stats</varname> reports total
        statement statistics, while the others report per-module statistics.
        <varname>log_statement_stats</varname> cannot be enabled together with
        any of the per-module options.  All of these options are disabled by
        default.   Only superusers can change these settings.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect2>
   </sect1>

   <sect1 id="runtime-config-autovacuum">
    <title>Automatic Vacuuming</title>

    <indexterm>
     <primary>autovacuum</primary>
     <secondary>configuration parameters</secondary>
    </indexterm>

     <para>
      These settings control the behavior of the <firstterm>autovacuum</firstterm>
      feature.  Refer to <xref linkend="autovacuum"/> for more information.
      Note that many of these settings can be overridden on a per-table
      basis; see <xref linkend="sql-createtable-storage-parameters"
      endterm="sql-createtable-storage-parameters-title"/>.
     </para>

    <variablelist>

     <varlistentry id="guc-autovacuum" xreflabel="autovacuum">
      <term><varname>autovacuum</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>autovacuum</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls whether the server should run the
        autovacuum launcher daemon.  This is on by default; however,
        <xref linkend="guc-track-counts"/> must also be enabled for
        autovacuum to work.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line; however, autovacuuming can be
        disabled for individual tables by changing table storage parameters.
       </para>
       <para>
        Note that even when this parameter is disabled, the system
        will launch autovacuum processes if necessary to
        prevent transaction ID wraparound.  See <xref
        linkend="vacuum-for-wraparound"/> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-autovacuum-min-duration" xreflabel="log_autovacuum_min_duration">
      <term><varname>log_autovacuum_min_duration</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>log_autovacuum_min_duration</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Causes each action executed by autovacuum to be logged if it ran for at
        least the specified number of milliseconds.  Setting this to zero logs
        all autovacuum actions. Minus-one (the default) disables logging
        autovacuum actions.  For example, if you set this to
        <literal>250ms</literal> then all automatic vacuums and analyzes that run
        250ms or longer will be logged.  In addition, when this parameter is
        set to any value other than <literal>-1</literal>, a message will be
        logged if an autovacuum action is skipped due to a conflicting lock or a
        concurrently dropped relation.  Enabling this parameter can be helpful
        in tracking autovacuum activity.  This parameter can only be set in
        the <filename>postgresql.conf</filename> file or on the server command line;
        but the setting can be overridden for individual tables by
        changing table storage parameters.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
      <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>autovacuum_max_workers</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the maximum number of autovacuum processes (other than the
        autovacuum launcher) that may be running at any one time.  The default
        is three.  This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-naptime" xreflabel="autovacuum_naptime">
      <term><varname>autovacuum_naptime</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>autovacuum_naptime</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the minimum delay between autovacuum runs on any given
        database.  In each round the daemon examines the
        database and issues <command>VACUUM</command> and <command>ANALYZE</command> commands
        as needed for tables in that database.  The delay is measured
        in seconds, and the default is one minute (<literal>1min</literal>).
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-vacuum-threshold" xreflabel="autovacuum_vacuum_threshold">
      <term><varname>autovacuum_vacuum_threshold</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>autovacuum_vacuum_threshold</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the minimum number of updated or deleted tuples needed
        to trigger a <command>VACUUM</command> in any one table.
        The default is 50 tuples.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line;
        but the setting can be overridden for individual tables by
        changing table storage parameters.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-analyze-threshold" xreflabel="autovacuum_analyze_threshold">
      <term><varname>autovacuum_analyze_threshold</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>autovacuum_analyze_threshold</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the minimum number of inserted, updated or deleted tuples
        needed to trigger an <command>ANALYZE</command> in any one table.
        The default is 50 tuples.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line;
        but the setting can be overridden for individual tables by
        changing table storage parameters.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-vacuum-scale-factor" xreflabel="autovacuum_vacuum_scale_factor">
      <term><varname>autovacuum_vacuum_scale_factor</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>autovacuum_vacuum_scale_factor</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies a fraction of the table size to add to
        <varname>autovacuum_vacuum_threshold</varname>
        when deciding whether to trigger a <command>VACUUM</command>.
        The default is 0.2 (20% of table size).
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line;
        but the setting can be overridden for individual tables by
        changing table storage parameters.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-analyze-scale-factor" xreflabel="autovacuum_analyze_scale_factor">
      <term><varname>autovacuum_analyze_scale_factor</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>autovacuum_analyze_scale_factor</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies a fraction of the table size to add to
        <varname>autovacuum_analyze_threshold</varname>
        when deciding whether to trigger an <command>ANALYZE</command>.
        The default is 0.1 (10% of table size).
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line;
        but the setting can be overridden for individual tables by
        changing table storage parameters.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-freeze-max-age" xreflabel="autovacuum_freeze_max_age">
      <term><varname>autovacuum_freeze_max_age</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>autovacuum_freeze_max_age</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the maximum age (in transactions) that a table's
        <structname>pg_class</structname>.<structfield>relfrozenxid</structfield> field can
        attain before a <command>VACUUM</command> operation is forced
        to prevent transaction ID wraparound within the table.
        Note that the system will launch autovacuum processes to
        prevent wraparound even when autovacuum is otherwise disabled.
       </para>

       <para>
        Vacuum also allows removal of old files from the
        <filename>pg_xact</filename> subdirectory, which is why the default
        is a relatively low 200 million transactions.
        This parameter can only be set at server start, but the setting
        can be reduced for individual tables by
        changing table storage parameters.
        For more information see <xref linkend="vacuum-for-wraparound"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-multixact-freeze-max-age" xreflabel="autovacuum_multixact_freeze_max_age">
      <term><varname>autovacuum_multixact_freeze_max_age</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>autovacuum_multixact_freeze_max_age</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the maximum age (in multixacts) that a table's
        <structname>pg_class</structname>.<structfield>relminmxid</structfield> field can
        attain before a <command>VACUUM</command> operation is forced to
        prevent multixact ID wraparound within the table.
        Note that the system will launch autovacuum processes to
        prevent wraparound even when autovacuum is otherwise disabled.
       </para>

       <para>
        Vacuuming multixacts also allows removal of old files from the
        <filename>pg_multixact/members</filename> and <filename>pg_multixact/offsets</filename>
        subdirectories, which is why the default is a relatively low
        400 million multixacts.
        This parameter can only be set at server start, but the setting can
        be reduced for individual tables by changing table storage parameters.
        For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-vacuum-cost-delay" xreflabel="autovacuum_vacuum_cost_delay">
      <term><varname>autovacuum_vacuum_cost_delay</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>autovacuum_vacuum_cost_delay</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the cost delay value that will be used in automatic
        <command>VACUUM</command> operations.  If -1 is specified, the regular
        <xref linkend="guc-vacuum-cost-delay"/> value will be used.
        The default value is 20 milliseconds.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line;
        but the setting can be overridden for individual tables by
        changing table storage parameters.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-vacuum-cost-limit" xreflabel="autovacuum_vacuum_cost_limit">
      <term><varname>autovacuum_vacuum_cost_limit</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>autovacuum_vacuum_cost_limit</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the cost limit value that will be used in automatic
        <command>VACUUM</command> operations.  If -1 is specified (which is the
        default), the regular
        <xref linkend="guc-vacuum-cost-limit"/> value will be used.  Note that
        the value is distributed proportionally among the running autovacuum
        workers, if there is more than one, so that the sum of the limits for
        each worker does not exceed the value of this variable.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line;
        but the setting can be overridden for individual tables by
        changing table storage parameters.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </sect1>

   <sect1 id="runtime-config-client">
    <title>Client Connection Defaults</title>

    <sect2 id="runtime-config-client-statement">
     <title>Statement Behavior</title>
     <variablelist>

     <varlistentry id="guc-client-min-messages" xreflabel="client_min_messages">
      <term><varname>client_min_messages</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>client_min_messages</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls which
        <link linkend="runtime-config-severity-levels">message levels</link>
        are sent to the client.
        Valid values are <literal>DEBUG5</literal>,
        <literal>DEBUG4</literal>, <literal>DEBUG3</literal>, <literal>DEBUG2</literal>,
        <literal>DEBUG1</literal>, <literal>LOG</literal>, <literal>NOTICE</literal>,
        <literal>WARNING</literal>, and <literal>ERROR</literal>.
        Each level includes all the levels that follow it.  The later the level,
        the fewer messages are sent.  The default is
        <literal>NOTICE</literal>.  Note that <literal>LOG</literal> has a different
        rank here than in <xref linkend="guc-log-min-messages"/>.
       </para>
       <para>
        <literal>INFO</literal> level messages are always sent to the client.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-search-path" xreflabel="search_path">
      <term><varname>search_path</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>search_path</varname> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>path</primary><secondary>for schemas</secondary></indexterm>
      </term>
      <listitem>
       <para>
        This variable specifies the order in which schemas are searched
        when an object (table, data type, function, etc.) is referenced by a
        simple name with no schema specified.  When there are objects of
        identical names in different schemas, the one found first
        in the search path is used.  An object that is not in any of the
        schemas in the search path can only be referenced by specifying
        its containing schema with a qualified (dotted) name.
       </para>

       <para>
        The value for <varname>search_path</varname> must be a comma-separated
        list of schema names.  Any name that is not an existing schema, or is
        a schema for which the user does not have <literal>USAGE</literal>
        permission, is silently ignored.
       </para>

       <para>
        If one of the list items is the special name
        <literal>$user</literal>, then the schema having the name returned by
        <function>CURRENT_USER</function> is substituted, if there is such a schema
        and the user has <literal>USAGE</literal> permission for it.
        (If not, <literal>$user</literal> is ignored.)
       </para>

       <para>
        The system catalog schema, <literal>pg_catalog</literal>, is always
        searched, whether it is mentioned in the path or not.  If it is
        mentioned in the path then it will be searched in the specified
        order.  If <literal>pg_catalog</literal> is not in the path then it will
        be searched <emphasis>before</emphasis> searching any of the path items.
       </para>

       <!-- To further split hairs, funcname('foo') does not use the temporary
            schema, even when it considers typname='funcname'.  This paragraph
            refers to function names in a loose sense, "pg_proc.proname or
            func_name grammar production". -->
       <para>
        Likewise, the current session's temporary-table schema,
        <literal>pg_temp_<replaceable>nnn</replaceable></literal>, is always searched if it
        exists.  It can be explicitly listed in the path by using the
        alias <literal>pg_temp</literal><indexterm><primary>pg_temp</primary></indexterm>.  If it is not listed in the path then
        it is searched first (even before <literal>pg_catalog</literal>).  However,
        the temporary schema is only searched for relation (table, view,
        sequence, etc) and data type names.  It is never searched for
        function or operator names.
       </para>

       <para>
        When objects are created without specifying a particular target
        schema, they will be placed in the first valid schema named in
        <varname>search_path</varname>.  An error is reported if the search
        path is empty.
       </para>

       <para>
        The default value for this parameter is
        <literal>"$user", public</literal>.
        This setting supports shared use of a database (where no users
        have private schemas, and all share use of <literal>public</literal>),
        private per-user schemas, and combinations of these.  Other
        effects can be obtained by altering the default search path
        setting, either globally or per-user.
       </para>

       <para>
        For more information on schema handling, see
        <xref linkend="ddl-schemas"/>.  In particular, the default
        configuration is suitable only when the database has a single user or
        a few mutually-trusting users.
       </para>

       <para>
        The current effective value of the search path can be examined
        via the <acronym>SQL</acronym> function
        <function>current_schemas</function>
        (see <xref linkend="functions-info"/>).
        This is not quite the same as
        examining the value of <varname>search_path</varname>, since
        <function>current_schemas</function> shows how the items
        appearing in <varname>search_path</varname> were resolved.
       </para>

      </listitem>
     </varlistentry>

     <varlistentry id="guc-row-security" xreflabel="row_security">
      <term><varname>row_security</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>row_security</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This variable controls whether to raise an error in lieu of applying a
        row security policy.  When set to <literal>on</literal>, policies apply
        normally.  When set to <literal>off</literal>, queries fail which would
        otherwise apply at least one policy.  The default is <literal>on</literal>.
        Change to <literal>off</literal> where limited row visibility could cause
        incorrect results; for example, <application>pg_dump</application> makes that
        change by default.  This variable has no effect on roles which bypass
        every row security policy, to wit, superusers and roles with
        the <literal>BYPASSRLS</literal> attribute.
       </para>

       <para>
        For more information on row security policies,
        see <xref linkend="sql-createpolicy"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-tablespace" xreflabel="default_tablespace">
      <term><varname>default_tablespace</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>default_tablespace</varname> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>tablespace</primary><secondary>default</secondary></indexterm>
      </term>
      <listitem>
       <para>
        This variable specifies the default tablespace in which to create
        objects (tables and indexes) when a <command>CREATE</command> command does
        not explicitly specify a tablespace.
       </para>

       <para>
        The value is either the name of a tablespace, or an empty string
        to specify using the default tablespace of the current database.
        If the value does not match the name of any existing tablespace,
        <productname>PostgreSQL</productname> will automatically use the default
        tablespace of the current database.  If a nondefault tablespace
        is specified, the user must have <literal>CREATE</literal> privilege
        for it, or creation attempts will fail.
       </para>

       <para>
        This variable is not used for temporary tables; for them,
        <xref linkend="guc-temp-tablespaces"/> is consulted instead.
       </para>

       <para>
        This variable is also not used when creating databases.
        By default, a new database inherits its tablespace setting from
        the template database it is copied from.
       </para>

       <para>
        For more information on tablespaces,
        see <xref linkend="manage-ag-tablespaces"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-temp-tablespaces" xreflabel="temp_tablespaces">
      <term><varname>temp_tablespaces</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>temp_tablespaces</varname> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>tablespace</primary><secondary>temporary</secondary></indexterm>
      </term>
      <listitem>
       <para>
        This variable specifies tablespaces in which to create temporary
        objects (temp tables and indexes on temp tables) when a
        <command>CREATE</command> command does not explicitly specify a tablespace.
        Temporary files for purposes such as sorting large data sets
        are also created in these tablespaces.
       </para>

       <para>
        The value is a list of names of tablespaces.  When there is more than
        one name in the list, <productname>PostgreSQL</productname> chooses a random
        member of the list each time a temporary object is to be created;
        except that within a transaction, successively created temporary
        objects are placed in successive tablespaces from the list.
        If the selected element of the list is an empty string,
        <productname>PostgreSQL</productname> will automatically use the default
        tablespace of the current database instead.
       </para>

       <para>
        When <varname>temp_tablespaces</varname> is set interactively, specifying a
        nonexistent tablespace is an error, as is specifying a tablespace for
        which the user does not have <literal>CREATE</literal> privilege.  However,
        when using a previously set value, nonexistent tablespaces are
        ignored, as are tablespaces for which the user lacks
        <literal>CREATE</literal> privilege.  In particular, this rule applies when
        using a value set in <filename>postgresql.conf</filename>.
       </para>

       <para>
        The default value is an empty string, which results in all temporary
        objects being created in the default tablespace of the current
        database.
       </para>

       <para>
        See also <xref linkend="guc-default-tablespace"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-check-function-bodies" xreflabel="check_function_bodies">
      <term><varname>check_function_bodies</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>check_function_bodies</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This parameter is normally on. When set to <literal>off</literal>, it
        disables validation of the function body string during <xref
        linkend="sql-createfunction"/>.  Disabling validation avoids side
        effects of the validation process and avoids false positives due
        to problems such as forward references.  Set this parameter
        to <literal>off</literal> before loading functions on behalf of other
        users; <application>pg_dump</application> does so automatically.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-transaction-isolation" xreflabel="default_transaction_isolation">
      <term><varname>default_transaction_isolation</varname> (<type>enum</type>)
      <indexterm>
       <primary>transaction isolation level</primary>
       <secondary>setting default</secondary>
      </indexterm>
      <indexterm>
       <primary><varname>default_transaction_isolation</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Each SQL transaction has an isolation level, which can be
        either <quote>read uncommitted</quote>, <quote>read
        committed</quote>, <quote>repeatable read</quote>, or
        <quote>serializable</quote>.  This parameter controls the
        default isolation level of each new transaction. The default
        is <quote>read committed</quote>.
       </para>

       <para>
        Consult <xref linkend="mvcc"/> and <xref
        linkend="sql-set-transaction"/> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-transaction-read-only" xreflabel="default_transaction_read_only">
      <term><varname>default_transaction_read_only</varname> (<type>boolean</type>)
      <indexterm>
       <primary>read-only transaction</primary>
       <secondary>setting default</secondary>
      </indexterm>
      <indexterm>
       <primary><varname>default_transaction_read_only</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        A read-only SQL transaction cannot alter non-temporary tables.
        This parameter controls the default read-only status of each new
        transaction. The default is <literal>off</literal> (read/write).
       </para>

       <para>
        Consult <xref linkend="sql-set-transaction"/> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-transaction-deferrable" xreflabel="default_transaction_deferrable">
      <term><varname>default_transaction_deferrable</varname> (<type>boolean</type>)
      <indexterm>
       <primary>deferrable transaction</primary>
       <secondary>setting default</secondary>
      </indexterm>
      <indexterm>
       <primary><varname>default_transaction_deferrable</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When running at the <literal>serializable</literal> isolation level,
        a deferrable read-only SQL transaction may be delayed before
        it is allowed to proceed.  However, once it begins executing
        it does not incur any of the overhead required to ensure
        serializability; so serialization code will have no reason to
        force it to abort because of concurrent updates, making this
        option suitable for long-running read-only transactions.
        </para>

        <para>
        This parameter controls the default deferrable status of each
        new transaction.  It currently has no effect on read-write
        transactions or those operating at isolation levels lower
        than <literal>serializable</literal>. The default is <literal>off</literal>.
       </para>

       <para>
        Consult <xref linkend="sql-set-transaction"/> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-transaction-isolation" xreflabel="transaction_isolation">
      <term><varname>transaction_isolation</varname> (<type>enum</type>)
      <indexterm>
       <primary>transaction isolation level</primary>
      </indexterm>
      <indexterm>
       <primary><varname>transaction_isolation</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This parameter reflects the current transaction's isolation level.
        At the beginning of each transaction, it is set to the current value
        of <xref linkend="guc-default-transaction-isolation"/>.
        Any subsequent attempt to change it is equivalent to a <xref
        linkend="sql-set-transaction"/> command.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-transaction-read-only" xreflabel="transaction_read_only">
      <term><varname>transaction_read_only</varname> (<type>boolean</type>)
      <indexterm>
       <primary>read-only transaction</primary>
      </indexterm>
      <indexterm>
       <primary><varname>transaction_read_only</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This parameter reflects the current transaction's read-only status.
        At the beginning of each transaction, it is set to the current value
        of <xref linkend="guc-default-transaction-read-only"/>.
        Any subsequent attempt to change it is equivalent to a <xref
        linkend="sql-set-transaction"/> command.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-transaction-deferrable" xreflabel="transaction_deferrable">
      <term><varname>transaction_deferrable</varname> (<type>boolean</type>)
      <indexterm>
       <primary>deferrable transaction</primary>
      </indexterm>
      <indexterm>
       <primary><varname>transaction_deferrable</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This parameter reflects the current transaction's deferrability status.
        At the beginning of each transaction, it is set to the current value
        of <xref linkend="guc-default-transaction-deferrable"/>.
        Any subsequent attempt to change it is equivalent to a <xref
        linkend="sql-set-transaction"/> command.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry id="guc-session-replication-role" xreflabel="session_replication_role">
      <term><varname>session_replication_role</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>session_replication_role</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Controls firing of replication-related triggers and rules for the
        current session.  Setting this variable requires
        superuser privilege and results in discarding any previously cached
        query plans.  Possible values are <literal>origin</literal> (the default),
        <literal>replica</literal> and <literal>local</literal>.
       </para>

       <para>
        The intended use of this setting is that logical replication systems
        set it to <literal>replica</literal> when they are applying replicated
        changes.  The effect of that will be that triggers and rules (that
        have not been altered from their default configuration) will not fire
        on the replica.  See the <xref linkend="sql-altertable"/> clauses
        <literal>ENABLE TRIGGER</literal> and <literal>ENABLE RULE</literal>
        for more information.
       </para>

       <para>
        PostgreSQL treats the settings <literal>origin</literal> and
        <literal>local</literal> the same internally.  Third-party replication
        systems may use these two values for their internal purposes, for
        example using <literal>local</literal> to designate a session whose
        changes should not be replicated.
       </para>

       <para>
        Since foreign keys are implemented as triggers, setting this parameter
        to <literal>replica</literal> also disables all foreign key checks,
        which can leave data in an inconsistent state if improperly used.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-statement-timeout" xreflabel="statement_timeout">
      <term><varname>statement_timeout</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>statement_timeout</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Abort any statement that takes more than the specified number of
        milliseconds, starting from the time the command arrives at the server
        from the client.  If <varname>log_min_error_statement</varname> is set to
        <literal>ERROR</literal> or lower, the statement that timed out will also be
        logged.  A value of zero (the default) turns this off.
       </para>

       <para>
        Setting <varname>statement_timeout</varname> in
        <filename>postgresql.conf</filename> is not recommended because it would
        affect all sessions.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lock-timeout" xreflabel="lock_timeout">
      <term><varname>lock_timeout</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>lock_timeout</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Abort any statement that waits longer than the specified number of
        milliseconds while attempting to acquire a lock on a table, index,
        row, or other database object.  The time limit applies separately to
        each lock acquisition attempt.  The limit applies both to explicit
        locking requests (such as <command>LOCK TABLE</command>, or <command>SELECT
        FOR UPDATE</command> without <literal>NOWAIT</literal>) and to implicitly-acquired
        locks.  A value of zero (the default) turns this off.
       </para>

       <para>
        Unlike <varname>statement_timeout</varname>, this timeout can only occur
        while waiting for locks.  Note that if <varname>statement_timeout</varname>
        is nonzero, it is rather pointless to set <varname>lock_timeout</varname> to
        the same or larger value, since the statement timeout would always
        trigger first.  If <varname>log_min_error_statement</varname> is set to
        <literal>ERROR</literal> or lower, the statement that timed out will be
        logged.  
       </para>

       <para>
        Setting <varname>lock_timeout</varname> in
        <filename>postgresql.conf</filename> is not recommended because it would
        affect all sessions.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-idle-in-transaction-session-timeout" xreflabel="idle_in_transaction_session_timeout">
      <term><varname>idle_in_transaction_session_timeout</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>idle_in_transaction_session_timeout</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
       Terminate any session with an open transaction that has been idle for
       longer than the specified duration in milliseconds. This allows any
       locks held by that session to be released and the connection slot to be reused;
       it also allows tuples visible only to this transaction to be vacuumed.  See
       <xref linkend="routine-vacuuming"/> for more details about this.
       </para>
       <para>
       The default value of 0 disables this feature.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-vacuum-freeze-table-age" xreflabel="vacuum_freeze_table_age">
      <term><varname>vacuum_freeze_table_age</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>vacuum_freeze_table_age</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        <command>VACUUM</command> performs an aggressive scan if the table's
        <structname>pg_class</structname>.<structfield>relfrozenxid</structfield> field has reached
        the age specified by this setting.  An aggressive scan differs from
        a regular <command>VACUUM</command> in that it visits every page that might
        contain unfrozen XIDs or MXIDs, not just those that might contain dead
        tuples.  The default is 150 million transactions.  Although users can
        set this value anywhere from zero to two billions, <command>VACUUM</command>
        will silently limit the effective value to 95% of
        <xref linkend="guc-autovacuum-freeze-max-age"/>, so that a
        periodical manual <command>VACUUM</command> has a chance to run before an
        anti-wraparound autovacuum is launched for the table. For more
        information see
        <xref linkend="vacuum-for-wraparound"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-vacuum-freeze-min-age" xreflabel="vacuum_freeze_min_age">
      <term><varname>vacuum_freeze_min_age</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>vacuum_freeze_min_age</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the cutoff age (in transactions) that <command>VACUUM</command>
        should use to decide whether to freeze row versions
        while scanning a table.
        The default is 50 million transactions.  Although
        users can set this value anywhere from zero to one billion,
        <command>VACUUM</command> will silently limit the effective value to half
        the value of <xref linkend="guc-autovacuum-freeze-max-age"/>, so
        that there is not an unreasonably short time between forced
        autovacuums.  For more information see <xref
        linkend="vacuum-for-wraparound"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-vacuum-multixact-freeze-table-age" xreflabel="vacuum_multixact_freeze_table_age">
      <term><varname>vacuum_multixact_freeze_table_age</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>vacuum_multixact_freeze_table_age</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        <command>VACUUM</command> performs an aggressive scan if the table's
        <structname>pg_class</structname>.<structfield>relminmxid</structfield> field has reached
        the age specified by this setting.  An aggressive scan differs from
        a regular <command>VACUUM</command> in that it visits every page that might
        contain unfrozen XIDs or MXIDs, not just those that might contain dead
        tuples.  The default is 150 million multixacts.
        Although users can set this value anywhere from zero to two billions,
        <command>VACUUM</command> will silently limit the effective value to 95% of
        <xref linkend="guc-autovacuum-multixact-freeze-max-age"/>, so that a
        periodical manual <command>VACUUM</command> has a chance to run before an
        anti-wraparound is launched for the table.
        For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-vacuum-multixact-freeze-min-age" xreflabel="vacuum_multixact_freeze_min_age">
      <term><varname>vacuum_multixact_freeze_min_age</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>vacuum_multixact_freeze_min_age</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the cutoff age (in multixacts) that <command>VACUUM</command>
        should use to decide whether to replace multixact IDs with a newer
        transaction ID or multixact ID while scanning a table.  The default
        is 5 million multixacts.
        Although users can set this value anywhere from zero to one billion,
        <command>VACUUM</command> will silently limit the effective value to half
        the value of <xref linkend="guc-autovacuum-multixact-freeze-max-age"/>,
        so that there is not an unreasonably short time between forced
        autovacuums.
        For more information see <xref linkend="vacuum-for-multixact-wraparound"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-vacuum-cleanup-index-scale-factor" xreflabel="vacuum_cleanup_index_scale_factor">
      <term><varname>vacuum_cleanup_index_scale_factor</varname> (<type>floating point</type>)
      <indexterm>
       <primary><varname>vacuum_cleanup_index_scale_factor</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Specifies the fraction of the total number of heap tuples counted in
        the previous statistics collection that can be inserted without
        incurring an index scan at the <command>VACUUM</command> cleanup stage.
        This setting currently applies to B-tree indexes only.
       </para>

       <para>
        If no tuples were deleted from the heap, B-tree indexes are still
        scanned at the <command>VACUUM</command> cleanup stage when at least one
        of the following conditions is met: the index statistics are stale, or
        the index contains deleted pages that can be recycled during cleanup.
        Index statistics are considered to be stale if the number of newly
        inserted tuples exceeds the <varname>vacuum_cleanup_index_scale_factor</varname>
        fraction of the total number of heap tuples detected by the previous
        statistics collection. The total number of heap tuples is stored in
        the index meta-page. Note that the meta-page does not include this data
        until <command>VACUUM</command> finds no dead tuples, so B-tree index
        scan at the cleanup stage can only be skipped if the second and
        subsequent <command>VACUUM</command> cycles detect no dead tuples.
       </para>

       <para>
        The value can range from <literal>0</literal> to
        <literal>10000000000</literal>.
        When <varname>vacuum_cleanup_index_scale_factor</varname> is set to
        <literal>0</literal>, index scans are never skipped during
        <command>VACUUM</command> cleanup. The default value is <literal>0.1</literal>.
       </para>

      </listitem>
     </varlistentry>

     <varlistentry id="guc-bytea-output" xreflabel="bytea_output">
      <term><varname>bytea_output</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>bytea_output</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the output format for values of type <type>bytea</type>.
        Valid values are <literal>hex</literal> (the default)
        and <literal>escape</literal> (the traditional PostgreSQL
        format).  See <xref linkend="datatype-binary"/> for more
        information.  The <type>bytea</type> type always
        accepts both formats on input, regardless of this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-xmlbinary" xreflabel="xmlbinary">
      <term><varname>xmlbinary</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>xmlbinary</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets how binary values are to be encoded in XML.  This applies
        for example when <type>bytea</type> values are converted to
        XML by the functions <function>xmlelement</function> or
        <function>xmlforest</function>.  Possible values are
        <literal>base64</literal> and <literal>hex</literal>, which
        are both defined in the XML Schema standard.  The default is
        <literal>base64</literal>.  For further information about
        XML-related functions, see <xref linkend="functions-xml"/>.
       </para>

       <para>
        The actual choice here is mostly a matter of taste,
        constrained only by possible restrictions in client
        applications.  Both methods support all possible values,
        although the hex encoding will be somewhat larger than the
        base64 encoding.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-xmloption" xreflabel="xmloption">
      <term><varname>xmloption</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>xmloption</varname> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>SET XML OPTION</varname></primary>
      </indexterm>
      <indexterm>
       <primary>XML option</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets whether <literal>DOCUMENT</literal> or
        <literal>CONTENT</literal> is implicit when converting between
        XML and character string values.  See <xref
        linkend="datatype-xml"/> for a description of this.  Valid
        values are <literal>DOCUMENT</literal> and
        <literal>CONTENT</literal>.  The default is
        <literal>CONTENT</literal>.
       </para>

       <para>
        According to the SQL standard, the command to set this option is
<synopsis>
SET XML OPTION { DOCUMENT | CONTENT };
</synopsis>
        This syntax is also available in PostgreSQL.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-gin-pending-list-limit" xreflabel="gin_pending_list_limit">
      <term><varname>gin_pending_list_limit</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>gin_pending_list_limit</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the maximum size of the GIN pending list which is used
        when <literal>fastupdate</literal> is enabled. If the list grows
        larger than this maximum size, it is cleaned up by moving
        the entries in it to the main GIN data structure in bulk.
        The default is four megabytes (<literal>4MB</literal>). This setting
        can be overridden for individual GIN indexes by changing
        index storage parameters.
         See <xref linkend="gin-fast-update"/> and <xref linkend="gin-tips"/>
         for more information.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
     <sect2 id="runtime-config-client-format">
     <title>Locale and Formatting</title>

     <variablelist>

     <varlistentry id="guc-datestyle" xreflabel="DateStyle">
      <term><varname>DateStyle</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>DateStyle</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the display format for date and time values, as well as the
        rules for interpreting ambiguous date input values. For
        historical reasons, this variable contains two independent
        components: the output format specification (<literal>ISO</literal>,
        <literal>Postgres</literal>, <literal>SQL</literal>, or <literal>German</literal>)
        and the input/output specification for year/month/day ordering
        (<literal>DMY</literal>, <literal>MDY</literal>, or <literal>YMD</literal>). These
        can be set separately or together. The keywords <literal>Euro</literal>
        and <literal>European</literal> are synonyms for <literal>DMY</literal>; the
        keywords <literal>US</literal>, <literal>NonEuro</literal>, and
        <literal>NonEuropean</literal> are synonyms for <literal>MDY</literal>. See
        <xref linkend="datatype-datetime"/> for more information. The
        built-in default is <literal>ISO, MDY</literal>, but
        <application>initdb</application> will initialize the
        configuration file with a setting that corresponds to the
        behavior of the chosen <varname>lc_time</varname> locale.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-intervalstyle" xreflabel="IntervalStyle">
      <term><varname>IntervalStyle</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>IntervalStyle</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the display format for interval values.
        The value <literal>sql_standard</literal> will produce
        output matching <acronym>SQL</acronym> standard interval literals.
        The value <literal>postgres</literal> (which is the default) will produce
        output matching <productname>PostgreSQL</productname> releases prior to 8.4
        when the <xref linkend="guc-datestyle"/>
        parameter was set to <literal>ISO</literal>.
        The value <literal>postgres_verbose</literal> will produce output
        matching <productname>PostgreSQL</productname> releases prior to 8.4
        when the <varname>DateStyle</varname>
        parameter was set to non-<literal>ISO</literal> output.
        The value <literal>iso_8601</literal> will produce output matching the time
        interval <quote>format with designators</quote> defined in section
        4.4.3.2 of ISO 8601.
       </para>
       <para>
        The <varname>IntervalStyle</varname> parameter also affects the
        interpretation of ambiguous interval input.  See
        <xref linkend="datatype-interval-input"/> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-timezone" xreflabel="TimeZone">
      <term><varname>TimeZone</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>TimeZone</varname> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>time zone</primary></indexterm>
      </term>
      <listitem>
       <para>
        Sets the time zone for displaying and interpreting time stamps.
        The built-in default is <literal>GMT</literal>, but that is typically
        overridden in <filename>postgresql.conf</filename>; <application>initdb</application>
        will install a setting there corresponding to its system environment.
        See <xref linkend="datatype-timezones"/> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-timezone-abbreviations" xreflabel="timezone_abbreviations">
      <term><varname>timezone_abbreviations</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>timezone_abbreviations</varname> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>time zone names</primary></indexterm>
      </term>
      <listitem>
       <para>
        Sets the collection of time zone abbreviations that will be accepted
        by the server for datetime input.  The default is <literal>'Default'</literal>,
        which is a collection that works in most of the world; there are
        also <literal>'Australia'</literal> and <literal>'India'</literal>,
        and other collections can be defined for a particular installation.
        See <xref linkend="datetime-config-files"/> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-extra-float-digits" xreflabel="extra_float_digits">
      <term><varname>extra_float_digits</varname> (<type>integer</type>)
      <indexterm>
       <primary>significant digits</primary>
      </indexterm>
      <indexterm>
       <primary>floating-point</primary>
       <secondary>display</secondary>
      </indexterm>
      <indexterm>
       <primary><varname>extra_float_digits</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This parameter adjusts the number of digits displayed for
        floating-point values, including <type>float4</type>, <type>float8</type>,
        and geometric data types.  The parameter value is added to the
        standard number of digits (<literal>FLT_DIG</literal> or <literal>DBL_DIG</literal>
        as appropriate).  The value can be set as high as 3, to include
        partially-significant digits; this is especially useful for dumping
        float data that needs to be restored exactly.  Or it can be set
        negative to suppress unwanted digits.
        See also <xref linkend="datatype-float"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-client-encoding" xreflabel="client_encoding">
      <term><varname>client_encoding</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>client_encoding</varname> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>character set</primary></indexterm>
      </term>
      <listitem>
       <para>
        Sets the client-side encoding (character set).
        The default is to use the database encoding.
        The character sets supported by the <productname>PostgreSQL</productname>
        server are described in <xref linkend="multibyte-charset-supported"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-messages" xreflabel="lc_messages">
      <term><varname>lc_messages</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>lc_messages</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the language in which messages are displayed.  Acceptable
        values are system-dependent; see <xref linkend="locale"/> for
        more information.  If this variable is set to the empty string
        (which is the default) then the value is inherited from the
        execution environment of the server in a system-dependent way.
       </para>

       <para>
        On some systems, this locale category does not exist.  Setting
        this variable will still work, but there will be no effect.
        Also, there is a chance that no translated messages for the
        desired language exist.  In that case you will continue to see
        the English messages.
       </para>

       <para>
        Only superusers can change this setting, because it affects the
        messages sent to the server log as well as to the client, and
        an improper value might obscure the readability of the server
        logs.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-monetary" xreflabel="lc_monetary">
      <term><varname>lc_monetary</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>lc_monetary</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the locale to use for formatting monetary amounts, for
        example with the <function>to_char</function> family of
        functions.  Acceptable values are system-dependent; see <xref
        linkend="locale"/> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-numeric" xreflabel="lc_numeric">
      <term><varname>lc_numeric</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>lc_numeric</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the locale to use for formatting numbers, for example
        with the <function>to_char</function> family of
        functions. Acceptable values are system-dependent; see <xref
        linkend="locale"/> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-time" xreflabel="lc_time">
      <term><varname>lc_time</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>lc_time</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Sets the locale to use for formatting dates and times, for example
        with the <function>to_char</function> family of
        functions. Acceptable values are system-dependent; see <xref
        linkend="locale"/> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-text-search-config" xreflabel="default_text_search_config">
      <term><varname>default_text_search_config</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>default_text_search_config</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Selects the text search configuration that is used by those variants
        of the text search functions that do not have an explicit argument
        specifying the configuration.
        See <xref linkend="textsearch"/> for further information.
        The built-in default is <literal>pg_catalog.simple</literal>, but
        <application>initdb</application> will initialize the
        configuration file with a setting that corresponds to the
        chosen <varname>lc_ctype</varname> locale, if a configuration
        matching that locale can be identified.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect2>

    <sect2 id="runtime-config-client-preload">
     <title>Shared Library Preloading</title>

     <para>
      Several settings are available for preloading shared libraries into the
      server, in order to load additional functionality or achieve performance
      benefits.  For example, a setting of
      <literal>'$libdir/mylib'</literal> would cause
      <literal>mylib.so</literal> (or on some platforms,
      <literal>mylib.sl</literal>) to be preloaded from the installation's standard
      library directory.  The differences between the settings are when they
      take effect and what privileges are required to change them.
     </para>

     <para>
      <productname>PostgreSQL</productname> procedural language libraries can
      be preloaded in this way, typically by using the
      syntax <literal>'$libdir/plXXX'</literal> where
      <literal>XXX</literal> is <literal>pgsql</literal>, <literal>perl</literal>,
      <literal>tcl</literal>, or <literal>python</literal>.
     </para>

     <para>
      Only shared libraries specifically intended to be used with PostgreSQL
      can be loaded this way.  Every PostgreSQL-supported library has
      a <quote>magic block</quote> that is checked to guarantee compatibility.  For
      this reason, non-PostgreSQL libraries cannot be loaded in this way.  You
      might be able to use operating-system facilities such
      as <envar>LD_PRELOAD</envar> for that.
     </para>

     <para>
      In general, refer to the documentation of a specific module for the
      recommended way to load that module.
     </para>

     <variablelist>
     <varlistentry id="guc-local-preload-libraries" xreflabel="local_preload_libraries">
      <term><varname>local_preload_libraries</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>local_preload_libraries</varname> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><filename>$libdir/plugins</filename></primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This variable specifies one or more shared libraries that are to be
        preloaded at connection start.
        It contains a comma-separated list of library names, where each name
        is interpreted as for the <xref linkend="sql-load"/> command.
        Whitespace between entries is ignored; surround a library name with
        double quotes if you need to include whitespace or commas in the name.
        The parameter value only takes effect at the start of the connection.
        Subsequent changes have no effect.  If a specified library is not
        found, the connection attempt will fail.
       </para>

       <para>
        This option can be set by any user.  Because of that, the libraries
        that can be loaded are restricted to those appearing in the
        <filename>plugins</filename> subdirectory of the installation's
        standard library directory.  (It is the database administrator's
        responsibility to ensure that only <quote>safe</quote> libraries
        are installed there.)  Entries in <varname>local_preload_libraries</varname>
        can specify this directory explicitly, for example
        <literal>$libdir/plugins/mylib</literal>, or just specify
        the library name &mdash; <literal>mylib</literal> would have
        the same effect as <literal>$libdir/plugins/mylib</literal>.
       </para>

       <para>
        The intent of this feature is to allow unprivileged users to load
        debugging or performance-measurement libraries into specific sessions
        without requiring an explicit <command>LOAD</command> command.  To that end,
        it would be typical to set this parameter using
        the <envar>PGOPTIONS</envar> environment variable on the client or by
        using
        <command>ALTER ROLE SET</command>.
       </para>

       <para>
        However, unless a module is specifically designed to be used in this way by
        non-superusers, this is usually not the right setting to use.  Look
        at <xref linkend="guc-session-preload-libraries"/> instead.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry id="guc-session-preload-libraries" xreflabel="session_preload_libraries">
      <term><varname>session_preload_libraries</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>session_preload_libraries</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This variable specifies one or more shared libraries that are to be
        preloaded at connection start.
        It contains a comma-separated list of library names, where each name
        is interpreted as for the <xref linkend="sql-load"/> command.
        Whitespace between entries is ignored; surround a library name with
        double quotes if you need to include whitespace or commas in the name.
        The parameter value only takes effect at the start of the connection.
        Subsequent changes have no effect.  If a specified library is not
        found, the connection attempt will fail.
        Only superusers can change this setting.
       </para>

       <para>
        The intent of this feature is to allow debugging or
        performance-measurement libraries to be loaded into specific sessions
        without an explicit
        <command>LOAD</command> command being given.  For
        example, <xref linkend="auto-explain"/> could be enabled for all
        sessions under a given user name by setting this parameter
        with <command>ALTER ROLE SET</command>.  Also, this parameter can be changed
        without restarting the server (but changes only take effect when a new
        session is started), so it is easier to add new modules this way, even
        if they should apply to all sessions.
       </para>

       <para>
        Unlike <xref linkend="guc-shared-preload-libraries"/>, there is no large
        performance advantage to loading a library at session start rather than
        when it is first used.  There is some advantage, however, when
        connection pooling is used.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-shared-preload-libraries" xreflabel="shared_preload_libraries">
      <term><varname>shared_preload_libraries</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>shared_preload_libraries</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This variable specifies one or more shared libraries to be preloaded at
        server start.
        It contains a comma-separated list of library names, where each name
        is interpreted as for the <xref linkend="sql-load"/> command.
        Whitespace between entries is ignored; surround a library name with
        double quotes if you need to include whitespace or commas in the name.
        This parameter can only be set at server start.  If a specified
        library is not found, the server will fail to start.
       </para>

       <para>
        Some libraries need to perform certain operations that can only take
        place at postmaster start, such as allocating shared memory, reserving
        light-weight locks, or starting background workers.  Those libraries
        must be loaded at server start through this parameter.  See the
        documentation of each library for details.
       </para>

       <para>
        Other libraries can also be preloaded.  By preloading a shared library,
        the library startup time is avoided when the library is first used.
        However, the time to start each new server process might increase
        slightly, even if that process never uses the library.  So this
        parameter is recommended only for libraries that will be used in most
        sessions.  Also, changing this parameter requires a server restart, so
        this is not the right setting to use for short-term debugging tasks,
        say.  Use <xref linkend="guc-session-preload-libraries"/> for that
        instead.
       </para>

      <note>
       <para>
        On Windows hosts, preloading a library at server start will not reduce
        the time required to start each new server process; each server process
        will re-load all preload libraries.  However, <varname>shared_preload_libraries
        </varname> is still useful on Windows hosts for libraries that need to
        perform operations at postmaster start time.
       </para>
      </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit-provider" xreflabel="jit_provider">
      <term><varname>jit_provider</varname> (<type>string</type>)
       <indexterm>
        <primary><varname>jit_provider</varname> configuration parameter</primary>
       </indexterm>
      </term>
      <listitem>
       <para>
        This variable is the name of the JIT provider library to be used
        (see <xref linkend="jit-pluggable"/>).
        The default is <literal>llvmjit</literal>.
        This parameter can only be set at server start.
       </para>

       <para>
        If set to a non-existent library, <acronym>JIT</acronym> will not be
        available, but no error will be raised. This allows JIT support to be
        installed separately from the main
        <productname>PostgreSQL</productname> package.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </sect2>

     <sect2 id="runtime-config-client-other">
     <title>Other Defaults</title>

     <variablelist>

     <varlistentry id="guc-dynamic-library-path" xreflabel="dynamic_library_path">
      <term><varname>dynamic_library_path</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>dynamic_library_path</varname> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>dynamic loading</primary></indexterm>
      </term>
      <listitem>
       <para>
        If a dynamically loadable module needs to be opened and the
        file name specified in the <command>CREATE FUNCTION</command> or
        <command>LOAD</command> command
        does not have a directory component (i.e., the
        name does not contain a slash), the system will search this
        path for the required file.
       </para>

       <para>
        The value for <varname>dynamic_library_path</varname> must be a
        list of absolute directory paths separated by colons (or semi-colons
        on Windows).  If a list element starts
        with the special string <literal>$libdir</literal>, the
        compiled-in <productname>PostgreSQL</productname> package
        library directory is substituted for <literal>$libdir</literal>; this
        is where the modules provided by the standard
        <productname>PostgreSQL</productname> distribution are installed.
        (Use <literal>pg_config --pkglibdir</literal> to find out the name of
        this directory.) For example:
<programlisting>
dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir'
</programlisting>
        or, in a Windows environment:
<programlisting>
dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
</programlisting>
       </para>

       <para>
        The default value for this parameter is
        <literal>'$libdir'</literal>. If the value is set to an empty
        string, the automatic path search is turned off.
       </para>

       <para>
        This parameter can be changed at run time by superusers, but a
        setting done that way will only persist until the end of the
        client connection, so this method should be reserved for
        development purposes. The recommended way to set this parameter
        is in the <filename>postgresql.conf</filename> configuration
        file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-gin-fuzzy-search-limit" xreflabel="gin_fuzzy_search_limit">
      <term><varname>gin_fuzzy_search_limit</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>gin_fuzzy_search_limit</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Soft upper limit of the size of the set returned by GIN index scans. For more
        information see <xref linkend="gin-tips"/>.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-locks">
    <title>Lock Management</title>

     <variablelist>

     <varlistentry id="guc-deadlock-timeout" xreflabel="deadlock_timeout">
      <term><varname>deadlock_timeout</varname> (<type>integer</type>)
      <indexterm>
       <primary>deadlock</primary>
       <secondary>timeout during</secondary>
      </indexterm>
      <indexterm>
       <primary>timeout</primary>
       <secondary>deadlock</secondary>
      </indexterm>
      <indexterm>
       <primary><varname>deadlock_timeout</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This is the amount of time, in milliseconds, to wait on a lock
        before checking to see if there is a deadlock condition. The
        check for deadlock is relatively expensive, so the server doesn't run
        it every time it waits for a lock. We optimistically assume
        that deadlocks are not common in production applications and
        just wait on the lock for a while before checking for a
        deadlock. Increasing this value reduces the amount of time
        wasted in needless deadlock checks, but slows down reporting of
        real deadlock errors. The default is one second (<literal>1s</literal>),
        which is probably about the smallest value you would want in
        practice. On a heavily loaded server you might want to raise it.
        Ideally the setting should exceed your typical transaction time,
        so as to improve the odds that a lock will be released before
        the waiter decides to check for deadlock.  Only superusers can change
        this setting.
       </para>

       <para>
        When <xref linkend="guc-log-lock-waits"/> is set,
        this parameter also determines the length of time to wait before
        a log message is issued about the lock wait.  If you are trying
        to investigate locking delays you might want to set a shorter than
        normal <varname>deadlock_timeout</varname>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-locks-per-transaction" xreflabel="max_locks_per_transaction">
      <term><varname>max_locks_per_transaction</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_locks_per_transaction</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        The shared lock table tracks locks on
        <varname>max_locks_per_transaction</varname> * (<xref
        linkend="guc-max-connections"/> + <xref
        linkend="guc-max-prepared-transactions"/>) objects (e.g.,  tables);
        hence, no more than this many distinct objects can be locked at
        any one time.  This parameter controls the average number of object
        locks allocated for each transaction;  individual transactions
        can lock more objects as long as the locks of all transactions
        fit in the lock table.  This is <emphasis>not</emphasis> the number of
        rows that can be locked; that value is unlimited.  The default,
        64, has historically proven sufficient, but you might need to
        raise this value if you have queries that touch many different
        tables in a single transaction, e.g., query of a parent table with
        many children.  This parameter can only be set at server start.
       </para>

       <para>
        When running a standby server, you must set this parameter to the
        same or higher value than on the master server. Otherwise, queries
        will not be allowed in the standby server.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-pred-locks-per-transaction" xreflabel="max_pred_locks_per_transaction">
      <term><varname>max_pred_locks_per_transaction</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_pred_locks_per_transaction</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        The shared predicate lock table tracks locks on
        <varname>max_pred_locks_per_transaction</varname> * (<xref
        linkend="guc-max-connections"/> + <xref
        linkend="guc-max-prepared-transactions"/>) objects (e.g., tables);
        hence, no more than this many distinct objects can be locked at
        any one time.  This parameter controls the average number of object
        locks allocated for each transaction;  individual transactions
        can lock more objects as long as the locks of all transactions
        fit in the lock table.  This is <emphasis>not</emphasis> the number of
        rows that can be locked; that value is unlimited.  The default,
        64, has generally been sufficient in testing, but you might need to
        raise this value if you have clients that touch many different
        tables in a single serializable transaction. This parameter can
        only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-pred-locks-per-relation" xreflabel="max_pred_locks_per_relation">
      <term><varname>max_pred_locks_per_relation</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_pred_locks_per_relation</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This controls how many pages or tuples of a single relation can be
        predicate-locked before the lock is promoted to covering the whole
        relation.  Values greater than or equal to zero mean an absolute
        limit, while negative values
        mean <xref linkend="guc-max-pred-locks-per-transaction"/> divided by
        the absolute value of this setting.  The default is -2, which keeps
        the behavior from previous versions of <productname>PostgreSQL</productname>.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-pred-locks-per-page" xreflabel="max_pred_locks_per_page">
      <term><varname>max_pred_locks_per_page</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_pred_locks_per_page</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This controls how many rows on a single page can be predicate-locked
        before the lock is promoted to covering the whole page.  The default
        is 2.  This parameter can only be set in
        the <filename>postgresql.conf</filename> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
   </sect1>

   <sect1 id="runtime-config-compatible">
    <title>Version and Platform Compatibility</title>

    <sect2 id="runtime-config-compatible-version">
     <title>Previous PostgreSQL Versions</title>

     <variablelist>

     <varlistentry id="guc-array-nulls" xreflabel="array_nulls">
      <term><varname>array_nulls</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>array_nulls</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This controls whether the array input parser recognizes
        unquoted <literal>NULL</literal> as specifying a null array element.
        By default, this is <literal>on</literal>, allowing array values containing
        null values to be entered.  However, <productname>PostgreSQL</productname> versions
        before 8.2 did not support null values in arrays, and therefore would
        treat <literal>NULL</literal> as specifying a normal array element with
        the string value <quote>NULL</quote>.  For backward compatibility with
        applications that require the old behavior, this variable can be
        turned <literal>off</literal>.
       </para>

       <para>
        Note that it is possible to create array values containing null values
        even when this variable is <literal>off</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-backslash-quote" xreflabel="backslash_quote">
      <term><varname>backslash_quote</varname> (<type>enum</type>)
      <indexterm><primary>strings</primary><secondary>backslash quotes</secondary></indexterm>
      <indexterm>
       <primary><varname>backslash_quote</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This controls whether a quote mark can be represented by
        <literal>\'</literal> in a string literal.  The preferred, SQL-standard way
        to represent a quote mark is by doubling it (<literal>''</literal>) but
        <productname>PostgreSQL</productname> has historically also accepted
        <literal>\'</literal>. However, use of <literal>\'</literal> creates security risks
        because in some client character set encodings, there are multibyte
        characters in which the last byte is numerically equivalent to ASCII
        <literal>\</literal>.  If client-side code does escaping incorrectly then a
        SQL-injection attack is possible.  This risk can be prevented by
        making the server reject queries in which a quote mark appears to be
        escaped by a backslash.
        The allowed values of <varname>backslash_quote</varname> are
        <literal>on</literal> (allow <literal>\'</literal> always),
        <literal>off</literal> (reject always), and
        <literal>safe_encoding</literal> (allow only if client encoding does not
        allow ASCII <literal>\</literal> within a multibyte character).
        <literal>safe_encoding</literal> is the default setting.
       </para>

       <para>
        Note that in a standard-conforming string literal, <literal>\</literal> just
        means <literal>\</literal> anyway.  This parameter only affects the handling of
        non-standard-conforming literals, including
        escape string syntax (<literal>E'...'</literal>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-with-oids" xreflabel="default_with_oids">
      <term><varname>default_with_oids</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>default_with_oids</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This controls whether <command>CREATE TABLE</command> and
        <command>CREATE TABLE AS</command> include an OID column in
        newly-created tables, if neither <literal>WITH OIDS</literal>
        nor <literal>WITHOUT OIDS</literal> is specified. It also
        determines whether OIDs will be included in tables created by
        <command>SELECT INTO</command>. The parameter is <literal>off</literal>
        by default; in <productname>PostgreSQL</productname> 8.0 and earlier, it
        was <literal>on</literal> by default.
       </para>

       <para>
        The use of OIDs in user tables is considered deprecated, so
        most installations should leave this variable disabled.
        Applications that require OIDs for a particular table should
        specify <literal>WITH OIDS</literal> when creating the
        table. This variable can be enabled for compatibility with old
        applications that do not follow this behavior.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-escape-string-warning" xreflabel="escape_string_warning">
      <term><varname>escape_string_warning</varname> (<type>boolean</type>)
      <indexterm><primary>strings</primary><secondary>escape warning</secondary></indexterm>
      <indexterm>
       <primary><varname>escape_string_warning</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When on, a warning is issued if a backslash (<literal>\</literal>)
        appears in an ordinary string literal (<literal>'...'</literal>
        syntax) and <varname>standard_conforming_strings</varname> is off.
        The default is <literal>on</literal>.
       </para>
       <para>
        Applications that wish to use backslash as escape should be
        modified to use escape string syntax (<literal>E'...'</literal>),
        because the default behavior of ordinary strings is now to treat
        backslash as an ordinary character, per SQL standard.  This variable
        can be enabled to help locate code that needs to be changed.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lo-compat-privileges" xreflabel="lo_compat_privileges">
      <term><varname>lo_compat_privileges</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>lo_compat_privileges</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        In <productname>PostgreSQL</productname> releases prior to 9.0, large objects
        did not have access privileges and were, therefore, always readable
        and writable by all users.  Setting this variable to <literal>on</literal>
        disables the new privilege checks, for compatibility with prior
        releases.  The default is <literal>off</literal>.
        Only superusers can change this setting.
       </para>
       <para>
        Setting this variable does not disable all security checks related to
        large objects &mdash; only those for which the default behavior has
        changed in <productname>PostgreSQL</productname> 9.0.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-operator-precedence-warning" xreflabel="operator_precedence_warning">
      <term><varname>operator_precedence_warning</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>operator_precedence_warning</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When on, the parser will emit a warning for any construct that might
        have changed meanings since <productname>PostgreSQL</productname> 9.4 as a result
        of changes in operator precedence.  This is useful for auditing
        applications to see if precedence changes have broken anything; but it
        is not meant to be kept turned on in production, since it will warn
        about some perfectly valid, standard-compliant SQL code.
        The default is <literal>off</literal>.
       </para>

       <para>
        See <xref linkend="sql-precedence"/> for more information.
       </para>
      </listitem>
     </varlistentry>

    <varlistentry id="guc-quote-all-identifiers" xreflabel="quote-all-identifiers">
      <term><varname>quote_all_identifiers</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>quote_all_identifiers</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When the database generates SQL, force all identifiers to be quoted,
        even if they are not (currently) keywords.  This will affect the
        output of <command>EXPLAIN</command> as well as the results of functions
        like <function>pg_get_viewdef</function>.  See also the
        <option>--quote-all-identifiers</option> option of
        <xref linkend="app-pgdump"/> and <xref linkend="app-pg-dumpall"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-standard-conforming-strings" xreflabel="standard_conforming_strings">
      <term><varname>standard_conforming_strings</varname> (<type>boolean</type>)
      <indexterm><primary>strings</primary><secondary>standard conforming</secondary></indexterm>
      <indexterm>
       <primary><varname>standard_conforming_strings</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This controls whether ordinary string literals
        (<literal>'...'</literal>) treat backslashes literally, as specified in
        the SQL standard.
        Beginning in <productname>PostgreSQL</productname> 9.1, the default is
        <literal>on</literal> (prior releases defaulted to <literal>off</literal>).
        Applications can check this
        parameter to determine how string literals will be processed.
        The presence of this parameter can also be taken as an indication
        that the escape string syntax (<literal>E'...'</literal>) is supported.
        Escape string syntax (<xref linkend="sql-syntax-strings-escape"/>)
        should be used if an application desires
        backslashes to be treated as escape characters.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-synchronize-seqscans" xreflabel="synchronize_seqscans">
      <term><varname>synchronize_seqscans</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>synchronize_seqscans</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This allows sequential scans of large tables to synchronize with each
        other, so that concurrent scans read the same block at about the
        same time and hence share the I/O workload.  When this is enabled,
        a scan might start in the middle of the table and then <quote>wrap
        around</quote> the end to cover all rows, so as to synchronize with the
        activity of scans already in progress.  This can result in
        unpredictable changes in the row ordering returned by queries that
        have no <literal>ORDER BY</literal> clause.  Setting this parameter to
        <literal>off</literal> ensures the pre-8.3 behavior in which a sequential
        scan always starts from the beginning of the table.  The default
        is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

    <sect2 id="runtime-config-compatible-clients">
     <title>Platform and Client Compatibility</title>
     <variablelist>

     <varlistentry id="guc-transform-null-equals" xreflabel="transform_null_equals">
      <term><varname>transform_null_equals</varname> (<type>boolean</type>)
      <indexterm><primary>IS NULL</primary></indexterm>
      <indexterm>
       <primary><varname>transform_null_equals</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When on, expressions of the form <literal><replaceable>expr</replaceable> =
        NULL</literal> (or <literal>NULL =
        <replaceable>expr</replaceable></literal>) are treated as
        <literal><replaceable>expr</replaceable> IS NULL</literal>, that is, they
        return true if <replaceable>expr</replaceable> evaluates to the null value,
        and false otherwise. The correct SQL-spec-compliant behavior of
        <literal><replaceable>expr</replaceable> = NULL</literal> is to always
        return null (unknown). Therefore this parameter defaults to
        <literal>off</literal>.
       </para>

       <para>
        However, filtered forms in <productname>Microsoft
        Access</productname> generate queries that appear to use
        <literal><replaceable>expr</replaceable> = NULL</literal> to test for
        null values, so if you use that interface to access the database you
        might want to turn this option on.  Since expressions of the
        form <literal><replaceable>expr</replaceable> = NULL</literal> always
        return the null value (using the SQL standard interpretation), they are not
        very useful and do not appear often in normal applications so
        this option does little harm in practice.  But new users are
        frequently confused about the semantics of expressions
        involving null values, so this option is off by default.
       </para>

       <para>
        Note that this option only affects the exact form <literal>= NULL</literal>,
        not other comparison operators or other expressions
        that are computationally equivalent to some expression
        involving the equals operator (such as <literal>IN</literal>).
        Thus, this option is not a general fix for bad programming.
       </para>

       <para>
        Refer to <xref linkend="functions-comparison"/> for related information.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-error-handling">
    <title>Error Handling</title>

    <variablelist>

     <varlistentry id="guc-exit-on-error" xreflabel="exit_on_error">
      <term><varname>exit_on_error</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>exit_on_error</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If true, any error will terminate the current session.  By default,
        this is set to false, so that only FATAL errors will terminate the
        session.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-restart-after-crash" xreflabel="restart_after_crash">
      <term><varname>restart_after_crash</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>restart_after_crash</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When set to true, which is the default, <productname>PostgreSQL</productname>
        will automatically reinitialize after a backend crash.  Leaving this
        value set to true is normally the best way to maximize the availability
        of the database.  However, in some circumstances, such as when
        <productname>PostgreSQL</productname> is being invoked by clusterware, it may be
        useful to disable the restart so that the clusterware can gain
        control and take any actions it deems appropriate.
       </para>

       <para>
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-data-sync-retry" xreflabel="data_sync_retry">
      <term><varname>data_sync_retry</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>data_sync_retry</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        When set to false, which is the default, <productname>PostgreSQL</productname>
        will raise a PANIC-level error on failure to flush modified data files
        to the filesystem.  This causes the database server to crash.  This
        parameter can only be set at server start.
       </para>
       <para>
        On some operating systems, the status of data in the kernel's page
        cache is unknown after a write-back failure.  In some cases it might
        have been entirely forgotten, making it unsafe to retry; the second
        attempt may be reported as successful, when in fact the data has been
        lost.  In these circumstances, the only way to avoid data loss is to
        recover from the WAL after any failure is reported, preferably
        after investigating the root cause of the failure and replacing any
        faulty hardware.
       </para>
       <para>
        If set to true, <productname>PostgreSQL</productname> will instead
        report an error but continue to run so that the data flushing
        operation can be retried in a later checkpoint.  Only set it to true
        after investigating the operating system's treatment of buffered data
        in case of write-back failure.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>

   </sect1>

   <sect1 id="runtime-config-preset">
    <title>Preset Options</title>

    <para>
     The following <quote>parameters</quote> are read-only, and are determined
     when <productname>PostgreSQL</productname> is compiled or when it is
     installed. As such, they have been excluded from the sample
     <filename>postgresql.conf</filename> file.  These options report
     various aspects of <productname>PostgreSQL</productname> behavior
     that might be of interest to certain applications, particularly
     administrative front-ends.
    </para>

    <variablelist>

     <varlistentry id="guc-block-size" xreflabel="block_size">
      <term><varname>block_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>block_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the size of a disk block.  It is determined by the value
        of <literal>BLCKSZ</literal> when building the server. The default
        value is 8192 bytes.  The meaning of some configuration
        variables (such as <xref linkend="guc-shared-buffers"/>) is
        influenced by <varname>block_size</varname>. See <xref
        linkend="runtime-config-resource"/> for information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-data-checksums" xreflabel="data_checksums">
      <term><varname>data_checksums</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>data_checksums</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports whether data checksums are enabled for this cluster.
        See <xref linkend="app-initdb-data-checksums"/> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-data-directory-mode" xreflabel="data_directory_mode">
      <term><varname>data_directory_mode</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>data_directory_mode</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        On Unix systems this parameter reports the permissions of the data
        directory defined by (<xref linkend="guc-data-directory"/>) at startup.
        (On Microsoft Windows this parameter will always display
        <literal>0700</literal>). See
        <xref linkend="app-initdb-allow-group-access"/> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-debug-assertions" xreflabel="debug_assertions">
      <term><varname>debug_assertions</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>debug_assertions</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports whether <productname>PostgreSQL</productname> has been built
        with assertions enabled. That is the case if the
        macro <symbol>USE_ASSERT_CHECKING</symbol> is defined
        when <productname>PostgreSQL</productname> is built (accomplished
        e.g., by the <command>configure</command> option
        <option>--enable-cassert</option>). By
        default <productname>PostgreSQL</productname> is built without
        assertions.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
      <term><varname>integer_datetimes</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>integer_datetimes</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports whether <productname>PostgreSQL</productname> was built with support for
        64-bit-integer dates and times.  As of <productname>PostgreSQL</productname> 10,
        this is always <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-collate" xreflabel="lc_collate">
      <term><varname>lc_collate</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>lc_collate</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the locale in which sorting of textual data is done.
        See <xref linkend="locale"/> for more information.
        This value is determined when a database is created.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-ctype" xreflabel="lc_ctype">
      <term><varname>lc_ctype</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>lc_ctype</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the locale that determines character classifications.
        See <xref linkend="locale"/> for more information.
        This value is determined when a database is created.
        Ordinarily this will be the same as <varname>lc_collate</varname>,
        but for special applications it might be set differently.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-function-args" xreflabel="max_function_args">
      <term><varname>max_function_args</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_function_args</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the maximum number of function arguments. It is determined by
        the value of <literal>FUNC_MAX_ARGS</literal> when building the server. The
        default value is 100 arguments.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-identifier-length" xreflabel="max_identifier_length">
      <term><varname>max_identifier_length</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_identifier_length</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the maximum identifier length. It is determined as one
        less than the value of <literal>NAMEDATALEN</literal> when building
        the server. The default value of <literal>NAMEDATALEN</literal> is
        64; therefore the default
        <varname>max_identifier_length</varname> is 63 bytes, which
        can be less than 63 characters when using multibyte encodings.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-index-keys" xreflabel="max_index_keys">
      <term><varname>max_index_keys</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>max_index_keys</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the maximum number of index keys. It is determined by
        the value of <literal>INDEX_MAX_KEYS</literal> when building the server. The
        default value is 32 keys.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-segment-size" xreflabel="segment_size">
      <term><varname>segment_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>segment_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the number of blocks (pages) that can be stored within a file
        segment.  It is determined by the value of <literal>RELSEG_SIZE</literal>
        when building the server.  The maximum size of a segment file in bytes
        is equal to <varname>segment_size</varname> multiplied by
        <varname>block_size</varname>; by default this is 1GB.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-server-encoding" xreflabel="server_encoding">
      <term><varname>server_encoding</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>server_encoding</varname> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>character set</primary></indexterm>
      </term>
      <listitem>
       <para>
        Reports the database encoding (character set).
        It is determined when the database is created.  Ordinarily,
        clients need only be concerned with the value of <xref
        linkend="guc-client-encoding"/>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-server-version" xreflabel="server_version">
      <term><varname>server_version</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>server_version</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the version number of the server. It is determined by the
        value of <literal>PG_VERSION</literal> when building the server.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-server-version-num" xreflabel="server_version_num">
      <term><varname>server_version_num</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>server_version_num</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the version number of the server as an integer. It is determined
        by the value of <literal>PG_VERSION_NUM</literal> when building the server.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-block-size" xreflabel="wal_block_size">
      <term><varname>wal_block_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>wal_block_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the size of a WAL disk block.  It is determined by the value
        of <literal>XLOG_BLCKSZ</literal> when building the server. The default value
        is 8192 bytes.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-segment-size" xreflabel="wal_segment_size">
      <term><varname>wal_segment_size</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>wal_segment_size</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Reports the size of write ahead log segments.  The default value is
        16MB. See <xref linkend="wal-configuration"/> for more information.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </sect1>

   <sect1 id="runtime-config-custom">
    <title>Customized Options</title>

    <para>
     This feature was designed to allow parameters not normally known to
     <productname>PostgreSQL</productname> to be added by add-on modules
     (such as procedural languages).  This allows extension modules to be
     configured in the standard ways.
    </para>

    <para>
     Custom options have two-part names: an extension name, then a dot, then
     the parameter name proper, much like qualified names in SQL.  An example
     is <literal>plpgsql.variable_conflict</literal>.
    </para>

    <para>
     Because custom options may need to be set in processes that have not
     loaded the relevant extension module, <productname>PostgreSQL</productname>
     will accept a setting for any two-part parameter name.  Such variables
     are treated as placeholders and have no function until the module that
     defines them is loaded. When an extension module is loaded, it will add
     its variable definitions, convert any placeholder values according to
     those definitions, and issue warnings for any unrecognized placeholders
     that begin with its extension name.
    </para>
   </sect1>

   <sect1 id="runtime-config-developer">
    <title>Developer Options</title>

    <para>
     The following parameters are intended for work on the
     <productname>PostgreSQL</productname> source code, and in some cases
     to assist with recovery of severely damaged databases.  There
     should be no reason to use them on a production database.
     As such, they have been excluded from the sample
     <filename>postgresql.conf</filename> file.  Note that many of these
     parameters require special source compilation flags to work at all.
    </para>

    <variablelist>
     <varlistentry id="guc-allow-in-place-tablespaces" xreflabel="allow_in_place_tablespaces">
      <term><varname>allow_in_place_tablespaces</varname> (<type>boolean</type>)
      <indexterm>
        <primary><varname>allow_in_place_tablespaces</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Allows tablespaces to be created as directories inside
        <filename>pg_tblspc</filename>, when an empty location string
        is provided to the <command>CREATE TABLESPACE</command> command.  This
        is intended to allow testing replication scenarios where primary and
        standby servers are running on the same machine.  Such directories
        are likely to confuse backup tools that expect to find only symbolic
        links in that location.  Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-allow-system-table-mods" xreflabel="allow_system_table_mods">
      <term><varname>allow_system_table_mods</varname> (<type>boolean</type>)
      <indexterm>
        <primary><varname>allow_system_table_mods</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Allows modification of the structure of system tables.
        This is used by <command>initdb</command>.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ignore-system-indexes" xreflabel="ignore_system_indexes">
      <term><varname>ignore_system_indexes</varname> (<type>boolean</type>)
      <indexterm>
        <primary><varname>ignore_system_indexes</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Ignore system indexes when reading system tables (but still
        update the indexes when modifying the tables).  This is useful
        when recovering from damaged system indexes.
        This parameter cannot be changed after session start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-post-auth-delay" xreflabel="post_auth_delay">
      <term><varname>post_auth_delay</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>post_auth_delay</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If nonzero, a delay of this many seconds occurs when a new
        server process is started, after it conducts the
        authentication procedure.  This is intended to give developers an
        opportunity to attach to the server process with a debugger.
        This parameter cannot be changed after session start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-pre-auth-delay" xreflabel="pre_auth_delay">
      <term><varname>pre_auth_delay</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>pre_auth_delay</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If nonzero, a delay of this many seconds occurs just after a
        new server process is forked, before it conducts the
        authentication procedure.  This is intended to give developers an
        opportunity to attach to the server process with a debugger to
        trace down misbehavior in authentication.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-trace-notify" xreflabel="trace_notify">
      <term><varname>trace_notify</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>trace_notify</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Generates a great amount of debugging output for the
        <command>LISTEN</command> and <command>NOTIFY</command>
        commands.  <xref linkend="guc-client-min-messages"/> or
        <xref linkend="guc-log-min-messages"/> must be
        <literal>DEBUG1</literal> or lower to send this output to the
        client or server logs, respectively.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-trace-recovery-messages" xreflabel="trace_recovery_messages">
      <term><varname>trace_recovery_messages</varname> (<type>enum</type>)
      <indexterm>
       <primary><varname>trace_recovery_messages</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Enables logging of recovery-related debugging output that otherwise
        would not be logged. This parameter allows the user to override the
        normal setting of <xref linkend="guc-log-min-messages"/>, but only for
        specific messages. This is intended for use in debugging Hot Standby.
        Valid values are <literal>DEBUG5</literal>, <literal>DEBUG4</literal>,
        <literal>DEBUG3</literal>, <literal>DEBUG2</literal>, <literal>DEBUG1</literal>, and
        <literal>LOG</literal>.  The default, <literal>LOG</literal>, does not affect
        logging decisions at all.  The other values cause recovery-related
        debug messages of that priority or higher to be logged as though they
        had <literal>LOG</literal> priority; for common settings of
        <varname>log_min_messages</varname> this results in unconditionally sending
        them to the server log.
        This parameter can only be set in the <filename>postgresql.conf</filename>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-trace-sort" xreflabel="trace_sort">
      <term><varname>trace_sort</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>trace_sort</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If on, emit information about resource usage during sort operations.
        This parameter is only available if the <symbol>TRACE_SORT</symbol> macro
        was defined when <productname>PostgreSQL</productname> was compiled.
        (However, <symbol>TRACE_SORT</symbol> is currently defined by default.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>trace_locks</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>trace_locks</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If on, emit information about lock usage.  Information dumped
        includes the type of lock operation, the type of lock and the unique
        identifier of the object being locked or unlocked.  Also included
        are bit masks for the lock types already granted on this object as
        well as for the lock types awaited on this object.  For each lock
        type a count of the number of granted locks and waiting locks is
        also dumped as well as the totals.  An example of the log file output
        is shown here:
<screen>
LOG:  LockAcquire: new: lock(0xb7acd844) id(24688,24696,0,0,0,1)
      grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0
      wait(0) type(AccessShareLock)
LOG:  GrantLock: lock(0xb7acd844) id(24688,24696,0,0,0,1)
      grantMask(2) req(1,0,0,0,0,0,0)=1 grant(1,0,0,0,0,0,0)=1
      wait(0) type(AccessShareLock)
LOG:  UnGrantLock: updated: lock(0xb7acd844) id(24688,24696,0,0,0,1)
      grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0
      wait(0) type(AccessShareLock)
LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
      grantMask(0) req(0,0,0,0,0,0,0)=0 grant(0,0,0,0,0,0,0)=0
      wait(0) type(INVALID)
</screen>
        Details of the structure being dumped may be found in
        <filename>src/include/storage/lock.h</filename>.
       </para>
       <para>
        This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
        macro was defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>trace_lwlocks</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>trace_lwlocks</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If on, emit information about lightweight lock usage.  Lightweight
        locks are intended primarily to provide mutual exclusion of access
        to shared-memory data structures.
       </para>
       <para>
        This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
        macro was defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>trace_userlocks</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>trace_userlocks</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If on, emit information about user lock usage.  Output is the same
        as for <symbol>trace_locks</symbol>, only for advisory locks.
       </para>
       <para>
        This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
        macro was defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>trace_lock_oidmin</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>trace_lock_oidmin</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If set, do not trace locks for tables below this OID (used to avoid
        output on system tables).
       </para>
       <para>
        This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
        macro was defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>trace_lock_table</varname> (<type>integer</type>)
      <indexterm>
       <primary><varname>trace_lock_table</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Unconditionally trace locks on this table (OID).
       </para>
       <para>
        This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
        macro was defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>debug_deadlocks</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>debug_deadlocks</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If set, dumps information about all current locks when a
        deadlock timeout occurs.
       </para>
       <para>
        This parameter is only available if the <symbol>LOCK_DEBUG</symbol>
        macro was defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>log_btree_build_stats</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>log_btree_build_stats</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If set, logs system resource usage statistics (memory and CPU) on
        various B-tree operations.
       </para>
       <para>
        This parameter is only available if the <symbol>BTREE_BUILD_STATS</symbol>
        macro was defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-consistency-checking" xreflabel="wal_consistency_checking">
      <term><varname>wal_consistency_checking</varname> (<type>string</type>)
      <indexterm>
       <primary><varname>wal_consistency_checking</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        This parameter is intended to be used to check for bugs in the WAL
        redo routines.  When enabled, full-page images of any buffers modified
        in conjunction with the WAL record are added to the record.
        If the record is subsequently replayed, the system will first apply
        each record and then test whether the buffers modified by the record
        match the stored images.  In certain cases (such as hint bits), minor
        variations are acceptable, and will be ignored.  Any unexpected
        differences will result in a fatal error, terminating recovery.
       </para>

       <para>
        The default value of this setting is the empty string, which disables
        the feature.  It can be set to <literal>all</literal> to check all
        records, or to a comma-separated list of resource managers to check
        only records originating from those resource managers.  Currently,
        the supported resource managers are <literal>heap</literal>,
        <literal>heap2</literal>, <literal>btree</literal>, <literal>hash</literal>,
        <literal>gin</literal>, <literal>gist</literal>, <literal>sequence</literal>,
        <literal>spgist</literal>, <literal>brin</literal>, and <literal>generic</literal>. Only
        superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-debug" xreflabel="wal_debug">
      <term><varname>wal_debug</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>wal_debug</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If on, emit WAL-related debugging output. This parameter is
        only available if the <symbol>WAL_DEBUG</symbol> macro was
        defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

    <varlistentry id="guc-ignore-checksum-failure" xreflabel="ignore_checksum_failure">
      <term><varname>ignore_checksum_failure</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>ignore_checksum_failure</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Only has effect if <xref linkend="app-initdb-data-checksums"/> are enabled.
       </para>
       <para>
        Detection of a checksum failure during a read normally causes
        <productname>PostgreSQL</productname> to report an error, aborting the current
        transaction.  Setting <varname>ignore_checksum_failure</varname> to on causes
        the system to ignore the failure (but still report a warning), and
        continue processing.  This behavior may <emphasis>cause crashes, propagate
        or hide corruption, or other serious problems</emphasis>.  However, it may allow
        you to get past the error and retrieve undamaged tuples that might still be
        present in the table if the block header is still sane. If the header is
        corrupt an error will be reported even if this option is enabled. The
        default setting is <literal>off</literal>, and it can only be changed by a superuser.
       </para>
      </listitem>
     </varlistentry>

    <varlistentry id="guc-zero-damaged-pages" xreflabel="zero_damaged_pages">
      <term><varname>zero_damaged_pages</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>zero_damaged_pages</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Detection of a damaged page header normally causes
        <productname>PostgreSQL</productname> to report an error, aborting the current
        transaction.  Setting <varname>zero_damaged_pages</varname> to on causes
        the system to instead report a warning, zero out the damaged
        page in memory, and continue processing.  This behavior <emphasis>will destroy data</emphasis>,
        namely all the rows on the damaged page.  However, it does allow you to get
        past the error and retrieve rows from any undamaged pages that might
        be present in the table.  It is useful for recovering data if
        corruption has occurred due to a hardware or software error.  You should
        generally not set this on until you have given up hope of recovering
        data from the damaged pages of a table.  Zeroed-out pages are not
        forced to disk so it is recommended to recreate the table or
        the index before turning this parameter off again.  The
        default setting is <literal>off</literal>, and it can only be changed
        by a superuser.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit-debugging-support" xreflabel="jit_debugging_support">
      <term><varname>jit_debugging_support</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>jit_debugging_support</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If LLVM has the required functionality, register generated functions
        with <productname>GDB</productname>.  This makes debugging easier.
        The default setting is <literal>off</literal>.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit-dump-bitcode" xreflabel="jit_dump_bitcode">
      <term><varname>jit_dump_bitcode</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>jit_dump_bitcode</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Writes the generated <productname>LLVM</productname> IR out to the
        file system, inside <xref linkend="guc-data-directory"/>. This is only
        useful for working on the internals of the JIT implementation.
        The default setting is <literal>off</literal>.
        This parameter can only be changed by a superuser.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit-expressions" xreflabel="jit_expressions">
      <term><varname>jit_expressions</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>jit_expressions</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Determines whether expressions are JIT compiled, when JIT compilation
        is activated (see <xref linkend="jit-decision"/>).  The default is
        <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit-profiling-support" xreflabel="jit_profiling_support">
      <term><varname>jit_profiling_support</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>jit_profiling_support</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        If LLVM has the required functionality, emit the data needed to allow
        <productname>perf</productname> to profile functions generated by JIT.
        This writes out files to <filename>$HOME/.debug/jit/</filename>; the
        user is responsible for performing cleanup when desired.
        The default setting is <literal>off</literal>.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-jit-tuple-deforming" xreflabel="jit_tuple_deforming">
      <term><varname>jit_tuple_deforming</varname> (<type>boolean</type>)
      <indexterm>
       <primary><varname>jit_tuple_deforming</varname> configuration parameter</primary>
      </indexterm>
      </term>
      <listitem>
       <para>
        Determines whether tuple deforming is JIT compiled, when JIT
        compilation is activated (see <xref linkend="jit-decision"/>).
        The default is <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
  </sect1>
  <sect1 id="runtime-config-short">
   <title>Short Options</title>

   <para>
    For convenience there are also single letter command-line option
    switches available for some parameters.  They are described in
    <xref linkend="runtime-config-short-table"/>.  Some of these
    options exist for historical reasons, and their presence as a
    single-letter option does not necessarily indicate an endorsement
    to use the option heavily.
   </para>

    <table id="runtime-config-short-table">
     <title>Short Option Key</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Short Option</entry>
        <entry>Equivalent</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><option>-B <replaceable>x</replaceable></option></entry>
        <entry><literal>shared_buffers = <replaceable>x</replaceable></literal></entry>
       </row>
       <row>
        <entry><option>-d <replaceable>x</replaceable></option></entry>
        <entry><literal>log_min_messages = DEBUG<replaceable>x</replaceable></literal></entry>
       </row>
       <row>
        <entry><option>-e</option></entry>
        <entry><literal>datestyle = euro</literal></entry>
       </row>
       <row>
        <entry>
          <option>-fb</option>, <option>-fh</option>, <option>-fi</option>,
          <option>-fm</option>, <option>-fn</option>, <option>-fo</option>,
          <option>-fs</option>, <option>-ft</option>
         </entry>
         <entry>
          <literal>enable_bitmapscan = off</literal>,
          <literal>enable_hashjoin = off</literal>,
          <literal>enable_indexscan = off</literal>,
          <literal>enable_mergejoin = off</literal>,
          <literal>enable_nestloop = off</literal>,
          <literal>enable_indexonlyscan = off</literal>,
          <literal>enable_seqscan = off</literal>,
          <literal>enable_tidscan = off</literal>
         </entry>
       </row>
       <row>
        <entry><option>-F</option></entry>
        <entry><literal>fsync = off</literal></entry>
       </row>
       <row>
        <entry><option>-h <replaceable>x</replaceable></option></entry>
        <entry><literal>listen_addresses = <replaceable>x</replaceable></literal></entry>
       </row>
       <row>
        <entry><option>-i</option></entry>
        <entry><literal>listen_addresses = '*'</literal></entry>
       </row>
       <row>
        <entry><option>-k <replaceable>x</replaceable></option></entry>
        <entry><literal>unix_socket_directories = <replaceable>x</replaceable></literal></entry>
       </row>
       <row>
        <entry><option>-l</option></entry>
        <entry><literal>ssl = on</literal></entry>
       </row>
       <row>
        <entry><option>-N <replaceable>x</replaceable></option></entry>
        <entry><literal>max_connections = <replaceable>x</replaceable></literal></entry>
       </row>
       <row>
        <entry><option>-O</option></entry>
        <entry><literal>allow_system_table_mods = on</literal></entry>
       </row>
       <row>
        <entry><option>-p <replaceable>x</replaceable></option></entry>
        <entry><literal>port = <replaceable>x</replaceable></literal></entry>
       </row>
       <row>
        <entry><option>-P</option></entry>
        <entry><literal>ignore_system_indexes = on</literal></entry>
       </row>
       <row>
        <entry><option>-s</option></entry>
        <entry><literal>log_statement_stats = on</literal></entry>
       </row>
       <row>
        <entry><option>-S <replaceable>x</replaceable></option></entry>
        <entry><literal>work_mem = <replaceable>x</replaceable></literal></entry>
       </row>
       <row>
        <entry><option>-tpa</option>, <option>-tpl</option>, <option>-te</option></entry>
        <entry><literal>log_parser_stats = on</literal>,
        <literal>log_planner_stats = on</literal>,
        <literal>log_executor_stats = on</literal></entry>
       </row>
       <row>
        <entry><option>-W <replaceable>x</replaceable></option></entry>
        <entry><literal>post_auth_delay = <replaceable>x</replaceable></literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  </sect1>
</chapter>
