nbtree.c

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/*-------------------------------------------------------------------------
 *
 * nbtree.c
 *    Implementation of Lehman and Yao's btree management algorithm for
 *    Postgres.
 *
 * NOTES
 *    This file contains only the public interface routines.
 *
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/nbtree/nbtree.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/nbtree.h"
#include "access/nbtxlog.h"
#include "access/relscan.h"
#include "access/xlog.h"
#include "commands/progress.h"
#include "commands/vacuum.h"
#include "miscadmin.h"
#include "nodes/execnodes.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "storage/condition_variable.h"
#include "storage/indexfsm.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"
#include "utils/builtins.h"
#include "utils/index_selfuncs.h"
#include "utils/memutils.h"


/* Working state needed by btvacuumpage */
typedef struct
{
    IndexVacuumInfo *info;
    IndexBulkDeleteResult *stats;
    IndexBulkDeleteCallback callback;
    void       *callback_state;
    BTCycleId   cycleid;
    BlockNumber totFreePages;   /* true total # of free pages */
    TransactionId oldestBtpoXact;
    MemoryContext pagedelcontext;
} BTVacState;

/*
 * BTPARALLEL_NOT_INITIALIZED indicates that the scan has not started.
 *
 * BTPARALLEL_ADVANCING indicates that some process is advancing the scan to
 * a new page; others must wait.
 *
 * BTPARALLEL_IDLE indicates that no backend is currently advancing the scan
 * to a new page; some process can start doing that.
 *
 * BTPARALLEL_DONE indicates that the scan is complete (including error exit).
 * We reach this state once for every distinct combination of array keys.
 */
typedef enum
{
    BTPARALLEL_NOT_INITIALIZED,
    BTPARALLEL_ADVANCING,
    BTPARALLEL_IDLE,
    BTPARALLEL_DONE
} BTPS_State;

/*
 * BTParallelScanDescData contains btree specific shared information required
 * for parallel scan.
 */
typedef struct BTParallelScanDescData
{
    BlockNumber btps_scanPage;  /* latest or next page to be scanned */
    BTPS_State  btps_pageStatus;    /* indicates whether next page is
                                     * available for scan. see above for
                                     * possible states of parallel scan. */
    int         btps_arrayKeyCount; /* count indicating number of array scan
                                     * keys processed by parallel scan */
    slock_t     btps_mutex;     /* protects above variables */
    ConditionVariable btps_cv;  /* used to synchronize parallel scan */
}           BTParallelScanDescData;

typedef struct BTParallelScanDescData *BTParallelScanDesc;


static void btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
                         IndexBulkDeleteCallback callback, void *callback_state,
                         BTCycleId cycleid);
static void btvacuumpage(BTVacState *vstate, BlockNumber scanblkno);
static BTVacuumPosting btreevacuumposting(BTVacState *vstate,
                                          IndexTuple posting,
                                          OffsetNumber updatedoffset,
                                          int *nremaining);


/*
 * Btree handler function: return IndexAmRoutine with access method parameters
 * and callbacks.
 */
Datum
bthandler(PG_FUNCTION_ARGS)
{
    IndexAmRoutine *amroutine = makeNode(IndexAmRoutine);

    amroutine->amstrategies = BTMaxStrategyNumber;
    amroutine->amsupport = BTNProcs;
    amroutine->amoptsprocnum = BTOPTIONS_PROC;
    amroutine->amcanorder = true;
    amroutine->amcanorderbyop = false;
    amroutine->amcanbackward = true;
    amroutine->amcanunique = true;
    amroutine->amcanmulticol = true;
    amroutine->amoptionalkey = true;
    amroutine->amsearcharray = true;
    amroutine->amsearchnulls = true;
    amroutine->amstorage = false;
    amroutine->amclusterable = true;
    amroutine->ampredlocks = true;
    amroutine->amcanparallel = true;
    amroutine->amcaninclude = true;
    amroutine->amusemaintenanceworkmem = false;
    amroutine->amparallelvacuumoptions =
        VACUUM_OPTION_PARALLEL_BULKDEL | VACUUM_OPTION_PARALLEL_COND_CLEANUP;
    amroutine->amkeytype = InvalidOid;

    amroutine->ambuild = btbuild;
    amroutine->ambuildempty = btbuildempty;
    amroutine->aminsert = btinsert;
    amroutine->ambulkdelete = btbulkdelete;
    amroutine->amvacuumcleanup = btvacuumcleanup;
    amroutine->amcanreturn = btcanreturn;
    amroutine->amcostestimate = btcostestimate;
    amroutine->amoptions = btoptions;
    amroutine->amproperty = btproperty;
    amroutine->ambuildphasename = btbuildphasename;
    amroutine->amvalidate = btvalidate;
    amroutine->ambeginscan = btbeginscan;
    amroutine->amrescan = btrescan;
    amroutine->amgettuple = btgettuple;
    amroutine->amgetbitmap = btgetbitmap;
    amroutine->amendscan = btendscan;
    amroutine->ammarkpos = btmarkpos;
    amroutine->amrestrpos = btrestrpos;
    amroutine->amestimateparallelscan = btestimateparallelscan;
    amroutine->aminitparallelscan = btinitparallelscan;
    amroutine->amparallelrescan = btparallelrescan;

    PG_RETURN_POINTER(amroutine);
}

/*
 *  btbuildempty() -- build an empty btree index in the initialization fork
 */
void
btbuildempty(Relation index)
{
    Page        metapage;

    /* Construct metapage. */
    metapage = (Page) palloc(BLCKSZ);
    _bt_initmetapage(metapage, P_NONE, 0, _bt_allequalimage(index, false));

    /*
     * Write the page and log it.  It might seem that an immediate sync would
     * be sufficient to guarantee that the file exists on disk, but recovery
     * itself might remove it while replaying, for example, an
     * XLOG_DBASE_CREATE or XLOG_TBLSPC_CREATE record.  Therefore, we need
     * this even when wal_level=minimal.
     */
    PageSetChecksumInplace(metapage, BTREE_METAPAGE);
    smgrwrite(index->rd_smgr, INIT_FORKNUM, BTREE_METAPAGE,
              (char *) metapage, true);
    log_newpage(&index->rd_smgr->smgr_rnode.node, INIT_FORKNUM,
                BTREE_METAPAGE, metapage, true);

    /*
     * An immediate sync is required even if we xlog'd the page, because the
     * write did not go through shared_buffers and therefore a concurrent
     * checkpoint may have moved the redo pointer past our xlog record.
     */
    smgrimmedsync(index->rd_smgr, INIT_FORKNUM);
}

/*
 *  btinsert() -- insert an index tuple into a btree.
 *
 *      Descend the tree recursively, find the appropriate location for our
 *      new tuple, and put it there.
 */
bool
btinsert(Relation rel, Datum *values, bool *isnull,
         ItemPointer ht_ctid, Relation heapRel,
         IndexUniqueCheck checkUnique,
         IndexInfo *indexInfo)
{
    bool        result;
    IndexTuple  itup;

    /* generate an index tuple */
    itup = index_form_tuple(RelationGetDescr(rel), values, isnull);
    itup->t_tid = *ht_ctid;

    result = _bt_doinsert(rel, itup, checkUnique, heapRel);

    pfree(itup);

    return result;
}

/*
 *  btgettuple() -- Get the next tuple in the scan.
 */
bool
btgettuple(IndexScanDesc scan, ScanDirection dir)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    bool        res;

    /* btree indexes are never lossy */
    scan->xs_recheck = false;

    /*
     * If we have any array keys, initialize them during first call for a
     * scan.  We can't do this in btrescan because we don't know the scan
     * direction at that time.
     */
    if (so->numArrayKeys && !BTScanPosIsValid(so->currPos))
    {
        /* punt if we have any unsatisfiable array keys */
        if (so->numArrayKeys < 0)
            return false;

        _bt_start_array_keys(scan, dir);
    }

    /* This loop handles advancing to the next array elements, if any */
    do
    {
        /*
         * If we've already initialized this scan, we can just advance it in
         * the appropriate direction.  If we haven't done so yet, we call
         * _bt_first() to get the first item in the scan.
         */
        if (!BTScanPosIsValid(so->currPos))
            res = _bt_first(scan, dir);
        else
        {
            /*
             * Check to see if we should kill the previously-fetched tuple.
             */
            if (scan->kill_prior_tuple)
            {
                /*
                 * Yes, remember it for later. (We'll deal with all such
                 * tuples at once right before leaving the index page.)  The
                 * test for numKilled overrun is not just paranoia: if the
                 * caller reverses direction in the indexscan then the same
                 * item might get entered multiple times. It's not worth
                 * trying to optimize that, so we don't detect it, but instead
                 * just forget any excess entries.
                 */
                if (so->killedItems == NULL)
                    so->killedItems = (int *)
                        palloc(MaxTIDsPerBTreePage * sizeof(int));
                if (so->numKilled < MaxTIDsPerBTreePage)
                    so->killedItems[so->numKilled++] = so->currPos.itemIndex;
            }

            /*
             * Now continue the scan.
             */
            res = _bt_next(scan, dir);
        }

        /* If we have a tuple, return it ... */
        if (res)
            break;
        /* ... otherwise see if we have more array keys to deal with */
    } while (so->numArrayKeys && _bt_advance_array_keys(scan, dir));

    return res;
}

/*
 * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
 */
int64
btgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    int64       ntids = 0;
    ItemPointer heapTid;

    /*
     * If we have any array keys, initialize them.
     */
    if (so->numArrayKeys)
    {
        /* punt if we have any unsatisfiable array keys */
        if (so->numArrayKeys < 0)
            return ntids;

        _bt_start_array_keys(scan, ForwardScanDirection);
    }

    /* This loop handles advancing to the next array elements, if any */
    do
    {
        /* Fetch the first page & tuple */
        if (_bt_first(scan, ForwardScanDirection))
        {
            /* Save tuple ID, and continue scanning */
            heapTid = &scan->xs_heaptid;
            tbm_add_tuples(tbm, heapTid, 1, false);
            ntids++;

            for (;;)
            {
                /*
                 * Advance to next tuple within page.  This is the same as the
                 * easy case in _bt_next().
                 */
                if (++so->currPos.itemIndex > so->currPos.lastItem)
                {
                    /* let _bt_next do the heavy lifting */
                    if (!_bt_next(scan, ForwardScanDirection))
                        break;
                }

                /* Save tuple ID, and continue scanning */
                heapTid = &so->currPos.items[so->currPos.itemIndex].heapTid;
                tbm_add_tuples(tbm, heapTid, 1, false);
                ntids++;
            }
        }
        /* Now see if we have more array keys to deal with */
    } while (so->numArrayKeys && _bt_advance_array_keys(scan, ForwardScanDirection));

    return ntids;
}

/*
 *  btbeginscan() -- start a scan on a btree index
 */
IndexScanDesc
btbeginscan(Relation rel, int nkeys, int norderbys)
{
    IndexScanDesc scan;
    BTScanOpaque so;

    /* no order by operators allowed */
    Assert(norderbys == 0);

    /* get the scan */
    scan = RelationGetIndexScan(rel, nkeys, norderbys);

    /* allocate private workspace */
    so = (BTScanOpaque) palloc(sizeof(BTScanOpaqueData));
    BTScanPosInvalidate(so->currPos);
    BTScanPosInvalidate(so->markPos);
    if (scan->numberOfKeys > 0)
        so->keyData = (ScanKey) palloc(scan->numberOfKeys * sizeof(ScanKeyData));
    else
        so->keyData = NULL;

    so->arrayKeyData = NULL;    /* assume no array keys for now */
    so->numArrayKeys = 0;
    so->arrayKeys = NULL;
    so->arrayContext = NULL;

    so->killedItems = NULL;     /* until needed */
    so->numKilled = 0;

    /*
     * We don't know yet whether the scan will be index-only, so we do not
     * allocate the tuple workspace arrays until btrescan.  However, we set up
     * scan->xs_itupdesc whether we'll need it or not, since that's so cheap.
     */
    so->currTuples = so->markTuples = NULL;

    scan->xs_itupdesc = RelationGetDescr(rel);

    scan->opaque = so;

    return scan;
}

/*
 *  btrescan() -- rescan an index relation
 */
void
btrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
         ScanKey orderbys, int norderbys)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;

    /* we aren't holding any read locks, but gotta drop the pins */
    if (BTScanPosIsValid(so->currPos))
    {
        /* Before leaving current page, deal with any killed items */
        if (so->numKilled > 0)
            _bt_killitems(scan);
        BTScanPosUnpinIfPinned(so->currPos);
        BTScanPosInvalidate(so->currPos);
    }

    so->markItemIndex = -1;
    so->arrayKeyCount = 0;
    BTScanPosUnpinIfPinned(so->markPos);
    BTScanPosInvalidate(so->markPos);

    /*
     * Allocate tuple workspace arrays, if needed for an index-only scan and
     * not already done in a previous rescan call.  To save on palloc
     * overhead, both workspaces are allocated as one palloc block; only this
     * function and btendscan know that.
     *
     * NOTE: this data structure also makes it safe to return data from a
     * "name" column, even though btree name_ops uses an underlying storage
     * datatype of cstring.  The risk there is that "name" is supposed to be
     * padded to NAMEDATALEN, but the actual index tuple is probably shorter.
     * However, since we only return data out of tuples sitting in the
     * currTuples array, a fetch of NAMEDATALEN bytes can at worst pull some
     * data out of the markTuples array --- running off the end of memory for
     * a SIGSEGV is not possible.  Yeah, this is ugly as sin, but it beats
     * adding special-case treatment for name_ops elsewhere.
     */
    if (scan->xs_want_itup && so->currTuples == NULL)
    {
        so->currTuples = (char *) palloc(BLCKSZ * 2);
        so->markTuples = so->currTuples + BLCKSZ;
    }

    /*
     * Reset the scan keys. Note that keys ordering stuff moved to _bt_first.
     * - vadim 05/05/97
     */
    if (scankey && scan->numberOfKeys > 0)
        memmove(scan->keyData,
                scankey,
                scan->numberOfKeys * sizeof(ScanKeyData));
    so->numberOfKeys = 0;       /* until _bt_preprocess_keys sets it */

    /* If any keys are SK_SEARCHARRAY type, set up array-key info */
    _bt_preprocess_array_keys(scan);
}

/*
 *  btendscan() -- close down a scan
 */
void
btendscan(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;

    /* we aren't holding any read locks, but gotta drop the pins */
    if (BTScanPosIsValid(so->currPos))
    {
        /* Before leaving current page, deal with any killed items */
        if (so->numKilled > 0)
            _bt_killitems(scan);
        BTScanPosUnpinIfPinned(so->currPos);
    }

    so->markItemIndex = -1;
    BTScanPosUnpinIfPinned(so->markPos);

    /* No need to invalidate positions, the RAM is about to be freed. */

    /* Release storage */
    if (so->keyData != NULL)
        pfree(so->keyData);
    /* so->arrayKeyData and so->arrayKeys are in arrayContext */
    if (so->arrayContext != NULL)
        MemoryContextDelete(so->arrayContext);
    if (so->killedItems != NULL)
        pfree(so->killedItems);
    if (so->currTuples != NULL)
        pfree(so->currTuples);
    /* so->markTuples should not be pfree'd, see btrescan */
    pfree(so);
}

/*
 *  btmarkpos() -- save current scan position
 */
void
btmarkpos(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;

    /* There may be an old mark with a pin (but no lock). */
    BTScanPosUnpinIfPinned(so->markPos);

    /*
     * Just record the current itemIndex.  If we later step to next page
     * before releasing the marked position, _bt_steppage makes a full copy of
     * the currPos struct in markPos.  If (as often happens) the mark is moved
     * before we leave the page, we don't have to do that work.
     */
    if (BTScanPosIsValid(so->currPos))
        so->markItemIndex = so->currPos.itemIndex;
    else
    {
        BTScanPosInvalidate(so->markPos);
        so->markItemIndex = -1;
    }

    /* Also record the current positions of any array keys */
    if (so->numArrayKeys)
        _bt_mark_array_keys(scan);
}

/*
 *  btrestrpos() -- restore scan to last saved position
 */
void
btrestrpos(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;

    /* Restore the marked positions of any array keys */
    if (so->numArrayKeys)
        _bt_restore_array_keys(scan);

    if (so->markItemIndex >= 0)
    {
        /*
         * The scan has never moved to a new page since the last mark.  Just
         * restore the itemIndex.
         *
         * NB: In this case we can't count on anything in so->markPos to be
         * accurate.
         */
        so->currPos.itemIndex = so->markItemIndex;
    }
    else
    {
        /*
         * The scan moved to a new page after last mark or restore, and we are
         * now restoring to the marked page.  We aren't holding any read
         * locks, but if we're still holding the pin for the current position,
         * we must drop it.
         */
        if (BTScanPosIsValid(so->currPos))
        {
            /* Before leaving current page, deal with any killed items */
            if (so->numKilled > 0)
                _bt_killitems(scan);
            BTScanPosUnpinIfPinned(so->currPos);
        }

        if (BTScanPosIsValid(so->markPos))
        {
            /* bump pin on mark buffer for assignment to current buffer */
            if (BTScanPosIsPinned(so->markPos))
                IncrBufferRefCount(so->markPos.buf);
            memcpy(&so->currPos, &so->markPos,
                   offsetof(BTScanPosData, items[1]) +
                   so->markPos.lastItem * sizeof(BTScanPosItem));
            if (so->currTuples)
                memcpy(so->currTuples, so->markTuples,
                       so->markPos.nextTupleOffset);
        }
        else
            BTScanPosInvalidate(so->currPos);
    }
}

/*
 * btestimateparallelscan -- estimate storage for BTParallelScanDescData
 */
Size
btestimateparallelscan(void)
{
    return sizeof(BTParallelScanDescData);
}

/*
 * btinitparallelscan -- initialize BTParallelScanDesc for parallel btree scan
 */
void
btinitparallelscan(void *target)
{
    BTParallelScanDesc bt_target = (BTParallelScanDesc) target;

    SpinLockInit(&bt_target->btps_mutex);
    bt_target->btps_scanPage = InvalidBlockNumber;
    bt_target->btps_pageStatus = BTPARALLEL_NOT_INITIALIZED;
    bt_target->btps_arrayKeyCount = 0;
    ConditionVariableInit(&bt_target->btps_cv);
}

/*
 *  btparallelrescan() -- reset parallel scan
 */
void
btparallelrescan(IndexScanDesc scan)
{
    BTParallelScanDesc btscan;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;

    Assert(parallel_scan);

    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);

    /*
     * In theory, we don't need to acquire the spinlock here, because there
     * shouldn't be any other workers running at this point, but we do so for
     * consistency.
     */
    SpinLockAcquire(&btscan->btps_mutex);
    btscan->btps_scanPage = InvalidBlockNumber;
    btscan->btps_pageStatus = BTPARALLEL_NOT_INITIALIZED;
    btscan->btps_arrayKeyCount = 0;
    SpinLockRelease(&btscan->btps_mutex);
}

/*
 * _bt_parallel_seize() -- Begin the process of advancing the scan to a new
 *      page.  Other scans must wait until we call _bt_parallel_release()
 *      or _bt_parallel_done().
 *
 * The return value is true if we successfully seized the scan and false
 * if we did not.  The latter case occurs if no pages remain for the current
 * set of scankeys.
 *
 * If the return value is true, *pageno returns the next or current page
 * of the scan (depending on the scan direction).  An invalid block number
 * means the scan hasn't yet started, and P_NONE means we've reached the end.
 * The first time a participating process reaches the last page, it will return
 * true and set *pageno to P_NONE; after that, further attempts to seize the
 * scan will return false.
 *
 * Callers should ignore the value of pageno if the return value is false.
 */
bool
_bt_parallel_seize(IndexScanDesc scan, BlockNumber *pageno)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    BTPS_State  pageStatus;
    bool        exit_loop = false;
    bool        status = true;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;

    *pageno = P_NONE;

    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);

    while (1)
    {
        SpinLockAcquire(&btscan->btps_mutex);
        pageStatus = btscan->btps_pageStatus;

        if (so->arrayKeyCount < btscan->btps_arrayKeyCount)
        {
            /* Parallel scan has already advanced to a new set of scankeys. */
            status = false;
        }
        else if (pageStatus == BTPARALLEL_DONE)
        {
            /*
             * We're done with this set of scankeys.  This may be the end, or
             * there could be more sets to try.
             */
            status = false;
        }
        else if (pageStatus != BTPARALLEL_ADVANCING)
        {
            /*
             * We have successfully seized control of the scan for the purpose
             * of advancing it to a new page!
             */
            btscan->btps_pageStatus = BTPARALLEL_ADVANCING;
            *pageno = btscan->btps_scanPage;
            exit_loop = true;
        }
        SpinLockRelease(&btscan->btps_mutex);
        if (exit_loop || !status)
            break;
        ConditionVariableSleep(&btscan->btps_cv, WAIT_EVENT_BTREE_PAGE);
    }
    ConditionVariableCancelSleep();

    return status;
}

/*
 * _bt_parallel_release() -- Complete the process of advancing the scan to a
 *      new page.  We now have the new value btps_scanPage; some other backend
 *      can now begin advancing the scan.
 */
void
_bt_parallel_release(IndexScanDesc scan, BlockNumber scan_page)
{
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;

    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);

    SpinLockAcquire(&btscan->btps_mutex);
    btscan->btps_scanPage = scan_page;
    btscan->btps_pageStatus = BTPARALLEL_IDLE;
    SpinLockRelease(&btscan->btps_mutex);
    ConditionVariableSignal(&btscan->btps_cv);
}

/*
 * _bt_parallel_done() -- Mark the parallel scan as complete.
 *
 * When there are no pages left to scan, this function should be called to
 * notify other workers.  Otherwise, they might wait forever for the scan to
 * advance to the next page.
 */
void
_bt_parallel_done(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;
    bool        status_changed = false;

    /* Do nothing, for non-parallel scans */
    if (parallel_scan == NULL)
        return;

    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);

    /*
     * Mark the parallel scan as done for this combination of scan keys,
     * unless some other process already did so.  See also
     * _bt_advance_array_keys.
     */
    SpinLockAcquire(&btscan->btps_mutex);
    if (so->arrayKeyCount >= btscan->btps_arrayKeyCount &&
        btscan->btps_pageStatus != BTPARALLEL_DONE)
    {
        btscan->btps_pageStatus = BTPARALLEL_DONE;
        status_changed = true;
    }
    SpinLockRelease(&btscan->btps_mutex);

    /* wake up all the workers associated with this parallel scan */
    if (status_changed)
        ConditionVariableBroadcast(&btscan->btps_cv);
}

/*
 * _bt_parallel_advance_array_keys() -- Advances the parallel scan for array
 *          keys.
 *
 * Updates the count of array keys processed for both local and parallel
 * scans.
 */
void
_bt_parallel_advance_array_keys(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;

    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);

    so->arrayKeyCount++;
    SpinLockAcquire(&btscan->btps_mutex);
    if (btscan->btps_pageStatus == BTPARALLEL_DONE)
    {
        btscan->btps_scanPage = InvalidBlockNumber;
        btscan->btps_pageStatus = BTPARALLEL_NOT_INITIALIZED;
        btscan->btps_arrayKeyCount++;
    }
    SpinLockRelease(&btscan->btps_mutex);
}

/*
 * _bt_vacuum_needs_cleanup() -- Checks if index needs cleanup
 *
 * Called by btvacuumcleanup when btbulkdelete was never called because no
 * tuples need to be deleted.
 *
 * When we return false, VACUUM can even skip the cleanup-only call to
 * btvacuumscan (i.e. there will be no btvacuumscan call for this index at
 * all).  Otherwise, a cleanup-only btvacuumscan call is required.
 */
static bool
_bt_vacuum_needs_cleanup(IndexVacuumInfo *info)
{
    Buffer      metabuf;
    Page        metapg;
    BTMetaPageData *metad;
    bool        result = false;

    metabuf = _bt_getbuf(info->index, BTREE_METAPAGE, BT_READ);
    metapg = BufferGetPage(metabuf);
    metad = BTPageGetMeta(metapg);

    if (metad->btm_version < BTREE_NOVAC_VERSION)
    {
        /*
         * Do cleanup if metapage needs upgrade, because we don't have
         * cleanup-related meta-information yet.
         */
        result = true;
    }
    else if (TransactionIdIsValid(metad->btm_oldest_btpo_xact) &&
             TransactionIdPrecedes(metad->btm_oldest_btpo_xact,
                                   RecentGlobalXmin))
    {
        /*
         * If any oldest btpo.xact from a previously deleted page in the index
         * is older than RecentGlobalXmin, then at least one deleted page can
         * be recycled -- don't skip cleanup.
         */
        result = true;
    }
    else
    {
        BTOptions  *relopts;
        float8      cleanup_scale_factor;
        float8      prev_num_heap_tuples;

        /*
         * If table receives enough insertions and no cleanup was performed,
         * then index would appear have stale statistics.  If scale factor is
         * set, we avoid that by performing cleanup if the number of inserted
         * tuples exceeds vacuum_cleanup_index_scale_factor fraction of
         * original tuples count.
         */
        relopts = (BTOptions *) info->index->rd_options;
        cleanup_scale_factor = (relopts &&
                                relopts->vacuum_cleanup_index_scale_factor >= 0)
            ? relopts->vacuum_cleanup_index_scale_factor
            : vacuum_cleanup_index_scale_factor;
        prev_num_heap_tuples = metad->btm_last_cleanup_num_heap_tuples;

        if (cleanup_scale_factor <= 0 ||
            prev_num_heap_tuples <= 0 ||
            (info->num_heap_tuples - prev_num_heap_tuples) /
            prev_num_heap_tuples >= cleanup_scale_factor)
            result = true;
    }

    _bt_relbuf(info->index, metabuf);
    return result;
}

/*
 * Bulk deletion of all index entries pointing to a set of heap tuples.
 * The set of target tuples is specified via a callback routine that tells
 * whether any given heap tuple (identified by ItemPointer) is being deleted.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
IndexBulkDeleteResult *
btbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
             IndexBulkDeleteCallback callback, void *callback_state)
{
    Relation    rel = info->index;
    BTCycleId   cycleid;

    /* allocate stats if first time through, else re-use existing struct */
    if (stats == NULL)
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));

    /* Establish the vacuum cycle ID to use for this scan */
    /* The ENSURE stuff ensures we clean up shared memory on failure */
    PG_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
    {
        cycleid = _bt_start_vacuum(rel);

        btvacuumscan(info, stats, callback, callback_state, cycleid);
    }
    PG_END_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
    _bt_end_vacuum(rel);

    return stats;
}

/*
 * Post-VACUUM cleanup.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
IndexBulkDeleteResult *
btvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
{
    /* No-op in ANALYZE ONLY mode */
    if (info->analyze_only)
        return stats;

    /*
     * If btbulkdelete was called, we need not do anything, just return the
     * stats from the latest btbulkdelete call.  If it wasn't called, we might
     * still need to do a pass over the index, to recycle any newly-recyclable
     * pages or to obtain index statistics.  _bt_vacuum_needs_cleanup
     * determines if either are needed.
     *
     * Since we aren't going to actually delete any leaf items, there's no
     * need to go through all the vacuum-cycle-ID pushups.
     */
    if (stats == NULL)
    {
        /* Check if we need a cleanup */
        if (!_bt_vacuum_needs_cleanup(info))
            return NULL;

        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
        btvacuumscan(info, stats, NULL, NULL, 0);
    }

    /*
     * It's quite possible for us to be fooled by concurrent page splits into
     * double-counting some index tuples, so disbelieve any total that exceeds
     * the underlying heap's count ... if we know that accurately.  Otherwise
     * this might just make matters worse.
     */
    if (!info->estimated_count)
    {
        if (stats->num_index_tuples > info->num_heap_tuples)
            stats->num_index_tuples = info->num_heap_tuples;
    }

    return stats;
}

/*
 * btvacuumscan --- scan the index for VACUUMing purposes
 *
 * This combines the functions of looking for leaf tuples that are deletable
 * according to the vacuum callback, looking for empty pages that can be
 * deleted, and looking for old deleted pages that can be recycled.  Both
 * btbulkdelete and btvacuumcleanup invoke this (the latter only if no
 * btbulkdelete call occurred and _bt_vacuum_needs_cleanup returned true).
 * Note that this is also where the metadata used by _bt_vacuum_needs_cleanup
 * is maintained.
 *
 * The caller is responsible for initially allocating/zeroing a stats struct
 * and for obtaining a vacuum cycle ID if necessary.
 */
static void
btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
             IndexBulkDeleteCallback callback, void *callback_state,
             BTCycleId cycleid)
{
    Relation    rel = info->index;
    BTVacState  vstate;
    BlockNumber num_pages;
    BlockNumber scanblkno;
    bool        needLock;

    /*
     * Reset counts that will be incremented during the scan; needed in case
     * of multiple scans during a single VACUUM command
     */
    stats->estimated_count = false;
    stats->num_index_tuples = 0;
    stats->pages_deleted = 0;

    /* Set up info to pass down to btvacuumpage */
    vstate.info = info;
    vstate.stats = stats;
    vstate.callback = callback;
    vstate.callback_state = callback_state;
    vstate.cycleid = cycleid;
    vstate.totFreePages = 0;
    vstate.oldestBtpoXact = InvalidTransactionId;

    /* Create a temporary memory context to run _bt_pagedel in */
    vstate.pagedelcontext = AllocSetContextCreate(CurrentMemoryContext,
                                                  "_bt_pagedel",
                                                  ALLOCSET_DEFAULT_SIZES);

    /*
     * The outer loop iterates over all index pages except the metapage, in
     * physical order (we hope the kernel will cooperate in providing
     * read-ahead for speed).  It is critical that we visit all leaf pages,
     * including ones added after we start the scan, else we might fail to
     * delete some deletable tuples.  Hence, we must repeatedly check the
     * relation length.  We must acquire the relation-extension lock while
     * doing so to avoid a race condition: if someone else is extending the
     * relation, there is a window where bufmgr/smgr have created a new
     * all-zero page but it hasn't yet been write-locked by _bt_getbuf(). If
     * we manage to scan such a page here, we'll improperly assume it can be
     * recycled.  Taking the lock synchronizes things enough to prevent a
     * problem: either num_pages won't include the new page, or _bt_getbuf
     * already has write lock on the buffer and it will be fully initialized
     * before we can examine it.  (See also vacuumlazy.c, which has the same
     * issue.)  Also, we need not worry if a page is added immediately after
     * we look; the page splitting code already has write-lock on the left
     * page before it adds a right page, so we must already have processed any
     * tuples due to be moved into such a page.
     *
     * We can skip locking for new or temp relations, however, since no one
     * else could be accessing them.
     */
    needLock = !RELATION_IS_LOCAL(rel);

    scanblkno = BTREE_METAPAGE + 1;
    for (;;)
    {
        /* Get the current relation length */
        if (needLock)
            LockRelationForExtension(rel, ExclusiveLock);
        num_pages = RelationGetNumberOfBlocks(rel);
        if (needLock)
            UnlockRelationForExtension(rel, ExclusiveLock);

        if (info->report_progress)
            pgstat_progress_update_param(PROGRESS_SCAN_BLOCKS_TOTAL,
                                         num_pages);

        /* Quit if we've scanned the whole relation */
        if (scanblkno >= num_pages)
            break;
        /* Iterate over pages, then loop back to recheck length */
        for (; scanblkno < num_pages; scanblkno++)
        {
            btvacuumpage(&vstate, scanblkno);
            if (info->report_progress)
                pgstat_progress_update_param(PROGRESS_SCAN_BLOCKS_DONE,
                                             scanblkno);
        }
    }

    MemoryContextDelete(vstate.pagedelcontext);

    /*
     * If we found any recyclable pages (and recorded them in the FSM), then
     * forcibly update the upper-level FSM pages to ensure that searchers can
     * find them.  It's possible that the pages were also found during
     * previous scans and so this is a waste of time, but it's cheap enough
     * relative to scanning the index that it shouldn't matter much, and
     * making sure that free pages are available sooner not later seems
     * worthwhile.
     *
     * Note that if no recyclable pages exist, we don't bother vacuuming the
     * FSM at all.
     */
    if (vstate.totFreePages > 0)
        IndexFreeSpaceMapVacuum(rel);

    /*
     * Maintain the oldest btpo.xact and a count of the current number of heap
     * tuples in the metapage (for the benefit of _bt_vacuum_needs_cleanup).
     *
     * The page with the oldest btpo.xact is typically a page deleted by this
     * VACUUM operation, since pages deleted by a previous VACUUM operation
     * tend to be placed in the FSM (by the current VACUUM operation) -- such
     * pages are not candidates to be the oldest btpo.xact.  (Note that pages
     * placed in the FSM are reported as deleted pages in the bulk delete
     * statistics, despite not counting as deleted pages for the purposes of
     * determining the oldest btpo.xact.)
     */
    _bt_update_meta_cleanup_info(rel, vstate.oldestBtpoXact,
                                 info->num_heap_tuples);

    /* update statistics */
    stats->num_pages = num_pages;
    stats->pages_free = vstate.totFreePages;
}

/*
 * btvacuumpage --- VACUUM one page
 *
 * This processes a single page for btvacuumscan().  In some cases we must
 * backtrack to re-examine and VACUUM pages that were the scanblkno during
 * a previous call here.  This is how we handle page splits (that happened
 * after our cycleid was acquired) whose right half page happened to reuse
 * a block that we might have processed at some point before it was
 * recycled (i.e. before the page split).
 */
static void
btvacuumpage(BTVacState *vstate, BlockNumber scanblkno)
{
    IndexVacuumInfo *info = vstate->info;
    IndexBulkDeleteResult *stats = vstate->stats;
    IndexBulkDeleteCallback callback = vstate->callback;
    void       *callback_state = vstate->callback_state;
    Relation    rel = info->index;
    bool        attempt_pagedel;
    BlockNumber blkno,
                backtrack_to;
    Buffer      buf;
    Page        page;
    BTPageOpaque opaque;

    blkno = scanblkno;

backtrack:

    attempt_pagedel = false;
    backtrack_to = P_NONE;

    /* call vacuum_delay_point while not holding any buffer lock */
    vacuum_delay_point();

    /*
     * We can't use _bt_getbuf() here because it always applies
     * _bt_checkpage(), which will barf on an all-zero page. We want to
     * recycle all-zero pages, not fail.  Also, we want to use a nondefault
     * buffer access strategy.
     */
    buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
                             info->strategy);
    LockBuffer(buf, BT_READ);
    page = BufferGetPage(buf);
    opaque = NULL;
    if (!PageIsNew(page))
    {
        _bt_checkpage(rel, buf);
        opaque = (BTPageOpaque) PageGetSpecialPointer(page);
    }

    Assert(blkno <= scanblkno);
    if (blkno != scanblkno)
    {
        /*
         * We're backtracking.
         *
         * We followed a right link to a sibling leaf page (a page that
         * happens to be from a block located before scanblkno).  The only
         * case we want to do anything with is a live leaf page having the
         * current vacuum cycle ID.
         *
         * The page had better be in a state that's consistent with what we
         * expect.  Check for conditions that imply corruption in passing.  It
         * can't be half-dead because only an interrupted VACUUM process can
         * leave pages in that state, so we'd definitely have dealt with it
         * back when the page was the scanblkno page (half-dead pages are
         * always marked fully deleted by _bt_pagedel()).  This assumes that
         * there can be only one vacuum process running at a time.
         */
        if (!opaque || !P_ISLEAF(opaque) || P_ISHALFDEAD(opaque))
        {
            Assert(false);
            ereport(LOG,
                    (errcode(ERRCODE_INDEX_CORRUPTED),
                     errmsg_internal("right sibling %u of scanblkno %u unexpectedly in an inconsistent state in index \"%s\"",
                                     blkno, scanblkno, RelationGetRelationName(rel))));
            _bt_relbuf(rel, buf);
            return;
        }

        /*
         * We may have already processed the page in an earlier call, when the
         * page was scanblkno.  This happens when the leaf page split occurred
         * after the scan began, but before the right sibling page became the
         * scanblkno.
         *
         * Page may also have been deleted by current btvacuumpage() call,
         * since _bt_pagedel() sometimes deletes the right sibling page of
         * scanblkno in passing (it does so after we decided where to
         * backtrack to).  We don't need to process this page as a deleted
         * page a second time now (in fact, it would be wrong to count it as a
         * deleted page in the bulk delete statistics a second time).
         */
        if (opaque->btpo_cycleid != vstate->cycleid || P_ISDELETED(opaque))
        {
            /* Done with current scanblkno (and all lower split pages) */
            _bt_relbuf(rel, buf);
            return;
        }
    }

    /* Page is valid, see what to do with it */
    if (_bt_page_recyclable(page))
    {
        /* Okay to recycle this page (which could be leaf or internal) */
        RecordFreeIndexPage(rel, blkno);
        vstate->totFreePages++;
        stats->pages_deleted++;
    }
    else if (P_ISDELETED(opaque))
    {
        /*
         * Already deleted page (which could be leaf or internal).  Can't
         * recycle yet.
         */
        stats->pages_deleted++;

        /* Maintain the oldest btpo.xact */
        if (!TransactionIdIsValid(vstate->oldestBtpoXact) ||
            TransactionIdPrecedes(opaque->btpo.xact, vstate->oldestBtpoXact))
            vstate->oldestBtpoXact = opaque->btpo.xact;
    }
    else if (P_ISHALFDEAD(opaque))
    {
        /*
         * Half-dead leaf page.  Try to delete now.  Might update
         * oldestBtpoXact and pages_deleted below.
         */
        attempt_pagedel = true;
    }
    else if (P_ISLEAF(opaque))
    {
        OffsetNumber deletable[MaxIndexTuplesPerPage];
        int         ndeletable;
        BTVacuumPosting updatable[MaxIndexTuplesPerPage];
        int         nupdatable;
        OffsetNumber offnum,
                    minoff,
                    maxoff;
        int         nhtidsdead,
                    nhtidslive;

        /*
         * Trade in the initial read lock for a super-exclusive write lock on
         * this page.  We must get such a lock on every leaf page over the
         * course of the vacuum scan, whether or not it actually contains any
         * deletable tuples --- see nbtree/README.
         */
        LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        LockBufferForCleanup(buf);

        /*
         * Check whether we need to backtrack to earlier pages.  What we are
         * concerned about is a page split that happened since we started the
         * vacuum scan.  If the split moved tuples on the right half of the
         * split (i.e. the tuples that sort high) to a block that we already
         * passed over, then we might have missed the tuples.  We need to
         * backtrack now.  (Must do this before possibly clearing btpo_cycleid
         * or deleting scanblkno page below!)
         */
        if (vstate->cycleid != 0 &&
            opaque->btpo_cycleid == vstate->cycleid &&
            !(opaque->btpo_flags & BTP_SPLIT_END) &&
            !P_RIGHTMOST(opaque) &&
            opaque->btpo_next < scanblkno)
            backtrack_to = opaque->btpo_next;

        /*
         * When each VACUUM begins, it determines an OldestXmin cutoff value.
         * Tuples before the cutoff are removed by VACUUM.  Scan over all
         * items to see which ones need to be deleted according to cutoff
         * point using callback.
         */
        ndeletable = 0;
        nupdatable = 0;
        minoff = P_FIRSTDATAKEY(opaque);
        maxoff = PageGetMaxOffsetNumber(page);
        nhtidsdead = 0;
        nhtidslive = 0;
        if (callback)
        {
            for (offnum = minoff;
                 offnum <= maxoff;
                 offnum = OffsetNumberNext(offnum))
            {
                IndexTuple  itup;

                itup = (IndexTuple) PageGetItem(page,
                                                PageGetItemId(page, offnum));

                /*
                 * Hot Standby assumes that it's okay that XLOG_BTREE_VACUUM
                 * records do not produce their own conflicts.  This is safe
                 * as long as the callback function only considers whether the
                 * index tuple refers to pre-cutoff heap tuples that were
                 * certainly already pruned away during VACUUM's initial heap
                 * scan by the time we get here. (XLOG_HEAP2_CLEANUP_INFO
                 * records produce conflicts using a latestRemovedXid value
                 * for the entire VACUUM, so there is no need to produce our
                 * own conflict now.)
                 *
                 * Backends with snapshots acquired after a VACUUM starts but
                 * before it finishes could have a RecentGlobalXmin with a
                 * later xid than the VACUUM's OldestXmin cutoff.  These
                 * backends might happen to opportunistically mark some index
                 * tuples LP_DEAD before we reach them, even though they may
                 * be after our cutoff.  We don't try to kill these "extra"
                 * index tuples in _bt_delitems_vacuum().  This keep things
                 * simple, and allows us to always avoid generating our own
                 * conflicts.
                 */
                Assert(!BTreeTupleIsPivot(itup));
                if (!BTreeTupleIsPosting(itup))
                {
                    /* Regular tuple, standard table TID representation */
                    if (callback(&itup->t_tid, callback_state))
                    {
                        deletable[ndeletable++] = offnum;
                        nhtidsdead++;
                    }
                    else
                        nhtidslive++;
                }
                else
                {
                    BTVacuumPosting vacposting;
                    int         nremaining;

                    /* Posting list tuple */
                    vacposting = btreevacuumposting(vstate, itup, offnum,
                                                    &nremaining);
                    if (vacposting == NULL)
                    {
                        /*
                         * All table TIDs from the posting tuple remain, so no
                         * delete or update required
                         */
                        Assert(nremaining == BTreeTupleGetNPosting(itup));
                    }
                    else if (nremaining > 0)
                    {

                        /*
                         * Store metadata about posting list tuple in
                         * updatable array for entire page.  Existing tuple
                         * will be updated during the later call to
                         * _bt_delitems_vacuum().
                         */
                        Assert(nremaining < BTreeTupleGetNPosting(itup));
                        updatable[nupdatable++] = vacposting;
                        nhtidsdead += BTreeTupleGetNPosting(itup) - nremaining;
                    }
                    else
                    {
                        /*
                         * All table TIDs from the posting list must be
                         * deleted.  We'll delete the index tuple completely
                         * (no update required).
                         */
                        Assert(nremaining == 0);
                        deletable[ndeletable++] = offnum;
                        nhtidsdead += BTreeTupleGetNPosting(itup);
                        pfree(vacposting);
                    }

                    nhtidslive += nremaining;
                }
            }
        }

        /*
         * Apply any needed deletes or updates.  We issue just one
         * _bt_delitems_vacuum() call per page, so as to minimize WAL traffic.
         */
        if (ndeletable > 0 || nupdatable > 0)
        {
            Assert(nhtidsdead >= ndeletable + nupdatable);
            _bt_delitems_vacuum(rel, buf, deletable, ndeletable, updatable,
                                nupdatable);

            stats->tuples_removed += nhtidsdead;
            /* must recompute maxoff */
            maxoff = PageGetMaxOffsetNumber(page);

            /* can't leak memory here */
            for (int i = 0; i < nupdatable; i++)
                pfree(updatable[i]);
        }
        else
        {
            /*
             * If the leaf page has been split during this vacuum cycle, it
             * seems worth expending a write to clear btpo_cycleid even if we
             * don't have any deletions to do.  (If we do, _bt_delitems_vacuum
             * takes care of this.)  This ensures we won't process the page
             * again.
             *
             * We treat this like a hint-bit update because there's no need to
             * WAL-log it.
             */
            Assert(nhtidsdead == 0);
            if (vstate->cycleid != 0 &&
                opaque->btpo_cycleid == vstate->cycleid)
            {
                opaque->btpo_cycleid = 0;
                MarkBufferDirtyHint(buf, true);
            }
        }

        /*
         * If the leaf page is now empty, try to delete it; else count the
         * live tuples (live table TIDs in posting lists are counted as
         * separate live tuples).  We don't delete when backtracking, though,
         * since that would require teaching _bt_pagedel() about backtracking
         * (doesn't seem worth adding more complexity to deal with that).
         */
        if (minoff > maxoff)
            attempt_pagedel = (blkno == scanblkno);
        else
            stats->num_index_tuples += nhtidslive;

        Assert(!attempt_pagedel || nhtidslive == 0);
    }

    if (attempt_pagedel)
    {
        MemoryContext oldcontext;

        /* Run pagedel in a temp context to avoid memory leakage */
        MemoryContextReset(vstate->pagedelcontext);
        oldcontext = MemoryContextSwitchTo(vstate->pagedelcontext);

        /*
         * We trust the _bt_pagedel return value because it does not include
         * any page that a future call here from btvacuumscan is expected to
         * count.  There will be no double-counting.
         */
        Assert(blkno == scanblkno);
        stats->pages_deleted += _bt_pagedel(rel, buf, &vstate->oldestBtpoXact);

        MemoryContextSwitchTo(oldcontext);
        /* pagedel released buffer, so we shouldn't */
    }
    else
        _bt_relbuf(rel, buf);

    if (backtrack_to != P_NONE)
    {
        blkno = backtrack_to;
        goto backtrack;
    }
}

/*
 * btreevacuumposting --- determine TIDs still needed in posting list
 *
 * Returns metadata describing how to build replacement tuple without the TIDs
 * that VACUUM needs to delete.  Returned value is NULL in the common case
 * where no changes are needed to caller's posting list tuple (we avoid
 * allocating memory here as an optimization).
 *
 * The number of TIDs that should remain in the posting list tuple is set for
 * caller in *nremaining.
 */
static BTVacuumPosting
btreevacuumposting(BTVacState *vstate, IndexTuple posting,
                   OffsetNumber updatedoffset, int *nremaining)
{
    int         live = 0;
    int         nitem = BTreeTupleGetNPosting(posting);
    ItemPointer items = BTreeTupleGetPosting(posting);
    BTVacuumPosting vacposting = NULL;

    for (int i = 0; i < nitem; i++)
    {
        if (!vstate->callback(items + i, vstate->callback_state))
        {
            /* Live table TID */
            live++;
        }
        else if (vacposting == NULL)
        {
            /*
             * First dead table TID encountered.
             *
             * It's now clear that we need to delete one or more dead table
             * TIDs, so start maintaining metadata describing how to update
             * existing posting list tuple.
             */
            vacposting = palloc(offsetof(BTVacuumPostingData, deletetids) +
                                nitem * sizeof(uint16));

            vacposting->itup = posting;
            vacposting->updatedoffset = updatedoffset;
            vacposting->ndeletedtids = 0;
            vacposting->deletetids[vacposting->ndeletedtids++] = i;
        }
        else
        {
            /* Second or subsequent dead table TID */
            vacposting->deletetids[vacposting->ndeletedtids++] = i;
        }
    }

    *nremaining = live;
    return vacposting;
}

/*
 *  btcanreturn() -- Check whether btree indexes support index-only scans.
 *
 * btrees always do, so this is trivial.
 */
bool
btcanreturn(Relation index, int attno)
{
    return true;
}