hash.c

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/*-------------------------------------------------------------------------
 *
 * hash.c
 *    Implementation of Margo Seltzer's Hashing package for postgres.
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/access/hash/hash.c
 *
 * NOTES
 *    This file contains only the public interface routines.
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/hash.h"
#include "access/hash_xlog.h"
#include "access/relscan.h"
#include "access/tableam.h"
#include "catalog/index.h"
#include "commands/progress.h"
#include "commands/vacuum.h"
#include "miscadmin.h"
#include "optimizer/plancat.h"
#include "pgstat.h"
#include "utils/builtins.h"
#include "utils/index_selfuncs.h"
#include "utils/rel.h"

/* Working state for hashbuild and its callback */
typedef struct
{
    HSpool     *spool;          /* NULL if not using spooling */
    double      indtuples;      /* # tuples accepted into index */
    Relation    heapRel;        /* heap relation descriptor */
} HashBuildState;

static void hashbuildCallback(Relation index,
                              ItemPointer tid,
                              Datum *values,
                              bool *isnull,
                              bool tupleIsAlive,
                              void *state);


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

    amroutine->amstrategies = HTMaxStrategyNumber;
    amroutine->amsupport = HASHNProcs;
    amroutine->amcanorder = false;
    amroutine->amcanorderbyop = false;
    amroutine->amcanbackward = true;
    amroutine->amcanunique = false;
    amroutine->amcanmulticol = false;
    amroutine->amoptionalkey = false;
    amroutine->amsearcharray = false;
    amroutine->amsearchnulls = false;
    amroutine->amstorage = false;
    amroutine->amclusterable = false;
    amroutine->ampredlocks = true;
    amroutine->amcanparallel = false;
    amroutine->amcaninclude = false;
    amroutine->amusemaintenanceworkmem = false;
    amroutine->amparallelvacuumoptions =
        VACUUM_OPTION_PARALLEL_BULKDEL;
    amroutine->amkeytype = INT4OID;

    amroutine->ambuild = hashbuild;
    amroutine->ambuildempty = hashbuildempty;
    amroutine->aminsert = hashinsert;
    amroutine->ambulkdelete = hashbulkdelete;
    amroutine->amvacuumcleanup = hashvacuumcleanup;
    amroutine->amcanreturn = NULL;
    amroutine->amcostestimate = hashcostestimate;
    amroutine->amoptions = hashoptions;
    amroutine->amproperty = NULL;
    amroutine->ambuildphasename = NULL;
    amroutine->amvalidate = hashvalidate;
    amroutine->ambeginscan = hashbeginscan;
    amroutine->amrescan = hashrescan;
    amroutine->amgettuple = hashgettuple;
    amroutine->amgetbitmap = hashgetbitmap;
    amroutine->amendscan = hashendscan;
    amroutine->ammarkpos = NULL;
    amroutine->amrestrpos = NULL;
    amroutine->amestimateparallelscan = NULL;
    amroutine->aminitparallelscan = NULL;
    amroutine->amparallelrescan = NULL;

    PG_RETURN_POINTER(amroutine);
}

/*
 *  hashbuild() -- build a new hash index.
 */
IndexBuildResult *
hashbuild(Relation heap, Relation index, IndexInfo *indexInfo)
{
    IndexBuildResult *result;
    BlockNumber relpages;
    double      reltuples;
    double      allvisfrac;
    uint32      num_buckets;
    long        sort_threshold;
    HashBuildState buildstate;

    /*
     * 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));

    /* Estimate the number of rows currently present in the table */
    estimate_rel_size(heap, NULL, &relpages, &reltuples, &allvisfrac);

    /* Initialize the hash index metadata page and initial buckets */
    num_buckets = _hash_init(index, reltuples, MAIN_FORKNUM);

    /*
     * If we just insert the tuples into the index in scan order, then
     * (assuming their hash codes are pretty random) there will be no locality
     * of access to the index, and if the index is bigger than available RAM
     * then we'll thrash horribly.  To prevent that scenario, we can sort the
     * tuples by (expected) bucket number.  However, such a sort is useless
     * overhead when the index does fit in RAM.  We choose to sort if the
     * initial index size exceeds maintenance_work_mem, or the number of
     * buffers usable for the index, whichever is less.  (Limiting by the
     * number of buffers should reduce thrashing between PG buffers and kernel
     * buffers, which seems useful even if no physical I/O results.  Limiting
     * by maintenance_work_mem is useful to allow easy testing of the sort
     * code path, and may be useful to DBAs as an additional control knob.)
     *
     * NOTE: this test will need adjustment if a bucket is ever different from
     * one page.  Also, "initial index size" accounting does not include the
     * metapage, nor the first bitmap page.
     */
    sort_threshold = (maintenance_work_mem * 1024L) / BLCKSZ;
    if (index->rd_rel->relpersistence != RELPERSISTENCE_TEMP)
        sort_threshold = Min(sort_threshold, NBuffers);
    else
        sort_threshold = Min(sort_threshold, NLocBuffer);

    if (num_buckets >= (uint32) sort_threshold)
        buildstate.spool = _h_spoolinit(heap, index, num_buckets);
    else
        buildstate.spool = NULL;

    /* prepare to build the index */
    buildstate.indtuples = 0;
    buildstate.heapRel = heap;

    /* do the heap scan */
    reltuples = table_index_build_scan(heap, index, indexInfo, true, true,
                                       hashbuildCallback,
                                       (void *) &buildstate, NULL);
    pgstat_progress_update_param(PROGRESS_CREATEIDX_TUPLES_TOTAL,
                                 buildstate.indtuples);

    if (buildstate.spool)
    {
        /* sort the tuples and insert them into the index */
        _h_indexbuild(buildstate.spool, buildstate.heapRel);
        _h_spooldestroy(buildstate.spool);
    }

    /*
     * Return statistics
     */
    result = (IndexBuildResult *) palloc(sizeof(IndexBuildResult));

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

    return result;
}

/*
 *  hashbuildempty() -- build an empty hash index in the initialization fork
 */
void
hashbuildempty(Relation index)
{
    _hash_init(index, 0, INIT_FORKNUM);
}

/*
 * Per-tuple callback for table_index_build_scan
 */
static void
hashbuildCallback(Relation index,
                  ItemPointer tid,
                  Datum *values,
                  bool *isnull,
                  bool tupleIsAlive,
                  void *state)
{
    HashBuildState *buildstate = (HashBuildState *) state;
    Datum       index_values[1];
    bool        index_isnull[1];
    IndexTuple  itup;

    /* convert data to a hash key; on failure, do not insert anything */
    if (!_hash_convert_tuple(index,
                             values, isnull,
                             index_values, index_isnull))
        return;

    /* Either spool the tuple for sorting, or just put it into the index */
    if (buildstate->spool)
        _h_spool(buildstate->spool, tid, index_values, index_isnull);
    else
    {
        /* form an index tuple and point it at the heap tuple */
        itup = index_form_tuple(RelationGetDescr(index),
                                index_values, index_isnull);
        itup->t_tid = *tid;
        _hash_doinsert(index, itup, buildstate->heapRel);
        pfree(itup);
    }

    buildstate->indtuples += 1;
}

/*
 *  hashinsert() -- insert an index tuple into a hash table.
 *
 *  Hash on the heap tuple's key, form an index tuple with hash code.
 *  Find the appropriate location for the new tuple, and put it there.
 */
bool
hashinsert(Relation rel, Datum *values, bool *isnull,
           ItemPointer ht_ctid, Relation heapRel,
           IndexUniqueCheck checkUnique,
           IndexInfo *indexInfo)
{
    Datum       index_values[1];
    bool        index_isnull[1];
    IndexTuple  itup;

    /* convert data to a hash key; on failure, do not insert anything */
    if (!_hash_convert_tuple(rel,
                             values, isnull,
                             index_values, index_isnull))
        return false;

    /* form an index tuple and point it at the heap tuple */
    itup = index_form_tuple(RelationGetDescr(rel), index_values, index_isnull);
    itup->t_tid = *ht_ctid;

    _hash_doinsert(rel, itup, heapRel);

    pfree(itup);

    return false;
}


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

    /* Hash indexes are always lossy since we store only the hash code */
    scan->xs_recheck = true;

    /*
     * 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 a routine to
     * get the first item in the scan.
     */
    if (!HashScanPosIsValid(so->currPos))
        res = _hash_first(scan, dir);
    else
    {
        /*
         * Check to see if we should kill the previously-fetched tuple.
         */
        if (scan->kill_prior_tuple)
        {
            /*
             * Yes, so remember it for later. (We'll deal with all such tuples
             * at once right after leaving the index page or at end of scan.)
             * In case if caller reverses the indexscan direction it is quite
             * possible that the same item might get entered multiple times.
             * But, we don't detect that; instead, we just forget any excess
             * entries.
             */
            if (so->killedItems == NULL)
                so->killedItems = (int *)
                    palloc(MaxIndexTuplesPerPage * sizeof(int));

            if (so->numKilled < MaxIndexTuplesPerPage)
                so->killedItems[so->numKilled++] = so->currPos.itemIndex;
        }

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

    return res;
}


/*
 *  hashgetbitmap() -- get all tuples at once
 */
int64
hashgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
{
    HashScanOpaque so = (HashScanOpaque) scan->opaque;
    bool        res;
    int64       ntids = 0;
    HashScanPosItem *currItem;

    res = _hash_first(scan, ForwardScanDirection);

    while (res)
    {
        currItem = &so->currPos.items[so->currPos.itemIndex];

        /*
         * _hash_first and _hash_next handle eliminate dead index entries
         * whenever scan->ignore_killed_tuples is true.  Therefore, there's
         * nothing to do here except add the results to the TIDBitmap.
         */
        tbm_add_tuples(tbm, &(currItem->heapTid), 1, true);
        ntids++;

        res = _hash_next(scan, ForwardScanDirection);
    }

    return ntids;
}


/*
 *  hashbeginscan() -- start a scan on a hash index
 */
IndexScanDesc
hashbeginscan(Relation rel, int nkeys, int norderbys)
{
    IndexScanDesc scan;
    HashScanOpaque so;

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

    scan = RelationGetIndexScan(rel, nkeys, norderbys);

    so = (HashScanOpaque) palloc(sizeof(HashScanOpaqueData));
    HashScanPosInvalidate(so->currPos);
    so->hashso_bucket_buf = InvalidBuffer;
    so->hashso_split_bucket_buf = InvalidBuffer;

    so->hashso_buc_populated = false;
    so->hashso_buc_split = false;

    so->killedItems = NULL;
    so->numKilled = 0;

    scan->opaque = so;

    return scan;
}

/*
 *  hashrescan() -- rescan an index relation
 */
void
hashrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
           ScanKey orderbys, int norderbys)
{
    HashScanOpaque so = (HashScanOpaque) scan->opaque;
    Relation    rel = scan->indexRelation;

    if (HashScanPosIsValid(so->currPos))
    {
        /* Before leaving current page, deal with any killed items */
        if (so->numKilled > 0)
            _hash_kill_items(scan);
    }

    _hash_dropscanbuf(rel, so);

    /* set position invalid (this will cause _hash_first call) */
    HashScanPosInvalidate(so->currPos);

    /* Update scan key, if a new one is given */
    if (scankey && scan->numberOfKeys > 0)
    {
        memmove(scan->keyData,
                scankey,
                scan->numberOfKeys * sizeof(ScanKeyData));
    }

    so->hashso_buc_populated = false;
    so->hashso_buc_split = false;
}

/*
 *  hashendscan() -- close down a scan
 */
void
hashendscan(IndexScanDesc scan)
{
    HashScanOpaque so = (HashScanOpaque) scan->opaque;
    Relation    rel = scan->indexRelation;

    if (HashScanPosIsValid(so->currPos))
    {
        /* Before leaving current page, deal with any killed items */
        if (so->numKilled > 0)
            _hash_kill_items(scan);
    }

    _hash_dropscanbuf(rel, so);

    if (so->killedItems != NULL)
        pfree(so->killedItems);
    pfree(so);
    scan->opaque = NULL;
}

/*
 * 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.
 *
 * This function also deletes the tuples that are moved by split to other
 * bucket.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
IndexBulkDeleteResult *
hashbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
               IndexBulkDeleteCallback callback, void *callback_state)
{
    Relation    rel = info->index;
    double      tuples_removed;
    double      num_index_tuples;
    double      orig_ntuples;
    Bucket      orig_maxbucket;
    Bucket      cur_maxbucket;
    Bucket      cur_bucket;
    Buffer      metabuf = InvalidBuffer;
    HashMetaPage metap;
    HashMetaPage cachedmetap;

    tuples_removed = 0;
    num_index_tuples = 0;

    /*
     * We need a copy of the metapage so that we can use its hashm_spares[]
     * values to compute bucket page addresses, but a cached copy should be
     * good enough.  (If not, we'll detect that further down and refresh the
     * cache as necessary.)
     */
    cachedmetap = _hash_getcachedmetap(rel, &metabuf, false);
    Assert(cachedmetap != NULL);

    orig_maxbucket = cachedmetap->hashm_maxbucket;
    orig_ntuples = cachedmetap->hashm_ntuples;

    /* Scan the buckets that we know exist */
    cur_bucket = 0;
    cur_maxbucket = orig_maxbucket;

loop_top:
    while (cur_bucket <= cur_maxbucket)
    {
        BlockNumber bucket_blkno;
        BlockNumber blkno;
        Buffer      bucket_buf;
        Buffer      buf;
        HashPageOpaque bucket_opaque;
        Page        page;
        bool        split_cleanup = false;

        /* Get address of bucket's start page */
        bucket_blkno = BUCKET_TO_BLKNO(cachedmetap, cur_bucket);

        blkno = bucket_blkno;

        /*
         * We need to acquire a cleanup lock on the primary bucket page to out
         * wait concurrent scans before deleting the dead tuples.
         */
        buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL, info->strategy);
        LockBufferForCleanup(buf);
        _hash_checkpage(rel, buf, LH_BUCKET_PAGE);

        page = BufferGetPage(buf);
        bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);

        /*
         * If the bucket contains tuples that are moved by split, then we need
         * to delete such tuples.  We can't delete such tuples if the split
         * operation on bucket is not finished as those are needed by scans.
         */
        if (!H_BUCKET_BEING_SPLIT(bucket_opaque) &&
            H_NEEDS_SPLIT_CLEANUP(bucket_opaque))
        {
            split_cleanup = true;

            /*
             * This bucket might have been split since we last held a lock on
             * the metapage.  If so, hashm_maxbucket, hashm_highmask and
             * hashm_lowmask might be old enough to cause us to fail to remove
             * tuples left behind by the most recent split.  To prevent that,
             * now that the primary page of the target bucket has been locked
             * (and thus can't be further split), check whether we need to
             * update our cached metapage data.
             */
            Assert(bucket_opaque->hasho_prevblkno != InvalidBlockNumber);
            if (bucket_opaque->hasho_prevblkno > cachedmetap->hashm_maxbucket)
            {
                cachedmetap = _hash_getcachedmetap(rel, &metabuf, true);
                Assert(cachedmetap != NULL);
            }
        }

        bucket_buf = buf;

        hashbucketcleanup(rel, cur_bucket, bucket_buf, blkno, info->strategy,
                          cachedmetap->hashm_maxbucket,
                          cachedmetap->hashm_highmask,
                          cachedmetap->hashm_lowmask, &tuples_removed,
                          &num_index_tuples, split_cleanup,
                          callback, callback_state);

        _hash_dropbuf(rel, bucket_buf);

        /* Advance to next bucket */
        cur_bucket++;
    }

    if (BufferIsInvalid(metabuf))
        metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_NOLOCK, LH_META_PAGE);

    /* Write-lock metapage and check for split since we started */
    LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);
    metap = HashPageGetMeta(BufferGetPage(metabuf));

    if (cur_maxbucket != metap->hashm_maxbucket)
    {
        /* There's been a split, so process the additional bucket(s) */
        LockBuffer(metabuf, BUFFER_LOCK_UNLOCK);
        cachedmetap = _hash_getcachedmetap(rel, &metabuf, true);
        Assert(cachedmetap != NULL);
        cur_maxbucket = cachedmetap->hashm_maxbucket;
        goto loop_top;
    }

    /* Okay, we're really done.  Update tuple count in metapage. */
    START_CRIT_SECTION();

    if (orig_maxbucket == metap->hashm_maxbucket &&
        orig_ntuples == metap->hashm_ntuples)
    {
        /*
         * No one has split or inserted anything since start of scan, so
         * believe our count as gospel.
         */
        metap->hashm_ntuples = num_index_tuples;
    }
    else
    {
        /*
         * Otherwise, our count is untrustworthy since we may have
         * double-scanned tuples in split buckets.  Proceed by dead-reckoning.
         * (Note: we still return estimated_count = false, because using this
         * count is better than not updating reltuples at all.)
         */
        if (metap->hashm_ntuples > tuples_removed)
            metap->hashm_ntuples -= tuples_removed;
        else
            metap->hashm_ntuples = 0;
        num_index_tuples = metap->hashm_ntuples;
    }

    MarkBufferDirty(metabuf);

    /* XLOG stuff */
    if (RelationNeedsWAL(rel))
    {
        xl_hash_update_meta_page xlrec;
        XLogRecPtr  recptr;

        xlrec.ntuples = metap->hashm_ntuples;

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, SizeOfHashUpdateMetaPage);

        XLogRegisterBuffer(0, metabuf, REGBUF_STANDARD);

        recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_UPDATE_META_PAGE);
        PageSetLSN(BufferGetPage(metabuf), recptr);
    }

    END_CRIT_SECTION();

    _hash_relbuf(rel, metabuf);

    /* return statistics */
    if (stats == NULL)
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
    stats->estimated_count = false;
    stats->num_index_tuples = num_index_tuples;
    stats->tuples_removed += tuples_removed;
    /* hashvacuumcleanup will fill in num_pages */

    return stats;
}

/*
 * Post-VACUUM cleanup.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
IndexBulkDeleteResult *
hashvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
{
    Relation    rel = info->index;
    BlockNumber num_pages;

    /* If hashbulkdelete wasn't called, return NULL signifying no change */
    /* Note: this covers the analyze_only case too */
    if (stats == NULL)
        return NULL;

    /* update statistics */
    num_pages = RelationGetNumberOfBlocks(rel);
    stats->num_pages = num_pages;

    return stats;
}

/*
 * Helper function to perform deletion of index entries from a bucket.
 *
 * This function expects that the caller has acquired a cleanup lock on the
 * primary bucket page, and will return with a write lock again held on the
 * primary bucket page.  The lock won't necessarily be held continuously,
 * though, because we'll release it when visiting overflow pages.
 *
 * There can't be any concurrent scans in progress when we first enter this
 * function because of the cleanup lock we hold on the primary bucket page,
 * but as soon as we release that lock, there might be.  If those scans got
 * ahead of our cleanup scan, they might see a tuple before we kill it and
 * wake up only after VACUUM has completed and the TID has been recycled for
 * an unrelated tuple.  To avoid that calamity, we prevent scans from passing
 * our cleanup scan by locking the next page in the bucket chain before
 * releasing the lock on the previous page.  (This type of lock chaining is not
 * ideal, so we might want to look for a better solution at some point.)
 *
 * We need to retain a pin on the primary bucket to ensure that no concurrent
 * split can start.
 */
void
hashbucketcleanup(Relation rel, Bucket cur_bucket, Buffer bucket_buf,
                  BlockNumber bucket_blkno, BufferAccessStrategy bstrategy,
                  uint32 maxbucket, uint32 highmask, uint32 lowmask,
                  double *tuples_removed, double *num_index_tuples,
                  bool split_cleanup,
                  IndexBulkDeleteCallback callback, void *callback_state)
{
    BlockNumber blkno;
    Buffer      buf;
    Bucket      new_bucket PG_USED_FOR_ASSERTS_ONLY = InvalidBucket;
    bool        bucket_dirty = false;

    blkno = bucket_blkno;
    buf = bucket_buf;

    if (split_cleanup)
        new_bucket = _hash_get_newbucket_from_oldbucket(rel, cur_bucket,
                                                        lowmask, maxbucket);

    /* Scan each page in bucket */
    for (;;)
    {
        HashPageOpaque opaque;
        OffsetNumber offno;
        OffsetNumber maxoffno;
        Buffer      next_buf;
        Page        page;
        OffsetNumber deletable[MaxOffsetNumber];
        int         ndeletable = 0;
        bool        retain_pin = false;
        bool        clear_dead_marking = false;

        vacuum_delay_point();

        page = BufferGetPage(buf);
        opaque = (HashPageOpaque) PageGetSpecialPointer(page);

        /* Scan each tuple in page */
        maxoffno = PageGetMaxOffsetNumber(page);
        for (offno = FirstOffsetNumber;
             offno <= maxoffno;
             offno = OffsetNumberNext(offno))
        {
            ItemPointer htup;
            IndexTuple  itup;
            Bucket      bucket;
            bool        kill_tuple = false;

            itup = (IndexTuple) PageGetItem(page,
                                            PageGetItemId(page, offno));
            htup = &(itup->t_tid);

            /*
             * To remove the dead tuples, we strictly want to rely on results
             * of callback function.  refer btvacuumpage for detailed reason.
             */
            if (callback && callback(htup, callback_state))
            {
                kill_tuple = true;
                if (tuples_removed)
                    *tuples_removed += 1;
            }
            else if (split_cleanup)
            {
                /* delete the tuples that are moved by split. */
                bucket = _hash_hashkey2bucket(_hash_get_indextuple_hashkey(itup),
                                              maxbucket,
                                              highmask,
                                              lowmask);
                /* mark the item for deletion */
                if (bucket != cur_bucket)
                {
                    /*
                     * We expect tuples to either belong to current bucket or
                     * new_bucket.  This is ensured because we don't allow
                     * further splits from bucket that contains garbage. See
                     * comments in _hash_expandtable.
                     */
                    Assert(bucket == new_bucket);
                    kill_tuple = true;
                }
            }

            if (kill_tuple)
            {
                /* mark the item for deletion */
                deletable[ndeletable++] = offno;
            }
            else
            {
                /* we're keeping it, so count it */
                if (num_index_tuples)
                    *num_index_tuples += 1;
            }
        }

        /* retain the pin on primary bucket page till end of bucket scan */
        if (blkno == bucket_blkno)
            retain_pin = true;
        else
            retain_pin = false;

        blkno = opaque->hasho_nextblkno;

        /*
         * Apply deletions, advance to next page and write page if needed.
         */
        if (ndeletable > 0)
        {
            /* No ereport(ERROR) until changes are logged */
            START_CRIT_SECTION();

            PageIndexMultiDelete(page, deletable, ndeletable);
            bucket_dirty = true;

            /*
             * Let us mark the page as clean if vacuum removes the DEAD tuples
             * from an index page. We do this by clearing
             * LH_PAGE_HAS_DEAD_TUPLES flag.
             */
            if (tuples_removed && *tuples_removed > 0 &&
                H_HAS_DEAD_TUPLES(opaque))
            {
                opaque->hasho_flag &= ~LH_PAGE_HAS_DEAD_TUPLES;
                clear_dead_marking = true;
            }

            MarkBufferDirty(buf);

            /* XLOG stuff */
            if (RelationNeedsWAL(rel))
            {
                xl_hash_delete xlrec;
                XLogRecPtr  recptr;

                xlrec.clear_dead_marking = clear_dead_marking;
                xlrec.is_primary_bucket_page = (buf == bucket_buf) ? true : false;

                XLogBeginInsert();
                XLogRegisterData((char *) &xlrec, SizeOfHashDelete);

                /*
                 * bucket buffer needs to be registered to ensure that we can
                 * acquire a cleanup lock on it during replay.
                 */
                if (!xlrec.is_primary_bucket_page)
                    XLogRegisterBuffer(0, bucket_buf, REGBUF_STANDARD | REGBUF_NO_IMAGE);

                XLogRegisterBuffer(1, buf, REGBUF_STANDARD);
                XLogRegisterBufData(1, (char *) deletable,
                                    ndeletable * sizeof(OffsetNumber));

                recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_DELETE);
                PageSetLSN(BufferGetPage(buf), recptr);
            }

            END_CRIT_SECTION();
        }

        /* bail out if there are no more pages to scan. */
        if (!BlockNumberIsValid(blkno))
            break;

        next_buf = _hash_getbuf_with_strategy(rel, blkno, HASH_WRITE,
                                              LH_OVERFLOW_PAGE,
                                              bstrategy);

        /*
         * release the lock on previous page after acquiring the lock on next
         * page
         */
        if (retain_pin)
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        else
            _hash_relbuf(rel, buf);

        buf = next_buf;
    }

    /*
     * lock the bucket page to clear the garbage flag and squeeze the bucket.
     * if the current buffer is same as bucket buffer, then we already have
     * lock on bucket page.
     */
    if (buf != bucket_buf)
    {
        _hash_relbuf(rel, buf);
        LockBuffer(bucket_buf, BUFFER_LOCK_EXCLUSIVE);
    }

    /*
     * Clear the garbage flag from bucket after deleting the tuples that are
     * moved by split.  We purposefully clear the flag before squeeze bucket,
     * so that after restart, vacuum shouldn't again try to delete the moved
     * by split tuples.
     */
    if (split_cleanup)
    {
        HashPageOpaque bucket_opaque;
        Page        page;

        page = BufferGetPage(bucket_buf);
        bucket_opaque = (HashPageOpaque) PageGetSpecialPointer(page);

        /* No ereport(ERROR) until changes are logged */
        START_CRIT_SECTION();

        bucket_opaque->hasho_flag &= ~LH_BUCKET_NEEDS_SPLIT_CLEANUP;
        MarkBufferDirty(bucket_buf);

        /* XLOG stuff */
        if (RelationNeedsWAL(rel))
        {
            XLogRecPtr  recptr;

            XLogBeginInsert();
            XLogRegisterBuffer(0, bucket_buf, REGBUF_STANDARD);

            recptr = XLogInsert(RM_HASH_ID, XLOG_HASH_SPLIT_CLEANUP);
            PageSetLSN(page, recptr);
        }

        END_CRIT_SECTION();
    }

    /*
     * If we have deleted anything, try to compact free space.  For squeezing
     * the bucket, we must have a cleanup lock, else it can impact the
     * ordering of tuples for a scan that has started before it.
     */
    if (bucket_dirty && IsBufferCleanupOK(bucket_buf))
        _hash_squeezebucket(rel, cur_bucket, bucket_blkno, bucket_buf,
                            bstrategy);
    else
        LockBuffer(bucket_buf, BUFFER_LOCK_UNLOCK);
}