nbtsort.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * 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) 1996-2020, 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/table.h"
#include "access/tableam.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "access/xloginsert.h"
#include "catalog/index.h"
#include "commands/progress.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)

/*
 * 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;

    /*
     * ParallelTableScanDescData data follows. Can't directly embed here, as
     * implementations of the parallel table scan desc interface might need
     * stronger alignment.
     */
} BTShared;

/*
 * Return pointer to a BTShared's parallel table scan.
 *
 * c.f. shm_toc_allocate as to why BUFFERALIGN is used, rather than just
 * MAXALIGN.
 */
#define ParallelTableScanFromBTShared(shared) \
    (ParallelTableScanDesc) ((char *) (shared) + BUFFERALIGN(sizeof(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;
} 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.
 */
typedef struct BTPageState
{
    Page        btps_page;      /* workspace for page building */
    BlockNumber btps_blkno;     /* block # to write this page at */
    IndexTuple  btps_lowkey;    /* page's strict lower bound pivot tuple */
    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;
    BTScanInsert inskey;        /* generic insertion scankey */
    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, ItemPointer tid, 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(Relation heap, 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,
                                       bool progress);


/*
 *  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;

    /* Report table scan phase started */
    pgstat_progress_update_param(PROGRESS_CREATEIDX_SUBPHASE,
                                 PROGRESS_BTREE_PHASE_INDEXBUILD_TABLESCAN);

    /* 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 = table_index_build_scan(heap, index, indexInfo, true, true,
                                           _bt_build_callback, (void *) buildstate,
                                           NULL);
    else
        reltuples = _bt_parallel_heapscan(buildstate,
                                          &indexInfo->ii_BrokenHotChain);

    /*
     * Set the progress target for the next phase.  Reset the block number
     * values set by table_index_build_scan
     */
    {
        const int   index[] = {
            PROGRESS_CREATEIDX_TUPLES_TOTAL,
            PROGRESS_SCAN_BLOCKS_TOTAL,
            PROGRESS_SCAN_BLOCKS_DONE
        };
        const int64 val[] = {
            buildstate->indtuples,
            0, 0
        };

        pgstat_progress_update_multi_param(3, index, val);
    }

    /* 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 */

    pgstat_progress_update_param(PROGRESS_CREATEIDX_SUBPHASE,
                                 PROGRESS_BTREE_PHASE_PERFORMSORT_1);
    tuplesort_performsort(btspool->sortstate);
    if (btspool2)
    {
        pgstat_progress_update_param(PROGRESS_CREATEIDX_SUBPHASE,
                                     PROGRESS_BTREE_PHASE_PERFORMSORT_2);
        tuplesort_performsort(btspool2->sortstate);
    }

    wstate.heap = btspool->heap;
    wstate.index = btspool->index;
    wstate.inskey = _bt_mkscankey(wstate.index, NULL);

    /*
     * 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 */

    pgstat_progress_update_param(PROGRESS_CREATEIDX_SUBPHASE,
                                 PROGRESS_BTREE_PHASE_LEAF_LOAD);
    _bt_load(&wstate, btspool, btspool2);
}

/*
 * Per-tuple callback for table_index_build_scan
 */
static void
_bt_build_callback(Relation index,
                   ItemPointer tid,
                   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, tid, values, isnull);
    else
    {
        /* dead tuples are put into spool2 */
        buildstate->havedead = true;
        _bt_spool(buildstate->spool2, tid, 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 XLOG_FPI 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_lowkey = 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 = BTGetTargetPageFreeSpace(wstate->index);

    /* 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 has a key that must be treated as minus infinity.  Therefore, it
 * truncates away all attributes.
 *
 * 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;
    bool        isleaf;

    /*
     * 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);
    /* Leaf case has slightly different rules due to suffix truncation */
    isleaf = (state->btps_level == 0);

    /*
     * Check whether the new item can fit on a btree page on current level at
     * all.
     *
     * Every newly built index will treat heap TID as part of the keyspace,
     * which imposes the requirement that new high keys must occasionally have
     * a heap TID appended within _bt_truncate().  That may leave a new pivot
     * tuple one or two MAXALIGN() quantums larger than the original first
     * right tuple it's derived from.  v4 deals with the problem by decreasing
     * the limit on the size of tuples inserted on the leaf level by the same
     * small amount.  Enforce the new v4+ limit on the leaf level, and the old
     * limit on internal levels, since pivot tuples may need to make use of
     * the reserved space.  This should never fail on internal pages.
     */
    if (unlikely(itupsz > BTMaxItemSize(npage)))
        _bt_check_third_page(wstate->index, wstate->heap, isleaf, npage,
                             itup);

    /*
     * Check to see if current page will fit new item, with space left over to
     * append a heap TID during suffix truncation when page is a leaf page.
     *
     * It is guaranteed that we can fit at least 2 non-pivot tuples plus a
     * high key with heap TID when finishing off a leaf page, since we rely on
     * _bt_check_third_page() rejecting oversized non-pivot tuples.  On
     * internal pages we can always fit 3 pivot tuples with larger internal
     * page tuple limit (includes page high key).
     *
     * Most of the time, a page is only "full" in the sense that the soft
     * fillfactor-wise limit has been exceeded.  However, we must always leave
     * at least two items plus a high key on each page before starting a new
     * page.  Disregard fillfactor and insert on "full" current page if we
     * don't have the minimum number of items yet.  (Note that we deliberately
     * assume that suffix truncation neither enlarges nor shrinks new high key
     * when applying soft limit.)
     */
    if (pgspc < itupsz + (isleaf ? MAXALIGN(sizeof(ItemPointerData)) : 0) ||
        (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;

        /* 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.  _bt_blnewpage()
         * allocated empty space for a line pointer when opage was first
         * created, so this is a matter of rearranging already-allocated space
         * on page, and initializing high key line pointer. (Actually, leaf
         * pages must also swap oitup with a truncated version of oitup, which
         * is sometimes larger than oitup, though never by more than the space
         * needed to append a heap TID.)
         */
        hii = PageGetItemId(opage, P_HIKEY);
        *hii = *ii;
        ItemIdSetUnused(ii);    /* redundant */
        ((PageHeader) opage)->pd_lower -= sizeof(ItemIdData);

        if (isleaf)
        {
            IndexTuple  lastleft;
            IndexTuple  truncated;

            /*
             * Truncate away any unneeded attributes from high key on leaf
             * level.  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().
             *
             * We don't try to bias our choice of split point to make it more
             * likely that _bt_truncate() can truncate away more attributes,
             * whereas the split point used within _bt_split() is chosen much
             * more delicately.  Suffix truncation is mostly useful because it
             * improves space utilization for workloads with random
             * insertions.  It doesn't seem worthwhile to add logic for
             * choosing a split point here for a benefit that is bound to be
             * much smaller.
             *
             * Overwrite the old item with new truncated high key directly.
             * oitup is already located at the physical beginning of tuple
             * space, so this should directly reuse the existing tuple space.
             */
            ii = PageGetItemId(opage, OffsetNumberPrev(last_off));
            lastleft = (IndexTuple) PageGetItem(opage, ii);

            truncated = _bt_truncate(wstate->index, lastleft, oitup,
                                     wstate->inskey);
            if (!PageIndexTupleOverwrite(opage, P_HIKEY, (Item) truncated,
                                         IndexTupleSize(truncated)))
                elog(ERROR, "failed to add high key to the index page");
            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 low 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_lowkey, wstate->index) <=
                IndexRelationGetNumberOfKeyAttributes(wstate->index) &&
                BTreeTupleGetNAtts(state->btps_lowkey, wstate->index) > 0) ||
               P_LEFTMOST((BTPageOpaque) PageGetSpecialPointer(opage)));
        Assert(BTreeTupleGetNAtts(state->btps_lowkey, wstate->index) == 0 ||
               !P_LEFTMOST((BTPageOpaque) PageGetSpecialPointer(opage)));
        BTreeTupleSetDownLink(state->btps_lowkey, oblkno);
        _bt_buildadd(wstate, state->btps_next, state->btps_lowkey);
        pfree(state->btps_lowkey);

        /*
         * Save a copy of the high key from the old page.  It is also the low
         * key for the new page.
         */
        state->btps_lowkey = 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;
    }

    /*
     * By here, either original page is still the current page, or a new page
     * was created that became the current page.  Either way, the current page
     * definitely has space for new item.
     *
     * If the new item is the first for its page, it must also be the first
     * item on its entire level.  On later same-level pages, a low key for a
     * page will be copied from the prior page in the code above.  Generate a
     * minus infinity low key here instead.
     */
    if (last_off == P_HIKEY)
    {
        Assert(state->btps_lowkey == NULL);
        state->btps_lowkey = palloc0(sizeof(IndexTupleData));
        state->btps_lowkey->t_info = sizeof(IndexTupleData);
        BTreeTupleSetNAtts(state->btps_lowkey, 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 low 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_lowkey, wstate->index) <=
                    IndexRelationGetNumberOfKeyAttributes(wstate->index) &&
                    BTreeTupleGetNAtts(s->btps_lowkey, wstate->index) > 0) ||
                   P_LEFTMOST(opaque));
            Assert(BTreeTupleGetNAtts(s->btps_lowkey, wstate->index) == 0 ||
                   !P_LEFTMOST(opaque));
            BTreeTupleSetDownLink(s->btps_lowkey, blkno);
            _bt_buildadd(wstate, s->btps_next, s->btps_lowkey);
            pfree(s->btps_lowkey);
            s->btps_lowkey = 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);
    SortSupport sortKeys;
    int64       tuples_done = 0;

    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);

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

        for (i = 0; i < keysz; i++)
        {
            SortSupport sortKey = sortKeys + i;
            ScanKey     scanKey = wstate->inskey->scankeys + 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);
        }

        for (;;)
        {
            load1 = true;       /* load BTSpool next ? */
            if (itup2 == NULL)
            {
                if (itup == NULL)
                    break;
            }
            else if (itup != NULL)
            {
                int32       compare = 0;

                for (i = 1; i <= keysz; i++)
                {
                    SortSupport entry;
                    Datum       attrDatum1,
                                attrDatum2;
                    bool        isNull1,
                                isNull2;

                    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;
                }

                /*
                 * If key values are equal, we sort on ItemPointer.  This is
                 * required for btree indexes, since heap TID is treated as an
                 * implicit last key attribute in order to ensure that all
                 * keys in the index are physically unique.
                 */
                if (compare == 0)
                {
                    compare = ItemPointerCompare(&itup->t_tid, &itup2->t_tid);
                    Assert(compare != 0);
                    if (compare > 0)
                        load1 = false;
                }
            }
            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);
            }

            /* Report progress */
            pgstat_progress_update_param(PROGRESS_CREATEIDX_TUPLES_DONE,
                                         ++tuples_done);
        }
        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);

            /* Report progress */
            pgstat_progress_update_param(PROGRESS_CREATEIDX_TUPLES_DONE,
                                         ++tuples_done);
        }
    }

    /* 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));
    bool        leaderparticipates = true;
    char       *sharedquery;
    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);

    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(btspool->heap, 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);
    }

    /* Finally, estimate PARALLEL_KEY_QUERY_TEXT space */
    querylen = strlen(debug_query_string);
    shm_toc_estimate_chunk(&pcxt->estimator, querylen + 1);
    shm_toc_estimate_keys(&pcxt->estimator, 1);

    /* 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;
    table_parallelscan_initialize(btspool->heap,
                                  ParallelTableScanFromBTShared(btshared),
                                  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 */
    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);

    /* 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;

    /* 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)
{
    /* Shutdown worker processes */
    WaitForParallelWorkersToFinish(btleader->pcxt);
    /* 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(Relation heap, Snapshot snapshot)
{
    /* c.f. shm_toc_allocate as to why BUFFERALIGN is used */
    return add_size(BUFFERALIGN(sizeof(BTShared)),
                    table_parallelscan_estimate(heap, 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, true);

#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;
    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, false);
    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 = table_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);
    }

    /* 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, false);

#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);
    table_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, bool progress)
{
    SortCoordinate coordinate;
    BTBuildState buildstate;
    TableScanDesc 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 = table_beginscan_parallel(btspool->heap,
                                    ParallelTableScanFromBTShared(btshared));
    reltuples = table_index_build_scan(btspool->heap, btspool->index, indexInfo,
                                       true, progress, _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);
}