/*-------------------------------------------------------------------------
 *
 * nbtsort.c
 *		Build a btree from sorted input by loading leaf pages sequentially.
 *
 * NOTES
 *
 * We use tuplesort.c to sort the given index tuples into order.
 * Then we scan the index tuples in order and build the btree pages
 * for each level.  We load source tuples into leaf-level pages.
 * Whenever we fill a page at one level, we add a link to it to its
 * parent level (starting a new parent level if necessary).  When
 * done, we write out each final page on each level, adding it to
 * its parent level.  When we have only one page on a level, it must be
 * the root -- it can be attached to the btree metapage and we are done.
 *
 * It is not wise to pack the pages entirely full, since then *any*
 * insertion would cause a split (and not only of the leaf page; the need
 * for a split would cascade right up the tree).  The steady-state load
 * factor for btrees is usually estimated at 70%.  We choose to pack leaf
 * pages to the user-controllable fill factor (default 90%) while upper pages
 * are always packed to 70%.  This gives us reasonable density (there aren't
 * many upper pages if the keys are reasonable-size) without risking a lot of
 * cascading splits during early insertions.
 *
 * Formerly the index pages being built were kept in shared buffers, but
 * that is of no value (since other backends have no interest in them yet)
 * and it created locking problems for CHECKPOINT, because the upper-level
 * pages were held exclusive-locked for long periods.  Now we just build
 * the pages in local memory and smgrwrite or smgrextend them as we finish
 * them.  They will need to be re-read into shared buffers on first use after
 * the build finishes.
 *
 * Since the index will never be used unless it is completely built,
 * from a crash-recovery point of view there is no need to WAL-log the
 * steps of the build.  After completing the index build, we can just sync
 * the whole file to disk using smgrimmedsync() before exiting this module.
 * This can be seen to be sufficient for crash recovery by considering that
 * it's effectively equivalent to what would happen if a CHECKPOINT occurred
 * just after the index build.  However, it is clearly not sufficient if the
 * DBA is using the WAL log for PITR or replication purposes, since another
 * machine would not be able to reconstruct the index from WAL.  Therefore,
 * we log the completed index pages to WAL if and only if WAL archiving is
 * active.
 *
 * This code isn't concerned about the FSM at all. The caller is responsible
 * for initializing that.
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *	  src/backend/access/nbtree/nbtsort.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/nbtree.h"
#include "access/parallel.h"
#include "access/relscan.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "catalog/index.h"
#include "executor/instrument.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/smgr.h"
#include "tcop/tcopprot.h"		/* pgrminclude ignore */
#include "utils/rel.h"
#include "utils/sortsupport.h"
#include "utils/tuplesort.h"


/* Magic numbers for parallel state sharing */
#define PARALLEL_KEY_BTREE_SHARED		UINT64CONST(0xA000000000000001)
#define PARALLEL_KEY_TUPLESORT			UINT64CONST(0xA000000000000002)
#define PARALLEL_KEY_TUPLESORT_SPOOL2	UINT64CONST(0xA000000000000003)
#define PARALLEL_KEY_QUERY_TEXT			UINT64CONST(0xA000000000000004)
#define PARALLEL_KEY_BUFFER_USAGE		UINT64CONST(0xA000000000000005)

/*
 * DISABLE_LEADER_PARTICIPATION disables the leader's participation in
 * parallel index builds.  This may be useful as a debugging aid.
#undef DISABLE_LEADER_PARTICIPATION
 */

/*
 * Status record for spooling/sorting phase.  (Note we may have two of
 * these due to the special requirements for uniqueness-checking with
 * dead tuples.)
 */
typedef struct BTSpool
{
	Tuplesortstate *sortstate;	/* state data for tuplesort.c */
	Relation	heap;
	Relation	index;
	bool		isunique;
} BTSpool;

/*
 * Status for index builds performed in parallel.  This is allocated in a
 * dynamic shared memory segment.  Note that there is a separate tuplesort TOC
 * entry, private to tuplesort.c but allocated by this module on its behalf.
 */
typedef struct BTShared
{
	/*
	 * These fields are not modified during the sort.  They primarily exist
	 * for the benefit of worker processes that need to create BTSpool state
	 * corresponding to that used by the leader.
	 */
	Oid			heaprelid;
	Oid			indexrelid;
	bool		isunique;
	bool		isconcurrent;
	int			scantuplesortstates;

	/*
	 * workersdonecv is used to monitor the progress of workers.  All parallel
	 * participants must indicate that they are done before leader can use
	 * mutable state that workers maintain during scan (and before leader can
	 * proceed to tuplesort_performsort()).
	 */
	ConditionVariable workersdonecv;

	/*
	 * mutex protects all fields before heapdesc.
	 *
	 * These fields contain status information of interest to B-Tree index
	 * builds that must work just the same when an index is built in parallel.
	 */
	slock_t		mutex;

	/*
	 * Mutable state that is maintained by workers, and reported back to
	 * leader at end of parallel scan.
	 *
	 * nparticipantsdone is number of worker processes finished.
	 *
	 * reltuples is the total number of input heap tuples.
	 *
	 * havedead indicates if RECENTLY_DEAD tuples were encountered during
	 * build.
	 *
	 * indtuples is the total number of tuples that made it into the index.
	 *
	 * brokenhotchain indicates if any worker detected a broken HOT chain
	 * during build.
	 */
	int			nparticipantsdone;
	double		reltuples;
	bool		havedead;
	double		indtuples;
	bool		brokenhotchain;

	/*
	 * This variable-sized field must come last.
	 *
	 * See _bt_parallel_estimate_shared().
	 */
	ParallelHeapScanDescData heapdesc;
} BTShared;

/*
 * Status for leader in parallel index build.
 */
typedef struct BTLeader
{
	/* parallel context itself */
	ParallelContext *pcxt;

	/*
	 * nparticipanttuplesorts is the exact number of worker processes
	 * successfully launched, plus one leader process if it participates as a
	 * worker (only DISABLE_LEADER_PARTICIPATION builds avoid leader
	 * participating as a worker).
	 */
	int			nparticipanttuplesorts;

	/*
	 * Leader process convenience pointers to shared state (leader avoids TOC
	 * lookups).
	 *
	 * btshared is the shared state for entire build.  sharedsort is the
	 * shared, tuplesort-managed state passed to each process tuplesort.
	 * sharedsort2 is the corresponding btspool2 shared state, used only when
	 * building unique indexes.  snapshot is the snapshot used by the scan iff
	 * an MVCC snapshot is required.
	 */
	BTShared   *btshared;
	Sharedsort *sharedsort;
	Sharedsort *sharedsort2;
	Snapshot	snapshot;
	BufferUsage *bufferusage;
} BTLeader;

/*
 * Working state for btbuild and its callback.
 *
 * When parallel CREATE INDEX is used, there is a BTBuildState for each
 * participant.
 */
typedef struct BTBuildState
{
	bool		isunique;
	bool		havedead;
	Relation	heap;
	BTSpool    *spool;

	/*
	 * spool2 is needed only when the index is a unique index. Dead tuples are
	 * put into spool2 instead of spool in order to avoid uniqueness check.
	 */
	BTSpool    *spool2;
	double		indtuples;

	/*
	 * btleader is only present when a parallel index build is performed, and
	 * only in the leader process. (Actually, only the leader has a
	 * BTBuildState.  Workers have their own spool and spool2, though.)
	 */
	BTLeader   *btleader;
} BTBuildState;

/*
 * Status record for a btree page being built.  We have one of these
 * for each active tree level.
 *
 * The reason we need to store a copy of the minimum key is that we'll
 * need to propagate it to the parent node when this page is linked
 * into its parent.  However, if the page is not a leaf page, the first
 * entry on the page doesn't need to contain a key, so we will not have
 * stored the key itself on the page.  (You might think we could skip
 * copying the minimum key on leaf pages, but actually we must have a
 * writable copy anyway because we'll poke the page's address into it
 * before passing it up to the parent...)
 */
typedef struct BTPageState
{
	Page		btps_page;		/* workspace for page building */
	BlockNumber btps_blkno;		/* block # to write this page at */
	IndexTuple	btps_minkey;	/* copy of minimum key (first item) on page */
	OffsetNumber btps_lastoff;	/* last item offset loaded */
	uint32		btps_level;		/* tree level (0 = leaf) */
	Size		btps_full;		/* "full" if less than this much free space */
	struct BTPageState *btps_next;	/* link to parent level, if any */
} BTPageState;

/*
 * Overall status record for index writing phase.
 */
typedef struct BTWriteState
{
	Relation	heap;
	Relation	index;
	bool		btws_use_wal;	/* dump pages to WAL? */
	BlockNumber btws_pages_alloced; /* # pages allocated */
	BlockNumber btws_pages_written; /* # pages written out */
	Page		btws_zeropage;	/* workspace for filling zeroes */
} BTWriteState;


static double _bt_spools_heapscan(Relation heap, Relation index,
					BTBuildState *buildstate, IndexInfo *indexInfo);
static void _bt_spooldestroy(BTSpool *btspool);
static void _bt_spool(BTSpool *btspool, ItemPointer self,
		  Datum *values, bool *isnull);
static void _bt_leafbuild(BTSpool *btspool, BTSpool *btspool2);
static void _bt_build_callback(Relation index, HeapTuple htup, Datum *values,
				   bool *isnull, bool tupleIsAlive, void *state);
static Page _bt_blnewpage(uint32 level);
static BTPageState *_bt_pagestate(BTWriteState *wstate, uint32 level);
static void _bt_slideleft(Page page);
static void _bt_sortaddtup(Page page, Size itemsize,
			   IndexTuple itup, OffsetNumber itup_off);
static void _bt_buildadd(BTWriteState *wstate, BTPageState *state,
			 IndexTuple itup);
static void _bt_uppershutdown(BTWriteState *wstate, BTPageState *state);
static void _bt_load(BTWriteState *wstate,
		 BTSpool *btspool, BTSpool *btspool2);
static void _bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent,
				   int request);
static void _bt_end_parallel(BTLeader *btleader);
static Size _bt_parallel_estimate_shared(Snapshot snapshot);
static double _bt_parallel_heapscan(BTBuildState *buildstate,
					  bool *brokenhotchain);
static void _bt_leader_participate_as_worker(BTBuildState *buildstate);
static void _bt_parallel_scan_and_sort(BTSpool *btspool, BTSpool *btspool2,
						   BTShared *btshared, Sharedsort *sharedsort,
						   Sharedsort *sharedsort2, int sortmem);


/*
 *	btbuild() -- build a new btree index.
 */
IndexBuildResult *
btbuild(Relation heap, Relation index, IndexInfo *indexInfo)
{
	IndexBuildResult *result;
	BTBuildState buildstate;
	double		reltuples;

#ifdef BTREE_BUILD_STATS
	if (log_btree_build_stats)
		ResetUsage();
#endif							/* BTREE_BUILD_STATS */

	buildstate.isunique = indexInfo->ii_Unique;
	buildstate.havedead = false;
	buildstate.heap = heap;
	buildstate.spool = NULL;
	buildstate.spool2 = NULL;
	buildstate.indtuples = 0;
	buildstate.btleader = NULL;

	/*
	 * We expect to be called exactly once for any index relation. If that's
	 * not the case, big trouble's what we have.
	 */
	if (RelationGetNumberOfBlocks(index) != 0)
		elog(ERROR, "index \"%s\" already contains data",
			 RelationGetRelationName(index));

	reltuples = _bt_spools_heapscan(heap, index, &buildstate, indexInfo);

	/*
	 * Finish the build by (1) completing the sort of the spool file, (2)
	 * inserting the sorted tuples into btree pages and (3) building the upper
	 * levels.  Finally, it may also be necessary to end use of parallelism.
	 */
	_bt_leafbuild(buildstate.spool, buildstate.spool2);
	_bt_spooldestroy(buildstate.spool);
	if (buildstate.spool2)
		_bt_spooldestroy(buildstate.spool2);
	if (buildstate.btleader)
		_bt_end_parallel(buildstate.btleader);

	result = (IndexBuildResult *) palloc(sizeof(IndexBuildResult));

	result->heap_tuples = reltuples;
	result->index_tuples = buildstate.indtuples;

#ifdef BTREE_BUILD_STATS
	if (log_btree_build_stats)
	{
		ShowUsage("BTREE BUILD STATS");
		ResetUsage();
	}
#endif							/* BTREE_BUILD_STATS */

	return result;
}

/*
 * Create and initialize one or two spool structures, and save them in caller's
 * buildstate argument.  May also fill-in fields within indexInfo used by index
 * builds.
 *
 * Scans the heap, possibly in parallel, filling spools with IndexTuples.  This
 * routine encapsulates all aspects of managing parallelism.  Caller need only
 * call _bt_end_parallel() in parallel case after it is done with spool/spool2.
 *
 * Returns the total number of heap tuples scanned.
 */
static double
_bt_spools_heapscan(Relation heap, Relation index, BTBuildState *buildstate,
					IndexInfo *indexInfo)
{
	BTSpool    *btspool = (BTSpool *) palloc0(sizeof(BTSpool));
	SortCoordinate coordinate = NULL;
	double		reltuples = 0;

	/*
	 * We size the sort area as maintenance_work_mem rather than work_mem to
	 * speed index creation.  This should be OK since a single backend can't
	 * run multiple index creations in parallel (see also: notes on
	 * parallelism and maintenance_work_mem below).
	 */
	btspool->heap = heap;
	btspool->index = index;
	btspool->isunique = indexInfo->ii_Unique;

	/* Save as primary spool */
	buildstate->spool = btspool;

	/* Attempt to launch parallel worker scan when required */
	if (indexInfo->ii_ParallelWorkers > 0)
		_bt_begin_parallel(buildstate, indexInfo->ii_Concurrent,
						   indexInfo->ii_ParallelWorkers);

	/*
	 * If parallel build requested and at least one worker process was
	 * successfully launched, set up coordination state
	 */
	if (buildstate->btleader)
	{
		coordinate = (SortCoordinate) palloc0(sizeof(SortCoordinateData));
		coordinate->isWorker = false;
		coordinate->nParticipants =
			buildstate->btleader->nparticipanttuplesorts;
		coordinate->sharedsort = buildstate->btleader->sharedsort;
	}

	/*
	 * Begin serial/leader tuplesort.
	 *
	 * In cases where parallelism is involved, the leader receives the same
	 * share of maintenance_work_mem as a serial sort (it is generally treated
	 * in the same way as a serial sort once we return).  Parallel worker
	 * Tuplesortstates will have received only a fraction of
	 * maintenance_work_mem, though.
	 *
	 * We rely on the lifetime of the Leader Tuplesortstate almost not
	 * overlapping with any worker Tuplesortstate's lifetime.  There may be
	 * some small overlap, but that's okay because we rely on leader
	 * Tuplesortstate only allocating a small, fixed amount of memory here.
	 * When its tuplesort_performsort() is called (by our caller), and
	 * significant amounts of memory are likely to be used, all workers must
	 * have already freed almost all memory held by their Tuplesortstates
	 * (they are about to go away completely, too).  The overall effect is
	 * that maintenance_work_mem always represents an absolute high watermark
	 * on the amount of memory used by a CREATE INDEX operation, regardless of
	 * the use of parallelism or any other factor.
	 */
	buildstate->spool->sortstate =
		tuplesort_begin_index_btree(heap, index, buildstate->isunique,
									maintenance_work_mem, coordinate,
									false);

	/*
	 * If building a unique index, put dead tuples in a second spool to keep
	 * them out of the uniqueness check.  We expect that the second spool (for
	 * dead tuples) won't get very full, so we give it only work_mem.
	 */
	if (indexInfo->ii_Unique)
	{
		BTSpool    *btspool2 = (BTSpool *) palloc0(sizeof(BTSpool));
		SortCoordinate coordinate2 = NULL;

		/* Initialize secondary spool */
		btspool2->heap = heap;
		btspool2->index = index;
		btspool2->isunique = false;
		/* Save as secondary spool */
		buildstate->spool2 = btspool2;

		if (buildstate->btleader)
		{
			/*
			 * Set up non-private state that is passed to
			 * tuplesort_begin_index_btree() about the basic high level
			 * coordination of a parallel sort.
			 */
			coordinate2 = (SortCoordinate) palloc0(sizeof(SortCoordinateData));
			coordinate2->isWorker = false;
			coordinate2->nParticipants =
				buildstate->btleader->nparticipanttuplesorts;
			coordinate2->sharedsort = buildstate->btleader->sharedsort2;
		}

		/*
		 * We expect that the second one (for dead tuples) won't get very
		 * full, so we give it only work_mem
		 */
		buildstate->spool2->sortstate =
			tuplesort_begin_index_btree(heap, index, false, work_mem,
										coordinate2, false);
	}

	/* Fill spool using either serial or parallel heap scan */
	if (!buildstate->btleader)
		reltuples = IndexBuildHeapScan(heap, index, indexInfo, true,
									   _bt_build_callback, (void *) buildstate,
									   NULL);
	else
		reltuples = _bt_parallel_heapscan(buildstate,
										  &indexInfo->ii_BrokenHotChain);

	/* okay, all heap tuples are spooled */
	if (buildstate->spool2 && !buildstate->havedead)
	{
		/* spool2 turns out to be unnecessary */
		_bt_spooldestroy(buildstate->spool2);
		buildstate->spool2 = NULL;
	}

	return reltuples;
}

/*
 * clean up a spool structure and its substructures.
 */
static void
_bt_spooldestroy(BTSpool *btspool)
{
	tuplesort_end(btspool->sortstate);
	pfree(btspool);
}

/*
 * spool an index entry into the sort file.
 */
static void
_bt_spool(BTSpool *btspool, ItemPointer self, Datum *values, bool *isnull)
{
	tuplesort_putindextuplevalues(btspool->sortstate, btspool->index,
								  self, values, isnull);
}

/*
 * given a spool loaded by successive calls to _bt_spool,
 * create an entire btree.
 */
static void
_bt_leafbuild(BTSpool *btspool, BTSpool *btspool2)
{
	BTWriteState wstate;

#ifdef BTREE_BUILD_STATS
	if (log_btree_build_stats)
	{
		ShowUsage("BTREE BUILD (Spool) STATISTICS");
		ResetUsage();
	}
#endif							/* BTREE_BUILD_STATS */

	tuplesort_performsort(btspool->sortstate);
	if (btspool2)
		tuplesort_performsort(btspool2->sortstate);

	wstate.heap = btspool->heap;
	wstate.index = btspool->index;

	/*
	 * We need to log index creation in WAL iff WAL archiving/streaming is
	 * enabled UNLESS the index isn't WAL-logged anyway.
	 */
	wstate.btws_use_wal = XLogIsNeeded() && RelationNeedsWAL(wstate.index);

	/* reserve the metapage */
	wstate.btws_pages_alloced = BTREE_METAPAGE + 1;
	wstate.btws_pages_written = 0;
	wstate.btws_zeropage = NULL;	/* until needed */

	_bt_load(&wstate, btspool, btspool2);
}

/*
 * Per-tuple callback from IndexBuildHeapScan
 */
static void
_bt_build_callback(Relation index,
				   HeapTuple htup,
				   Datum *values,
				   bool *isnull,
				   bool tupleIsAlive,
				   void *state)
{
	BTBuildState *buildstate = (BTBuildState *) state;

	/*
	 * insert the index tuple into the appropriate spool file for subsequent
	 * processing
	 */
	if (tupleIsAlive || buildstate->spool2 == NULL)
		_bt_spool(buildstate->spool, &htup->t_self, values, isnull);
	else
	{
		/* dead tuples are put into spool2 */
		buildstate->havedead = true;
		_bt_spool(buildstate->spool2, &htup->t_self, values, isnull);
	}

	buildstate->indtuples += 1;
}

/*
 * allocate workspace for a new, clean btree page, not linked to any siblings.
 */
static Page
_bt_blnewpage(uint32 level)
{
	Page		page;
	BTPageOpaque opaque;

	page = (Page) palloc(BLCKSZ);

	/* Zero the page and set up standard page header info */
	_bt_pageinit(page, BLCKSZ);

	/* Initialize BT opaque state */
	opaque = (BTPageOpaque) PageGetSpecialPointer(page);
	opaque->btpo_prev = opaque->btpo_next = P_NONE;
	opaque->btpo.level = level;
	opaque->btpo_flags = (level > 0) ? 0 : BTP_LEAF;
	opaque->btpo_cycleid = 0;

	/* Make the P_HIKEY line pointer appear allocated */
	((PageHeader) page)->pd_lower += sizeof(ItemIdData);

	return page;
}

/*
 * emit a completed btree page, and release the working storage.
 */
static void
_bt_blwritepage(BTWriteState *wstate, Page page, BlockNumber blkno)
{
	/* Ensure rd_smgr is open (could have been closed by relcache flush!) */
	RelationOpenSmgr(wstate->index);

	/* XLOG stuff */
	if (wstate->btws_use_wal)
	{
		/* We use the heap NEWPAGE record type for this */
		log_newpage(&wstate->index->rd_node, MAIN_FORKNUM, blkno, page, true);
	}

	/*
	 * If we have to write pages nonsequentially, fill in the space with
	 * zeroes until we come back and overwrite.  This is not logically
	 * necessary on standard Unix filesystems (unwritten space will read as
	 * zeroes anyway), but it should help to avoid fragmentation. The dummy
	 * pages aren't WAL-logged though.
	 */
	while (blkno > wstate->btws_pages_written)
	{
		if (!wstate->btws_zeropage)
			wstate->btws_zeropage = (Page) palloc0(BLCKSZ);
		/* don't set checksum for all-zero page */
		smgrextend(wstate->index->rd_smgr, MAIN_FORKNUM,
				   wstate->btws_pages_written++,
				   (char *) wstate->btws_zeropage,
				   true);
	}

	PageSetChecksumInplace(page, blkno);

	/*
	 * Now write the page.  There's no need for smgr to schedule an fsync for
	 * this write; we'll do it ourselves before ending the build.
	 */
	if (blkno == wstate->btws_pages_written)
	{
		/* extending the file... */
		smgrextend(wstate->index->rd_smgr, MAIN_FORKNUM, blkno,
				   (char *) page, true);
		wstate->btws_pages_written++;
	}
	else
	{
		/* overwriting a block we zero-filled before */
		smgrwrite(wstate->index->rd_smgr, MAIN_FORKNUM, blkno,
				  (char *) page, true);
	}

	pfree(page);
}

/*
 * allocate and initialize a new BTPageState.  the returned structure
 * is suitable for immediate use by _bt_buildadd.
 */
static BTPageState *
_bt_pagestate(BTWriteState *wstate, uint32 level)
{
	BTPageState *state = (BTPageState *) palloc0(sizeof(BTPageState));

	/* create initial page for level */
	state->btps_page = _bt_blnewpage(level);

	/* and assign it a page position */
	state->btps_blkno = wstate->btws_pages_alloced++;

	state->btps_minkey = NULL;
	/* initialize lastoff so first item goes into P_FIRSTKEY */
	state->btps_lastoff = P_HIKEY;
	state->btps_level = level;
	/* set "full" threshold based on level.  See notes at head of file. */
	if (level > 0)
		state->btps_full = (BLCKSZ * (100 - BTREE_NONLEAF_FILLFACTOR) / 100);
	else
		state->btps_full = RelationGetTargetPageFreeSpace(wstate->index,
														  BTREE_DEFAULT_FILLFACTOR);
	/* no parent level, yet */
	state->btps_next = NULL;

	return state;
}

/*
 * slide an array of ItemIds back one slot (from P_FIRSTKEY to
 * P_HIKEY, overwriting P_HIKEY).  we need to do this when we discover
 * that we have built an ItemId array in what has turned out to be a
 * P_RIGHTMOST page.
 */
static void
_bt_slideleft(Page page)
{
	OffsetNumber off;
	OffsetNumber maxoff;
	ItemId		previi;
	ItemId		thisii;

	if (!PageIsEmpty(page))
	{
		maxoff = PageGetMaxOffsetNumber(page);
		previi = PageGetItemId(page, P_HIKEY);
		for (off = P_FIRSTKEY; off <= maxoff; off = OffsetNumberNext(off))
		{
			thisii = PageGetItemId(page, off);
			*previi = *thisii;
			previi = thisii;
		}
		((PageHeader) page)->pd_lower -= sizeof(ItemIdData);
	}
}

/*
 * Add an item to a page being built.
 *
 * The main difference between this routine and a bare PageAddItem call
 * is that this code knows that the leftmost data item on a non-leaf
 * btree page doesn't need to have a key.  Therefore, it strips such
 * items down to just the item header.
 *
 * This is almost like nbtinsert.c's _bt_pgaddtup(), but we can't use
 * that because it assumes that P_RIGHTMOST() will return the correct
 * answer for the page.  Here, we don't know yet if the page will be
 * rightmost.  Offset P_FIRSTKEY is always the first data key.
 */
static void
_bt_sortaddtup(Page page,
			   Size itemsize,
			   IndexTuple itup,
			   OffsetNumber itup_off)
{
	BTPageOpaque opaque = (BTPageOpaque) PageGetSpecialPointer(page);
	IndexTupleData trunctuple;

	if (!P_ISLEAF(opaque) && itup_off == P_FIRSTKEY)
	{
		trunctuple = *itup;
		trunctuple.t_info = sizeof(IndexTupleData);
		BTreeTupleSetNAtts(&trunctuple, 0);
		itup = &trunctuple;
		itemsize = sizeof(IndexTupleData);
	}

	if (PageAddItem(page, (Item) itup, itemsize, itup_off,
					false, false) == InvalidOffsetNumber)
		elog(ERROR, "failed to add item to the index page");
}

/*----------
 * Add an item to a disk page from the sort output.
 *
 * We must be careful to observe the page layout conventions of nbtsearch.c:
 * - rightmost pages start data items at P_HIKEY instead of at P_FIRSTKEY.
 * - on non-leaf pages, the key portion of the first item need not be
 *	 stored, we should store only the link.
 *
 * A leaf page being built looks like:
 *
 * +----------------+---------------------------------+
 * | PageHeaderData | linp0 linp1 linp2 ...           |
 * +-----------+----+---------------------------------+
 * | ... linpN |									  |
 * +-----------+--------------------------------------+
 * |	 ^ last										  |
 * |												  |
 * +-------------+------------------------------------+
 * |			 | itemN ...                          |
 * +-------------+------------------+-----------------+
 * |		  ... item3 item2 item1 | "special space" |
 * +--------------------------------+-----------------+
 *
 * Contrast this with the diagram in bufpage.h; note the mismatch
 * between linps and items.  This is because we reserve linp0 as a
 * placeholder for the pointer to the "high key" item; when we have
 * filled up the page, we will set linp0 to point to itemN and clear
 * linpN.  On the other hand, if we find this is the last (rightmost)
 * page, we leave the items alone and slide the linp array over.  If
 * the high key is to be truncated, offset 1 is deleted, and we insert
 * the truncated high key at offset 1.
 *
 * 'last' pointer indicates the last offset added to the page.
 *----------
 */
static void
_bt_buildadd(BTWriteState *wstate, BTPageState *state, IndexTuple itup)
{
	Page		npage;
	BlockNumber nblkno;
	OffsetNumber last_off;
	Size		pgspc;
	Size		itupsz;
	int			indnatts = IndexRelationGetNumberOfAttributes(wstate->index);
	int			indnkeyatts = IndexRelationGetNumberOfKeyAttributes(wstate->index);

	/*
	 * This is a handy place to check for cancel interrupts during the btree
	 * load phase of index creation.
	 */
	CHECK_FOR_INTERRUPTS();

	npage = state->btps_page;
	nblkno = state->btps_blkno;
	last_off = state->btps_lastoff;

	pgspc = PageGetFreeSpace(npage);
	itupsz = IndexTupleSize(itup);
	itupsz = MAXALIGN(itupsz);

	/*
	 * Check whether the item can fit on a btree page at all. (Eventually, we
	 * ought to try to apply TOAST methods if not.) We actually need to be
	 * able to fit three items on every page, so restrict any one item to 1/3
	 * the per-page available space. Note that at this point, itupsz doesn't
	 * include the ItemId.
	 *
	 * NOTE: similar code appears in _bt_insertonpg() to defend against
	 * oversize items being inserted into an already-existing index. But
	 * during creation of an index, we don't go through there.
	 */
	if (itupsz > BTMaxItemSize(npage))
		ereport(ERROR,
				(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
				 errmsg("index row size %zu exceeds maximum %zu for index \"%s\"",
						itupsz, BTMaxItemSize(npage),
						RelationGetRelationName(wstate->index)),
				 errhint("Values larger than 1/3 of a buffer page cannot be indexed.\n"
						 "Consider a function index of an MD5 hash of the value, "
						 "or use full text indexing."),
				 errtableconstraint(wstate->heap,
									RelationGetRelationName(wstate->index))));

	/*
	 * Check to see if page is "full".  It's definitely full if the item won't
	 * fit.  Otherwise, compare to the target freespace derived from the
	 * fillfactor.  However, we must put at least two items on each page, so
	 * disregard fillfactor if we don't have that many.
	 */
	if (pgspc < itupsz || (pgspc < state->btps_full && last_off > P_FIRSTKEY))
	{
		/*
		 * Finish off the page and write it out.
		 */
		Page		opage = npage;
		BlockNumber oblkno = nblkno;
		ItemId		ii;
		ItemId		hii;
		IndexTuple	oitup;
		BTPageOpaque opageop = (BTPageOpaque) PageGetSpecialPointer(opage);

		/* Create new page of same level */
		npage = _bt_blnewpage(state->btps_level);

		/* and assign it a page position */
		nblkno = wstate->btws_pages_alloced++;

		/*
		 * We copy the last item on the page into the new page, and then
		 * rearrange the old page so that the 'last item' becomes its high key
		 * rather than a true data item.  There had better be at least two
		 * items on the page already, else the page would be empty of useful
		 * data.
		 */
		Assert(last_off > P_FIRSTKEY);
		ii = PageGetItemId(opage, last_off);
		oitup = (IndexTuple) PageGetItem(opage, ii);
		_bt_sortaddtup(npage, ItemIdGetLength(ii), oitup, P_FIRSTKEY);

		/*
		 * Move 'last' into the high key position on opage
		 */
		hii = PageGetItemId(opage, P_HIKEY);
		*hii = *ii;
		ItemIdSetUnused(ii);	/* redundant */
		((PageHeader) opage)->pd_lower -= sizeof(ItemIdData);

		if (indnkeyatts != indnatts && P_ISLEAF(opageop))
		{
			IndexTuple	truncated;
			Size		truncsz;

			/*
			 * Truncate any non-key attributes from high key on leaf level
			 * (i.e. truncate on leaf level if we're building an INCLUDE
			 * index).  This is only done at the leaf level because downlinks
			 * in internal pages are either negative infinity items, or get
			 * their contents from copying from one level down.  See also:
			 * _bt_split().
			 *
			 * Since the truncated tuple is probably smaller than the
			 * original, it cannot just be copied in place (besides, we want
			 * to actually save space on the leaf page).  We delete the
			 * original high key, and add our own truncated high key at the
			 * same offset.
			 *
			 * Note that the page layout won't be changed very much.  oitup is
			 * already located at the physical beginning of tuple space, so we
			 * only shift the line pointer array back and forth, and overwrite
			 * the latter portion of the space occupied by the original tuple.
			 * This is fairly cheap.
			 */
			truncated = _bt_nonkey_truncate(wstate->index, oitup);
			truncsz = IndexTupleSize(truncated);
			PageIndexTupleDelete(opage, P_HIKEY);
			_bt_sortaddtup(opage, truncsz, truncated, P_HIKEY);
			pfree(truncated);

			/* oitup should continue to point to the page's high key */
			hii = PageGetItemId(opage, P_HIKEY);
			oitup = (IndexTuple) PageGetItem(opage, hii);
		}

		/*
		 * Link the old page into its parent, using its minimum key. If we
		 * don't have a parent, we have to create one; this adds a new btree
		 * level.
		 */
		if (state->btps_next == NULL)
			state->btps_next = _bt_pagestate(wstate, state->btps_level + 1);

		Assert(BTreeTupleGetNAtts(state->btps_minkey, wstate->index) ==
			   IndexRelationGetNumberOfKeyAttributes(wstate->index) ||
			   P_LEFTMOST(opageop));
		Assert(BTreeTupleGetNAtts(state->btps_minkey, wstate->index) == 0 ||
			   !P_LEFTMOST(opageop));
		BTreeInnerTupleSetDownLink(state->btps_minkey, oblkno);
		_bt_buildadd(wstate, state->btps_next, state->btps_minkey);
		pfree(state->btps_minkey);

		/*
		 * Save a copy of the minimum key for the new page.  We have to copy
		 * it off the old page, not the new one, in case we are not at leaf
		 * level.
		 */
		state->btps_minkey = CopyIndexTuple(oitup);

		/*
		 * Set the sibling links for both pages.
		 */
		{
			BTPageOpaque oopaque = (BTPageOpaque) PageGetSpecialPointer(opage);
			BTPageOpaque nopaque = (BTPageOpaque) PageGetSpecialPointer(npage);

			oopaque->btpo_next = nblkno;
			nopaque->btpo_prev = oblkno;
			nopaque->btpo_next = P_NONE;	/* redundant */
		}

		/*
		 * Write out the old page.  We never need to touch it again, so we can
		 * free the opage workspace too.
		 */
		_bt_blwritepage(wstate, opage, oblkno);

		/*
		 * Reset last_off to point to new page
		 */
		last_off = P_FIRSTKEY;
	}

	/*
	 * If the new item is the first for its page, stash a copy for later. Note
	 * this will only happen for the first item on a level; on later pages,
	 * the first item for a page is copied from the prior page in the code
	 * above.  Since the minimum key for an entire level is only used as a
	 * minus infinity downlink, and never as a high key, there is no need to
	 * truncate away non-key attributes at this point.
	 */
	if (last_off == P_HIKEY)
	{
		Assert(state->btps_minkey == NULL);
		state->btps_minkey = CopyIndexTuple(itup);
		/* _bt_sortaddtup() will perform full truncation later */
		BTreeTupleSetNAtts(state->btps_minkey, 0);
	}

	/*
	 * Add the new item into the current page.
	 */
	last_off = OffsetNumberNext(last_off);
	_bt_sortaddtup(npage, itupsz, itup, last_off);

	state->btps_page = npage;
	state->btps_blkno = nblkno;
	state->btps_lastoff = last_off;
}

/*
 * Finish writing out the completed btree.
 */
static void
_bt_uppershutdown(BTWriteState *wstate, BTPageState *state)
{
	BTPageState *s;
	BlockNumber rootblkno = P_NONE;
	uint32		rootlevel = 0;
	Page		metapage;

	/*
	 * Each iteration of this loop completes one more level of the tree.
	 */
	for (s = state; s != NULL; s = s->btps_next)
	{
		BlockNumber blkno;
		BTPageOpaque opaque;

		blkno = s->btps_blkno;
		opaque = (BTPageOpaque) PageGetSpecialPointer(s->btps_page);

		/*
		 * We have to link the last page on this level to somewhere.
		 *
		 * If we're at the top, it's the root, so attach it to the metapage.
		 * Otherwise, add an entry for it to its parent using its minimum key.
		 * This may cause the last page of the parent level to split, but
		 * that's not a problem -- we haven't gotten to it yet.
		 */
		if (s->btps_next == NULL)
		{
			opaque->btpo_flags |= BTP_ROOT;
			rootblkno = blkno;
			rootlevel = s->btps_level;
		}
		else
		{
			Assert(BTreeTupleGetNAtts(s->btps_minkey, wstate->index) ==
				   IndexRelationGetNumberOfKeyAttributes(wstate->index) ||
				   P_LEFTMOST(opaque));
			Assert(BTreeTupleGetNAtts(s->btps_minkey, wstate->index) == 0 ||
				   !P_LEFTMOST(opaque));
			BTreeInnerTupleSetDownLink(s->btps_minkey, blkno);
			_bt_buildadd(wstate, s->btps_next, s->btps_minkey);
			pfree(s->btps_minkey);
			s->btps_minkey = NULL;
		}

		/*
		 * This is the rightmost page, so the ItemId array needs to be slid
		 * back one slot.  Then we can dump out the page.
		 */
		_bt_slideleft(s->btps_page);
		_bt_blwritepage(wstate, s->btps_page, s->btps_blkno);
		s->btps_page = NULL;	/* writepage freed the workspace */
	}

	/*
	 * As the last step in the process, construct the metapage and make it
	 * point to the new root (unless we had no data at all, in which case it's
	 * set to point to "P_NONE").  This changes the index to the "valid" state
	 * by filling in a valid magic number in the metapage.
	 */
	metapage = (Page) palloc(BLCKSZ);
	_bt_initmetapage(metapage, rootblkno, rootlevel);
	_bt_blwritepage(wstate, metapage, BTREE_METAPAGE);
}

/*
 * Read tuples in correct sort order from tuplesort, and load them into
 * btree leaves.
 */
static void
_bt_load(BTWriteState *wstate, BTSpool *btspool, BTSpool *btspool2)
{
	BTPageState *state = NULL;
	bool		merge = (btspool2 != NULL);
	IndexTuple	itup,
				itup2 = NULL;
	bool		load1;
	TupleDesc	tupdes = RelationGetDescr(wstate->index);
	int			i,
				keysz = IndexRelationGetNumberOfKeyAttributes(wstate->index);
	ScanKey		indexScanKey = NULL;
	SortSupport sortKeys;

	if (merge)
	{
		/*
		 * Another BTSpool for dead tuples exists. Now we have to merge
		 * btspool and btspool2.
		 */

		/* the preparation of merge */
		itup = tuplesort_getindextuple(btspool->sortstate, true);
		itup2 = tuplesort_getindextuple(btspool2->sortstate, true);
		indexScanKey = _bt_mkscankey_nodata(wstate->index);

		/* Prepare SortSupport data for each column */
		sortKeys = (SortSupport) palloc0(keysz * sizeof(SortSupportData));

		for (i = 0; i < keysz; i++)
		{
			SortSupport sortKey = sortKeys + i;
			ScanKey		scanKey = indexScanKey + i;
			int16		strategy;

			sortKey->ssup_cxt = CurrentMemoryContext;
			sortKey->ssup_collation = scanKey->sk_collation;
			sortKey->ssup_nulls_first =
				(scanKey->sk_flags & SK_BT_NULLS_FIRST) != 0;
			sortKey->ssup_attno = scanKey->sk_attno;
			/* Abbreviation is not supported here */
			sortKey->abbreviate = false;

			AssertState(sortKey->ssup_attno != 0);

			strategy = (scanKey->sk_flags & SK_BT_DESC) != 0 ?
				BTGreaterStrategyNumber : BTLessStrategyNumber;

			PrepareSortSupportFromIndexRel(wstate->index, strategy, sortKey);
		}

		_bt_freeskey(indexScanKey);

		for (;;)
		{
			load1 = true;		/* load BTSpool next ? */
			if (itup2 == NULL)
			{
				if (itup == NULL)
					break;
			}
			else if (itup != NULL)
			{
				for (i = 1; i <= keysz; i++)
				{
					SortSupport entry;
					Datum		attrDatum1,
								attrDatum2;
					bool		isNull1,
								isNull2;
					int32		compare;

					entry = sortKeys + i - 1;
					attrDatum1 = index_getattr(itup, i, tupdes, &isNull1);
					attrDatum2 = index_getattr(itup2, i, tupdes, &isNull2);

					compare = ApplySortComparator(attrDatum1, isNull1,
												  attrDatum2, isNull2,
												  entry);
					if (compare > 0)
					{
						load1 = false;
						break;
					}
					else if (compare < 0)
						break;
				}
			}
			else
				load1 = false;

			/* When we see first tuple, create first index page */
			if (state == NULL)
				state = _bt_pagestate(wstate, 0);

			if (load1)
			{
				_bt_buildadd(wstate, state, itup);
				itup = tuplesort_getindextuple(btspool->sortstate, true);
			}
			else
			{
				_bt_buildadd(wstate, state, itup2);
				itup2 = tuplesort_getindextuple(btspool2->sortstate, true);
			}
		}
		pfree(sortKeys);
	}
	else
	{
		/* merge is unnecessary */
		while ((itup = tuplesort_getindextuple(btspool->sortstate,
											   true)) != NULL)
		{
			/* When we see first tuple, create first index page */
			if (state == NULL)
				state = _bt_pagestate(wstate, 0);

			_bt_buildadd(wstate, state, itup);
		}
	}

	/* Close down final pages and write the metapage */
	_bt_uppershutdown(wstate, state);

	/*
	 * If the index is WAL-logged, we must fsync it down to disk before it's
	 * safe to commit the transaction.  (For a non-WAL-logged index we don't
	 * care since the index will be uninteresting after a crash anyway.)
	 *
	 * It's obvious that we must do this when not WAL-logging the build. It's
	 * less obvious that we have to do it even if we did WAL-log the index
	 * pages.  The reason is that since we're building outside shared buffers,
	 * a CHECKPOINT occurring during the build has no way to flush the
	 * previously written data to disk (indeed it won't know the index even
	 * exists).  A crash later on would replay WAL from the checkpoint,
	 * therefore it wouldn't replay our earlier WAL entries. If we do not
	 * fsync those pages here, they might still not be on disk when the crash
	 * occurs.
	 */
	if (RelationNeedsWAL(wstate->index))
	{
		RelationOpenSmgr(wstate->index);
		smgrimmedsync(wstate->index->rd_smgr, MAIN_FORKNUM);
	}
}

/*
 * Create parallel context, and launch workers for leader.
 *
 * buildstate argument should be initialized (with the exception of the
 * tuplesort state in spools, which may later be created based on shared
 * state initially set up here).
 *
 * isconcurrent indicates if operation is CREATE INDEX CONCURRENTLY.
 *
 * request is the target number of parallel worker processes to launch.
 *
 * Sets buildstate's BTLeader, which caller must use to shut down parallel
 * mode by passing it to _bt_end_parallel() at the very end of its index
 * build.  If not even a single worker process can be launched, this is
 * never set, and caller should proceed with a serial index build.
 */
static void
_bt_begin_parallel(BTBuildState *buildstate, bool isconcurrent, int request)
{
	ParallelContext *pcxt;
	int			scantuplesortstates;
	Snapshot	snapshot;
	Size		estbtshared;
	Size		estsort;
	BTShared   *btshared;
	Sharedsort *sharedsort;
	Sharedsort *sharedsort2;
	BTSpool    *btspool = buildstate->spool;
	BTLeader   *btleader = (BTLeader *) palloc0(sizeof(BTLeader));
	BufferUsage *bufferusage;
	bool		leaderparticipates = true;
	int			querylen;

#ifdef DISABLE_LEADER_PARTICIPATION
	leaderparticipates = false;
#endif

	/*
	 * Enter parallel mode, and create context for parallel build of btree
	 * index
	 */
	EnterParallelMode();
	Assert(request > 0);
	pcxt = CreateParallelContext("postgres", "_bt_parallel_build_main",
								 request, true);

	scantuplesortstates = leaderparticipates ? request + 1 : request;

	/*
	 * Prepare for scan of the base relation.  In a normal index build, we use
	 * SnapshotAny because we must retrieve all tuples and do our own time
	 * qual checks (because we have to index RECENTLY_DEAD tuples).  In a
	 * concurrent build, we take a regular MVCC snapshot and index whatever's
	 * live according to that.
	 */
	if (!isconcurrent)
		snapshot = SnapshotAny;
	else
		snapshot = RegisterSnapshot(GetTransactionSnapshot());

	/*
	 * Estimate size for our own PARALLEL_KEY_BTREE_SHARED workspace, and
	 * PARALLEL_KEY_TUPLESORT tuplesort workspace
	 */
	estbtshared = _bt_parallel_estimate_shared(snapshot);
	shm_toc_estimate_chunk(&pcxt->estimator, estbtshared);
	estsort = tuplesort_estimate_shared(scantuplesortstates);
	shm_toc_estimate_chunk(&pcxt->estimator, estsort);

	/*
	 * Unique case requires a second spool, and so we may have to account for
	 * another shared workspace for that -- PARALLEL_KEY_TUPLESORT_SPOOL2
	 */
	if (!btspool->isunique)
		shm_toc_estimate_keys(&pcxt->estimator, 2);
	else
	{
		shm_toc_estimate_chunk(&pcxt->estimator, estsort);
		shm_toc_estimate_keys(&pcxt->estimator, 3);
	}

	/*
	 * Estimate space for BufferUsage -- PARALLEL_KEY_BUFFER_USAGE.
	 *
	 * If there are no extensions loaded that care, we could skip this.  We
	 * have no way of knowing whether anyone's looking at pgBufferUsage, so do
	 * it unconditionally.
	 */
	shm_toc_estimate_chunk(&pcxt->estimator,
						   mul_size(sizeof(BufferUsage), pcxt->nworkers));
	shm_toc_estimate_keys(&pcxt->estimator, 1);

	/* Finally, estimate PARALLEL_KEY_QUERY_TEXT space */
	if (debug_query_string)
	{
		querylen = strlen(debug_query_string);
		shm_toc_estimate_chunk(&pcxt->estimator, querylen + 1);
		shm_toc_estimate_keys(&pcxt->estimator, 1);
	}
	else
		querylen = 0;			/* keep compiler quiet */

	/* Everyone's had a chance to ask for space, so now create the DSM */
	InitializeParallelDSM(pcxt);

	/* If no DSM segment was available, back out (do serial build) */
	if (pcxt->seg == NULL)
	{
		if (IsMVCCSnapshot(snapshot))
			UnregisterSnapshot(snapshot);
		DestroyParallelContext(pcxt);
		ExitParallelMode();
		return;
	}

	/* Store shared build state, for which we reserved space */
	btshared = (BTShared *) shm_toc_allocate(pcxt->toc, estbtshared);
	/* Initialize immutable state */
	btshared->heaprelid = RelationGetRelid(btspool->heap);
	btshared->indexrelid = RelationGetRelid(btspool->index);
	btshared->isunique = btspool->isunique;
	btshared->isconcurrent = isconcurrent;
	btshared->scantuplesortstates = scantuplesortstates;
	ConditionVariableInit(&btshared->workersdonecv);
	SpinLockInit(&btshared->mutex);
	/* Initialize mutable state */
	btshared->nparticipantsdone = 0;
	btshared->reltuples = 0.0;
	btshared->havedead = false;
	btshared->indtuples = 0.0;
	btshared->brokenhotchain = false;
	heap_parallelscan_initialize(&btshared->heapdesc, btspool->heap, snapshot);

	/*
	 * Store shared tuplesort-private state, for which we reserved space.
	 * Then, initialize opaque state using tuplesort routine.
	 */
	sharedsort = (Sharedsort *) shm_toc_allocate(pcxt->toc, estsort);
	tuplesort_initialize_shared(sharedsort, scantuplesortstates,
								pcxt->seg);

	shm_toc_insert(pcxt->toc, PARALLEL_KEY_BTREE_SHARED, btshared);
	shm_toc_insert(pcxt->toc, PARALLEL_KEY_TUPLESORT, sharedsort);

	/* Unique case requires a second spool, and associated shared state */
	if (!btspool->isunique)
		sharedsort2 = NULL;
	else
	{
		/*
		 * Store additional shared tuplesort-private state, for which we
		 * reserved space.  Then, initialize opaque state using tuplesort
		 * routine.
		 */
		sharedsort2 = (Sharedsort *) shm_toc_allocate(pcxt->toc, estsort);
		tuplesort_initialize_shared(sharedsort2, scantuplesortstates,
									pcxt->seg);

		shm_toc_insert(pcxt->toc, PARALLEL_KEY_TUPLESORT_SPOOL2, sharedsort2);
	}

	/* Store query string for workers */
	if (debug_query_string)
	{
		char	   *sharedquery;

		sharedquery = (char *) shm_toc_allocate(pcxt->toc, querylen + 1);
		memcpy(sharedquery, debug_query_string, querylen + 1);
		shm_toc_insert(pcxt->toc, PARALLEL_KEY_QUERY_TEXT, sharedquery);
	}

	/* Allocate space for each worker's BufferUsage; no need to initialize */
	bufferusage = shm_toc_allocate(pcxt->toc,
								   mul_size(sizeof(BufferUsage), pcxt->nworkers));
	shm_toc_insert(pcxt->toc, PARALLEL_KEY_BUFFER_USAGE, bufferusage);

	/* Launch workers, saving status for leader/caller */
	LaunchParallelWorkers(pcxt);
	btleader->pcxt = pcxt;
	btleader->nparticipanttuplesorts = pcxt->nworkers_launched;
	if (leaderparticipates)
		btleader->nparticipanttuplesorts++;
	btleader->btshared = btshared;
	btleader->sharedsort = sharedsort;
	btleader->sharedsort2 = sharedsort2;
	btleader->snapshot = snapshot;
	btleader->bufferusage = bufferusage;

	/* If no workers were successfully launched, back out (do serial build) */
	if (pcxt->nworkers_launched == 0)
	{
		_bt_end_parallel(btleader);
		return;
	}

	/* Save leader state now that it's clear build will be parallel */
	buildstate->btleader = btleader;

	/* Join heap scan ourselves */
	if (leaderparticipates)
		_bt_leader_participate_as_worker(buildstate);

	/*
	 * Caller needs to wait for all launched workers when we return.  Make
	 * sure that the failure-to-start case will not hang forever.
	 */
	WaitForParallelWorkersToAttach(pcxt);
}

/*
 * Shut down workers, destroy parallel context, and end parallel mode.
 */
static void
_bt_end_parallel(BTLeader *btleader)
{
	int			i;

	/* Shutdown worker processes */
	WaitForParallelWorkersToFinish(btleader->pcxt);

	/*
	 * Next, accumulate buffer usage.  (This must wait for the workers to
	 * finish, or we might get incomplete data.)
	 */
	for (i = 0; i < btleader->pcxt->nworkers_launched; i++)
		InstrAccumParallelQuery(&btleader->bufferusage[i]);

	/* Free last reference to MVCC snapshot, if one was used */
	if (IsMVCCSnapshot(btleader->snapshot))
		UnregisterSnapshot(btleader->snapshot);
	DestroyParallelContext(btleader->pcxt);
	ExitParallelMode();
}

/*
 * Returns size of shared memory required to store state for a parallel
 * btree index build based on the snapshot its parallel scan will use.
 */
static Size
_bt_parallel_estimate_shared(Snapshot snapshot)
{
	if (!IsMVCCSnapshot(snapshot))
	{
		Assert(snapshot == SnapshotAny);
		return sizeof(BTShared);
	}

	return add_size(offsetof(BTShared, heapdesc) +
					offsetof(ParallelHeapScanDescData, phs_snapshot_data),
					EstimateSnapshotSpace(snapshot));
}

/*
 * Within leader, wait for end of heap scan.
 *
 * When called, parallel heap scan started by _bt_begin_parallel() will
 * already be underway within worker processes (when leader participates
 * as a worker, we should end up here just as workers are finishing).
 *
 * Fills in fields needed for ambuild statistics, and lets caller set
 * field indicating that some worker encountered a broken HOT chain.
 *
 * Returns the total number of heap tuples scanned.
 */
static double
_bt_parallel_heapscan(BTBuildState *buildstate, bool *brokenhotchain)
{
	BTShared   *btshared = buildstate->btleader->btshared;
	int			nparticipanttuplesorts;
	double		reltuples;

	nparticipanttuplesorts = buildstate->btleader->nparticipanttuplesorts;
	for (;;)
	{
		SpinLockAcquire(&btshared->mutex);
		if (btshared->nparticipantsdone == nparticipanttuplesorts)
		{
			buildstate->havedead = btshared->havedead;
			buildstate->indtuples = btshared->indtuples;
			*brokenhotchain = btshared->brokenhotchain;
			reltuples = btshared->reltuples;
			SpinLockRelease(&btshared->mutex);
			break;
		}
		SpinLockRelease(&btshared->mutex);

		ConditionVariableSleep(&btshared->workersdonecv,
							   WAIT_EVENT_PARALLEL_CREATE_INDEX_SCAN);
	}

	ConditionVariableCancelSleep();

	return reltuples;
}

/*
 * Within leader, participate as a parallel worker.
 */
static void
_bt_leader_participate_as_worker(BTBuildState *buildstate)
{
	BTLeader   *btleader = buildstate->btleader;
	BTSpool    *leaderworker;
	BTSpool    *leaderworker2;
	int			sortmem;

	/* Allocate memory and initialize private spool */
	leaderworker = (BTSpool *) palloc0(sizeof(BTSpool));
	leaderworker->heap = buildstate->spool->heap;
	leaderworker->index = buildstate->spool->index;
	leaderworker->isunique = buildstate->spool->isunique;

	/* Initialize second spool, if required */
	if (!btleader->btshared->isunique)
		leaderworker2 = NULL;
	else
	{
		/* Allocate memory for worker's own private secondary spool */
		leaderworker2 = (BTSpool *) palloc0(sizeof(BTSpool));

		/* Initialize worker's own secondary spool */
		leaderworker2->heap = leaderworker->heap;
		leaderworker2->index = leaderworker->index;
		leaderworker2->isunique = false;
	}

	/*
	 * Might as well use reliable figure when doling out maintenance_work_mem
	 * (when requested number of workers were not launched, this will be
	 * somewhat higher than it is for other workers).
	 */
	sortmem = maintenance_work_mem / btleader->nparticipanttuplesorts;

	/* Perform work common to all participants */
	_bt_parallel_scan_and_sort(leaderworker, leaderworker2, btleader->btshared,
							   btleader->sharedsort, btleader->sharedsort2,
							   sortmem);

#ifdef BTREE_BUILD_STATS
	if (log_btree_build_stats)
	{
		ShowUsage("BTREE BUILD (Leader Partial Spool) STATISTICS");
		ResetUsage();
	}
#endif							/* BTREE_BUILD_STATS */
}

/*
 * Perform work within a launched parallel process.
 */
void
_bt_parallel_build_main(dsm_segment *seg, shm_toc *toc)
{
	char	   *sharedquery;
	BTSpool    *btspool;
	BTSpool    *btspool2;
	BTShared   *btshared;
	Sharedsort *sharedsort;
	Sharedsort *sharedsort2;
	Relation	heapRel;
	Relation	indexRel;
	LOCKMODE	heapLockmode;
	LOCKMODE	indexLockmode;
	BufferUsage *bufferusage;
	int			sortmem;

#ifdef BTREE_BUILD_STATS
	if (log_btree_build_stats)
		ResetUsage();
#endif							/* BTREE_BUILD_STATS */

	/* Set debug_query_string for individual workers first */
	sharedquery = shm_toc_lookup(toc, PARALLEL_KEY_QUERY_TEXT, true);
	debug_query_string = sharedquery;

	/* Report the query string from leader */
	pgstat_report_activity(STATE_RUNNING, debug_query_string);

	/* Look up nbtree shared state */
	btshared = shm_toc_lookup(toc, PARALLEL_KEY_BTREE_SHARED, false);

	/* Open relations using lock modes known to be obtained by index.c */
	if (!btshared->isconcurrent)
	{
		heapLockmode = ShareLock;
		indexLockmode = AccessExclusiveLock;
	}
	else
	{
		heapLockmode = ShareUpdateExclusiveLock;
		indexLockmode = RowExclusiveLock;
	}

	/* Open relations within worker */
	heapRel = heap_open(btshared->heaprelid, heapLockmode);
	indexRel = index_open(btshared->indexrelid, indexLockmode);

	/* Initialize worker's own spool */
	btspool = (BTSpool *) palloc0(sizeof(BTSpool));
	btspool->heap = heapRel;
	btspool->index = indexRel;
	btspool->isunique = btshared->isunique;

	/* Look up shared state private to tuplesort.c */
	sharedsort = shm_toc_lookup(toc, PARALLEL_KEY_TUPLESORT, false);
	tuplesort_attach_shared(sharedsort, seg);
	if (!btshared->isunique)
	{
		btspool2 = NULL;
		sharedsort2 = NULL;
	}
	else
	{
		/* Allocate memory for worker's own private secondary spool */
		btspool2 = (BTSpool *) palloc0(sizeof(BTSpool));

		/* Initialize worker's own secondary spool */
		btspool2->heap = btspool->heap;
		btspool2->index = btspool->index;
		btspool2->isunique = false;
		/* Look up shared state private to tuplesort.c */
		sharedsort2 = shm_toc_lookup(toc, PARALLEL_KEY_TUPLESORT_SPOOL2, false);
		tuplesort_attach_shared(sharedsort2, seg);
	}

	/* Prepare to track buffer usage during parallel execution */
	InstrStartParallelQuery();

	/* Perform sorting of spool, and possibly a spool2 */
	sortmem = maintenance_work_mem / btshared->scantuplesortstates;
	_bt_parallel_scan_and_sort(btspool, btspool2, btshared, sharedsort,
							   sharedsort2, sortmem);

	/* Report buffer usage during parallel execution */
	bufferusage = shm_toc_lookup(toc, PARALLEL_KEY_BUFFER_USAGE, false);
	InstrEndParallelQuery(&bufferusage[ParallelWorkerNumber]);

#ifdef BTREE_BUILD_STATS
	if (log_btree_build_stats)
	{
		ShowUsage("BTREE BUILD (Worker Partial Spool) STATISTICS");
		ResetUsage();
	}
#endif							/* BTREE_BUILD_STATS */

	index_close(indexRel, indexLockmode);
	heap_close(heapRel, heapLockmode);
}

/*
 * Perform a worker's portion of a parallel sort.
 *
 * This generates a tuplesort for passed btspool, and a second tuplesort
 * state if a second btspool is need (i.e. for unique index builds).  All
 * other spool fields should already be set when this is called.
 *
 * sortmem is the amount of working memory to use within each worker,
 * expressed in KBs.
 *
 * When this returns, workers are done, and need only release resources.
 */
static void
_bt_parallel_scan_and_sort(BTSpool *btspool, BTSpool *btspool2,
						   BTShared *btshared, Sharedsort *sharedsort,
						   Sharedsort *sharedsort2, int sortmem)
{
	SortCoordinate coordinate;
	BTBuildState buildstate;
	HeapScanDesc scan;
	double		reltuples;
	IndexInfo  *indexInfo;

	/* Initialize local tuplesort coordination state */
	coordinate = palloc0(sizeof(SortCoordinateData));
	coordinate->isWorker = true;
	coordinate->nParticipants = -1;
	coordinate->sharedsort = sharedsort;

	/* Begin "partial" tuplesort */
	btspool->sortstate = tuplesort_begin_index_btree(btspool->heap,
													 btspool->index,
													 btspool->isunique,
													 sortmem, coordinate,
													 false);

	/*
	 * Just as with serial case, there may be a second spool.  If so, a
	 * second, dedicated spool2 partial tuplesort is required.
	 */
	if (btspool2)
	{
		SortCoordinate coordinate2;

		/*
		 * We expect that the second one (for dead tuples) won't get very
		 * full, so we give it only work_mem (unless sortmem is less for
		 * worker).  Worker processes are generally permitted to allocate
		 * work_mem independently.
		 */
		coordinate2 = palloc0(sizeof(SortCoordinateData));
		coordinate2->isWorker = true;
		coordinate2->nParticipants = -1;
		coordinate2->sharedsort = sharedsort2;
		btspool2->sortstate =
			tuplesort_begin_index_btree(btspool->heap, btspool->index, false,
										Min(sortmem, work_mem), coordinate2,
										false);
	}

	/* Fill in buildstate for _bt_build_callback() */
	buildstate.isunique = btshared->isunique;
	buildstate.havedead = false;
	buildstate.heap = btspool->heap;
	buildstate.spool = btspool;
	buildstate.spool2 = btspool2;
	buildstate.indtuples = 0;
	buildstate.btleader = NULL;

	/* Join parallel scan */
	indexInfo = BuildIndexInfo(btspool->index);
	indexInfo->ii_Concurrent = btshared->isconcurrent;
	scan = heap_beginscan_parallel(btspool->heap, &btshared->heapdesc);
	reltuples = IndexBuildHeapScan(btspool->heap, btspool->index, indexInfo,
								   true, _bt_build_callback,
								   (void *) &buildstate, scan);

	/*
	 * Execute this worker's part of the sort.
	 *
	 * Unlike leader and serial cases, we cannot avoid calling
	 * tuplesort_performsort() for spool2 if it ends up containing no dead
	 * tuples (this is disallowed for workers by tuplesort).
	 */
	tuplesort_performsort(btspool->sortstate);
	if (btspool2)
		tuplesort_performsort(btspool2->sortstate);

	/*
	 * Done.  Record ambuild statistics, and whether we encountered a broken
	 * HOT chain.
	 */
	SpinLockAcquire(&btshared->mutex);
	btshared->nparticipantsdone++;
	btshared->reltuples += reltuples;
	if (buildstate.havedead)
		btshared->havedead = true;
	btshared->indtuples += buildstate.indtuples;
	if (indexInfo->ii_BrokenHotChain)
		btshared->brokenhotchain = true;
	SpinLockRelease(&btshared->mutex);

	/* Notify leader */
	ConditionVariableSignal(&btshared->workersdonecv);

	/* We can end tuplesorts immediately */
	tuplesort_end(btspool->sortstate);
	if (btspool2)
		tuplesort_end(btspool2->sortstate);
}
