spgvacuum.c

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * spgvacuum.c
 *    vacuum for SP-GiST
 *
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *          src/backend/access/spgist/spgvacuum.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/genam.h"
#include "access/spgist_private.h"
#include "access/spgxlog.h"
#include "access/transam.h"
#include "access/xloginsert.h"
#include "commands/vacuum.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/indexfsm.h"
#include "storage/lmgr.h"
#include "utils/snapmgr.h"
 
 
/* Entry in pending-list of TIDs we need to revisit */
typedef struct spgVacPendingItem
{
    ItemPointerData tid;        /* redirection target to visit */
    bool        done;           /* have we dealt with this? */
    struct spgVacPendingItem *next; /* list link */
} spgVacPendingItem;
 
/* Local state for vacuum operations */
typedef struct spgBulkDeleteState
{
    /* Parameters passed in to spgvacuumscan */
    IndexVacuumInfo *info;
    IndexBulkDeleteResult *stats;
    IndexBulkDeleteCallback callback;
    void       *callback_state;
 
    /* Additional working state */
    SpGistState spgstate;       /* for SPGiST operations that need one */
    spgVacPendingItem *pendingList; /* TIDs we need to (re)visit */
    TransactionId myXmin;       /* for detecting newly-added redirects */
    BlockNumber lastFilledBlock;    /* last non-deletable block */
} spgBulkDeleteState;
 
 
/*
 * Add TID to pendingList, but only if not already present.
 *
 * Note that new items are always appended at the end of the list; this
 * ensures that scans of the list don't miss items added during the scan.
 */
static void
spgAddPendingTID(spgBulkDeleteState *bds, ItemPointer tid)
{
    spgVacPendingItem *pitem;
    spgVacPendingItem **listLink;
 
    /* search the list for pre-existing entry */
    listLink = &bds->pendingList;
    while (*listLink != NULL)
    {
        pitem = *listLink;
        if (ItemPointerEquals(tid, &pitem->tid))
            return;             /* already in list, do nothing */
        listLink = &pitem->next;
    }
    /* not there, so append new entry */
    pitem = (spgVacPendingItem *) palloc(sizeof(spgVacPendingItem));
    pitem->tid = *tid;
    pitem->done = false;
    pitem->next = NULL;
    *listLink = pitem;
}
 
/*
 * Clear pendingList
 */
static void
spgClearPendingList(spgBulkDeleteState *bds)
{
    spgVacPendingItem *pitem;
    spgVacPendingItem *nitem;
 
    for (pitem = bds->pendingList; pitem != NULL; pitem = nitem)
    {
        nitem = pitem->next;
        /* All items in list should have been dealt with */
        Assert(pitem->done);
        pfree(pitem);
    }
    bds->pendingList = NULL;
}
 
/*
 * Vacuum a regular (non-root) leaf page
 *
 * We must delete tuples that are targeted for deletion by the VACUUM,
 * but not move any tuples that are referenced by outside links; we assume
 * those are the ones that are heads of chains.
 *
 * If we find a REDIRECT that was made by a concurrently-running transaction,
 * we must add its target TID to pendingList.  (We don't try to visit the
 * target immediately, first because we don't want VACUUM locking more than
 * one buffer at a time, and second because the duplicate-filtering logic
 * in spgAddPendingTID is useful to ensure we can't get caught in an infinite
 * loop in the face of continuous concurrent insertions.)
 *
 * If forPending is true, we are examining the page as a consequence of
 * chasing a redirect link, not as part of the normal sequential scan.
 * We still vacuum the page normally, but we don't increment the stats
 * about live tuples; else we'd double-count those tuples, since the page
 * has been or will be visited in the sequential scan as well.
 */
static void
vacuumLeafPage(spgBulkDeleteState *bds, Relation index, Buffer buffer,
               bool forPending)
{
    Page        page = BufferGetPage(buffer);
    spgxlogVacuumLeaf xlrec;
    OffsetNumber toDead[MaxIndexTuplesPerPage];
    OffsetNumber toPlaceholder[MaxIndexTuplesPerPage];
    OffsetNumber moveSrc[MaxIndexTuplesPerPage];
    OffsetNumber moveDest[MaxIndexTuplesPerPage];
    OffsetNumber chainSrc[MaxIndexTuplesPerPage];
    OffsetNumber chainDest[MaxIndexTuplesPerPage];
    OffsetNumber predecessor[MaxIndexTuplesPerPage + 1];
    bool        deletable[MaxIndexTuplesPerPage + 1];
    int         nDeletable;
    OffsetNumber i,
                max = PageGetMaxOffsetNumber(page);
 
    memset(predecessor, 0, sizeof(predecessor));
    memset(deletable, 0, sizeof(deletable));
    nDeletable = 0;
 
    /* Scan page, identify tuples to delete, accumulate stats */
    for (i = FirstOffsetNumber; i <= max; i++)
    {
        SpGistLeafTuple lt;
 
        lt = (SpGistLeafTuple) PageGetItem(page,
                                           PageGetItemId(page, i));
        if (lt->tupstate == SPGIST_LIVE)
        {
            Assert(ItemPointerIsValid(&lt->heapPtr));
 
            if (bds->callback(&lt->heapPtr, bds->callback_state))
            {
                bds->stats->tuples_removed += 1;
                deletable[i] = true;
                nDeletable++;
            }
            else
            {
                if (!forPending)
                    bds->stats->num_index_tuples += 1;
            }
 
            /* Form predecessor map, too */
            if (SGLT_GET_NEXTOFFSET(lt) != InvalidOffsetNumber)
            {
                /* paranoia about corrupted chain links */
                if (SGLT_GET_NEXTOFFSET(lt) < FirstOffsetNumber ||
                    SGLT_GET_NEXTOFFSET(lt) > max ||
                    predecessor[SGLT_GET_NEXTOFFSET(lt)] != InvalidOffsetNumber)
                    elog(ERROR, "inconsistent tuple chain links in page %u of index \"%s\"",
                         BufferGetBlockNumber(buffer),
                         RelationGetRelationName(index));
                predecessor[SGLT_GET_NEXTOFFSET(lt)] = i;
            }
        }
        else if (lt->tupstate == SPGIST_REDIRECT)
        {
            SpGistDeadTuple dt = (SpGistDeadTuple) lt;
 
            Assert(SGLT_GET_NEXTOFFSET(dt) == InvalidOffsetNumber);
            Assert(ItemPointerIsValid(&dt->pointer));
 
            /*
             * Add target TID to pending list if the redirection could have
             * happened since VACUUM started.  (If xid is invalid, assume it
             * must have happened before VACUUM started, since REINDEX
             * CONCURRENTLY locks out VACUUM.)
             *
             * Note: we could make a tighter test by seeing if the xid is
             * "running" according to the active snapshot; but snapmgr.c
             * doesn't currently export a suitable API, and it's not entirely
             * clear that a tighter test is worth the cycles anyway.
             */
            if (TransactionIdFollowsOrEquals(dt->xid, bds->myXmin))
                spgAddPendingTID(bds, &dt->pointer);
        }
        else
        {
            Assert(SGLT_GET_NEXTOFFSET(lt) == InvalidOffsetNumber);
        }
    }
 
    if (nDeletable == 0)
        return;                 /* nothing more to do */
 
    /*----------
     * Figure out exactly what we have to do.  We do this separately from
     * actually modifying the page, mainly so that we have a representation
     * that can be dumped into WAL and then the replay code can do exactly
     * the same thing.  The output of this step consists of six arrays
     * describing four kinds of operations, to be performed in this order:
     *
     * toDead[]: tuple numbers to be replaced with DEAD tuples
     * toPlaceholder[]: tuple numbers to be replaced with PLACEHOLDER tuples
     * moveSrc[]: tuple numbers that need to be relocated to another offset
     * (replacing the tuple there) and then replaced with PLACEHOLDER tuples
     * moveDest[]: new locations for moveSrc tuples
     * chainSrc[]: tuple numbers whose chain links (nextOffset) need updates
     * chainDest[]: new values of nextOffset for chainSrc members
     *
     * It's easiest to figure out what we have to do by processing tuple
     * chains, so we iterate over all the tuples (not just the deletable
     * ones!) to identify chain heads, then chase down each chain and make
     * work item entries for deletable tuples within the chain.
     *----------
     */
    xlrec.nDead = xlrec.nPlaceholder = xlrec.nMove = xlrec.nChain = 0;
 
    for (i = FirstOffsetNumber; i <= max; i++)
    {
        SpGistLeafTuple head;
        bool        interveningDeletable;
        OffsetNumber prevLive;
        OffsetNumber j;
 
        head = (SpGistLeafTuple) PageGetItem(page,
                                             PageGetItemId(page, i));
        if (head->tupstate != SPGIST_LIVE)
            continue;           /* can't be a chain member */
        if (predecessor[i] != 0)
            continue;           /* not a chain head */
 
        /* initialize ... */
        interveningDeletable = false;
        prevLive = deletable[i] ? InvalidOffsetNumber : i;
 
        /* scan down the chain ... */
        j = SGLT_GET_NEXTOFFSET(head);
        while (j != InvalidOffsetNumber)
        {
            SpGistLeafTuple lt;
 
            lt = (SpGistLeafTuple) PageGetItem(page,
                                               PageGetItemId(page, j));
            if (lt->tupstate != SPGIST_LIVE)
            {
                /* all tuples in chain should be live */
                elog(ERROR, "unexpected SPGiST tuple state: %d",
                     lt->tupstate);
            }
 
            if (deletable[j])
            {
                /* This tuple should be replaced by a placeholder */
                toPlaceholder[xlrec.nPlaceholder] = j;
                xlrec.nPlaceholder++;
                /* previous live tuple's chain link will need an update */
                interveningDeletable = true;
            }
            else if (prevLive == InvalidOffsetNumber)
            {
                /*
                 * This is the first live tuple in the chain.  It has to move
                 * to the head position.
                 */
                moveSrc[xlrec.nMove] = j;
                moveDest[xlrec.nMove] = i;
                xlrec.nMove++;
                /* Chain updates will be applied after the move */
                prevLive = i;
                interveningDeletable = false;
            }
            else
            {
                /*
                 * Second or later live tuple.  Arrange to re-chain it to the
                 * previous live one, if there was a gap.
                 */
                if (interveningDeletable)
                {
                    chainSrc[xlrec.nChain] = prevLive;
                    chainDest[xlrec.nChain] = j;
                    xlrec.nChain++;
                }
                prevLive = j;
                interveningDeletable = false;
            }
 
            j = SGLT_GET_NEXTOFFSET(lt);
        }
 
        if (prevLive == InvalidOffsetNumber)
        {
            /* The chain is entirely removable, so we need a DEAD tuple */
            toDead[xlrec.nDead] = i;
            xlrec.nDead++;
        }
        else if (interveningDeletable)
        {
            /* One or more deletions at end of chain, so close it off */
            chainSrc[xlrec.nChain] = prevLive;
            chainDest[xlrec.nChain] = InvalidOffsetNumber;
            xlrec.nChain++;
        }
    }
 
    /* sanity check ... */
    if (nDeletable != xlrec.nDead + xlrec.nPlaceholder + xlrec.nMove)
        elog(ERROR, "inconsistent counts of deletable tuples");
 
    /* Do the updates */
    START_CRIT_SECTION();
 
    spgPageIndexMultiDelete(&bds->spgstate, page,
                            toDead, xlrec.nDead,
                            SPGIST_DEAD, SPGIST_DEAD,
                            InvalidBlockNumber, InvalidOffsetNumber);
 
    spgPageIndexMultiDelete(&bds->spgstate, page,
                            toPlaceholder, xlrec.nPlaceholder,
                            SPGIST_PLACEHOLDER, SPGIST_PLACEHOLDER,
                            InvalidBlockNumber, InvalidOffsetNumber);
 
    /*
     * We implement the move step by swapping the line pointers of the source
     * and target tuples, then replacing the newly-source tuples with
     * placeholders.  This is perhaps unduly friendly with the page data
     * representation, but it's fast and doesn't risk page overflow when a
     * tuple to be relocated is large.
     */
    for (i = 0; i < xlrec.nMove; i++)
    {
        ItemId      idSrc = PageGetItemId(page, moveSrc[i]);
        ItemId      idDest = PageGetItemId(page, moveDest[i]);
        ItemIdData  tmp;
 
        tmp = *idSrc;
        *idSrc = *idDest;
        *idDest = tmp;
    }
 
    spgPageIndexMultiDelete(&bds->spgstate, page,
                            moveSrc, xlrec.nMove,
                            SPGIST_PLACEHOLDER, SPGIST_PLACEHOLDER,
                            InvalidBlockNumber, InvalidOffsetNumber);
 
    for (i = 0; i < xlrec.nChain; i++)
    {
        SpGistLeafTuple lt;
 
        lt = (SpGistLeafTuple) PageGetItem(page,
                                           PageGetItemId(page, chainSrc[i]));
        Assert(lt->tupstate == SPGIST_LIVE);
        SGLT_SET_NEXTOFFSET(lt, chainDest[i]);
    }
 
    MarkBufferDirty(buffer);
 
    if (RelationNeedsWAL(index))
    {
        XLogRecPtr  recptr;
 
        XLogBeginInsert();
 
        STORE_STATE(&bds->spgstate, xlrec.stateSrc);
 
        XLogRegisterData((char *) &xlrec, SizeOfSpgxlogVacuumLeaf);
        /* sizeof(xlrec) should be a multiple of sizeof(OffsetNumber) */
        XLogRegisterData((char *) toDead, sizeof(OffsetNumber) * xlrec.nDead);
        XLogRegisterData((char *) toPlaceholder, sizeof(OffsetNumber) * xlrec.nPlaceholder);
        XLogRegisterData((char *) moveSrc, sizeof(OffsetNumber) * xlrec.nMove);
        XLogRegisterData((char *) moveDest, sizeof(OffsetNumber) * xlrec.nMove);
        XLogRegisterData((char *) chainSrc, sizeof(OffsetNumber) * xlrec.nChain);
        XLogRegisterData((char *) chainDest, sizeof(OffsetNumber) * xlrec.nChain);
 
        XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
 
        recptr = XLogInsert(RM_SPGIST_ID, XLOG_SPGIST_VACUUM_LEAF);
 
        PageSetLSN(page, recptr);
    }
 
    END_CRIT_SECTION();
}
 
/*
 * Vacuum a root page when it is also a leaf
 *
 * On the root, we just delete any dead leaf tuples; no fancy business
 */
static void
vacuumLeafRoot(spgBulkDeleteState *bds, Relation index, Buffer buffer)
{
    Page        page = BufferGetPage(buffer);
    spgxlogVacuumRoot xlrec;
    OffsetNumber toDelete[MaxIndexTuplesPerPage];
    OffsetNumber i,
                max = PageGetMaxOffsetNumber(page);
 
    xlrec.nDelete = 0;
 
    /* Scan page, identify tuples to delete, accumulate stats */
    for (i = FirstOffsetNumber; i <= max; i++)
    {
        SpGistLeafTuple lt;
 
        lt = (SpGistLeafTuple) PageGetItem(page,
                                           PageGetItemId(page, i));
        if (lt->tupstate == SPGIST_LIVE)
        {
            Assert(ItemPointerIsValid(&lt->heapPtr));
 
            if (bds->callback(&lt->heapPtr, bds->callback_state))
            {
                bds->stats->tuples_removed += 1;
                toDelete[xlrec.nDelete] = i;
                xlrec.nDelete++;
            }
            else
            {
                bds->stats->num_index_tuples += 1;
            }
        }
        else
        {
            /* all tuples on root should be live */
            elog(ERROR, "unexpected SPGiST tuple state: %d",
                 lt->tupstate);
        }
    }
 
    if (xlrec.nDelete == 0)
        return;                 /* nothing more to do */
 
    /* Do the update */
    START_CRIT_SECTION();
 
    /* The tuple numbers are in order, so we can use PageIndexMultiDelete */
    PageIndexMultiDelete(page, toDelete, xlrec.nDelete);
 
    MarkBufferDirty(buffer);
 
    if (RelationNeedsWAL(index))
    {
        XLogRecPtr  recptr;
 
        XLogBeginInsert();
 
        /* Prepare WAL record */
        STORE_STATE(&bds->spgstate, xlrec.stateSrc);
 
        XLogRegisterData((char *) &xlrec, SizeOfSpgxlogVacuumRoot);
        /* sizeof(xlrec) should be a multiple of sizeof(OffsetNumber) */
        XLogRegisterData((char *) toDelete,
                         sizeof(OffsetNumber) * xlrec.nDelete);
 
        XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
 
        recptr = XLogInsert(RM_SPGIST_ID, XLOG_SPGIST_VACUUM_ROOT);
 
        PageSetLSN(page, recptr);
    }
 
    END_CRIT_SECTION();
}
 
/*
 * Clean up redirect and placeholder tuples on the given page
 *
 * Redirect tuples can be marked placeholder once they're old enough.
 * Placeholder tuples can be removed if it won't change the offsets of
 * non-placeholder ones.
 *
 * Unlike the routines above, this works on both leaf and inner pages.
 */
static void
vacuumRedirectAndPlaceholder(Relation index, Relation heaprel, Buffer buffer)
{
    Page        page = BufferGetPage(buffer);
    SpGistPageOpaque opaque = SpGistPageGetOpaque(page);
    OffsetNumber i,
                max = PageGetMaxOffsetNumber(page),
                firstPlaceholder = InvalidOffsetNumber;
    bool        hasNonPlaceholder = false;
    bool        hasUpdate = false;
    OffsetNumber itemToPlaceholder[MaxIndexTuplesPerPage];
    OffsetNumber itemnos[MaxIndexTuplesPerPage];
    spgxlogVacuumRedirect xlrec;
    GlobalVisState *vistest;
 
    xlrec.isCatalogRel = RelationIsAccessibleInLogicalDecoding(heaprel);
    xlrec.nToPlaceholder = 0;
    xlrec.snapshotConflictHorizon = InvalidTransactionId;
 
    vistest = GlobalVisTestFor(heaprel);
 
    START_CRIT_SECTION();
 
    /*
     * Scan backwards to convert old redirection tuples to placeholder tuples,
     * and identify location of last non-placeholder tuple while at it.
     */
    for (i = max;
         i >= FirstOffsetNumber &&
         (opaque->nRedirection > 0 || !hasNonPlaceholder);
         i--)
    {
        SpGistDeadTuple dt;
 
        dt = (SpGistDeadTuple) PageGetItem(page, PageGetItemId(page, i));
 
        /*
         * We can convert a REDIRECT to a PLACEHOLDER if there could no longer
         * be any index scans "in flight" to it.  Such an index scan would
         * have to be in a transaction whose snapshot sees the REDIRECT's XID
         * as still running, so comparing the XID against global xmin is a
         * conservatively safe test.  If the XID is invalid, it must have been
         * inserted by REINDEX CONCURRENTLY, so we can zap it immediately.
         */
        if (dt->tupstate == SPGIST_REDIRECT &&
            (!TransactionIdIsValid(dt->xid) ||
             GlobalVisTestIsRemovableXid(vistest, dt->xid)))
        {
            dt->tupstate = SPGIST_PLACEHOLDER;
            Assert(opaque->nRedirection > 0);
            opaque->nRedirection--;
            opaque->nPlaceholder++;
 
            /* remember newest XID among the removed redirects */
            if (!TransactionIdIsValid(xlrec.snapshotConflictHorizon) ||
                TransactionIdPrecedes(xlrec.snapshotConflictHorizon, dt->xid))
                xlrec.snapshotConflictHorizon = dt->xid;
 
            ItemPointerSetInvalid(&dt->pointer);
 
            itemToPlaceholder[xlrec.nToPlaceholder] = i;
            xlrec.nToPlaceholder++;
 
            hasUpdate = true;
        }
 
        if (dt->tupstate == SPGIST_PLACEHOLDER)
        {
            if (!hasNonPlaceholder)
                firstPlaceholder = i;
        }
        else
        {
            hasNonPlaceholder = true;
        }
    }
 
    /*
     * Any placeholder tuples at the end of page can safely be removed.  We
     * can't remove ones before the last non-placeholder, though, because we
     * can't alter the offset numbers of non-placeholder tuples.
     */
    if (firstPlaceholder != InvalidOffsetNumber)
    {
        /*
         * We do not store this array to rdata because it's easy to recreate.
         */
        for (i = firstPlaceholder; i <= max; i++)
            itemnos[i - firstPlaceholder] = i;
 
        i = max - firstPlaceholder + 1;
        Assert(opaque->nPlaceholder >= i);
        opaque->nPlaceholder -= i;
 
        /* The array is surely sorted, so can use PageIndexMultiDelete */
        PageIndexMultiDelete(page, itemnos, i);
 
        hasUpdate = true;
    }
 
    xlrec.firstPlaceholder = firstPlaceholder;
 
    if (hasUpdate)
        MarkBufferDirty(buffer);
 
    if (hasUpdate && RelationNeedsWAL(index))
    {
        XLogRecPtr  recptr;
 
        XLogBeginInsert();
 
        XLogRegisterData((char *) &xlrec, SizeOfSpgxlogVacuumRedirect);
        XLogRegisterData((char *) itemToPlaceholder,
                         sizeof(OffsetNumber) * xlrec.nToPlaceholder);
 
        XLogRegisterBuffer(0, buffer, REGBUF_STANDARD);
 
        recptr = XLogInsert(RM_SPGIST_ID, XLOG_SPGIST_VACUUM_REDIRECT);
 
        PageSetLSN(page, recptr);
    }
 
    END_CRIT_SECTION();
}
 
/*
 * Process one page during a bulkdelete scan
 */
static void
spgvacuumpage(spgBulkDeleteState *bds, BlockNumber blkno)
{
    Relation    index = bds->info->index;
    Buffer      buffer;
    Page        page;
 
    /* call vacuum_delay_point while not holding any buffer lock */
    vacuum_delay_point();
 
    buffer = ReadBufferExtended(index, MAIN_FORKNUM, blkno,
                                RBM_NORMAL, bds->info->strategy);
    LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
    page = (Page) BufferGetPage(buffer);
 
    if (PageIsNew(page))
    {
        /*
         * We found an all-zero page, which could happen if the database
         * crashed just after extending the file.  Recycle it.
         */
    }
    else if (PageIsEmpty(page))
    {
        /* nothing to do */
    }
    else if (SpGistPageIsLeaf(page))
    {
        if (SpGistBlockIsRoot(blkno))
        {
            vacuumLeafRoot(bds, index, buffer);
            /* no need for vacuumRedirectAndPlaceholder */
        }
        else
        {
            vacuumLeafPage(bds, index, buffer, false);
            vacuumRedirectAndPlaceholder(index, bds->info->heaprel, buffer);
        }
    }
    else
    {
        /* inner page */
        vacuumRedirectAndPlaceholder(index, bds->info->heaprel, buffer);
    }
 
    /*
     * The root pages must never be deleted, nor marked as available in FSM,
     * because we don't want them ever returned by a search for a place to put
     * a new tuple.  Otherwise, check for empty page, and make sure the FSM
     * knows about it.
     */
    if (!SpGistBlockIsRoot(blkno))
    {
        if (PageIsNew(page) || PageIsEmpty(page))
        {
            RecordFreeIndexPage(index, blkno);
            bds->stats->pages_deleted++;
        }
        else
        {
            SpGistSetLastUsedPage(index, buffer);
            bds->lastFilledBlock = blkno;
        }
    }
 
    UnlockReleaseBuffer(buffer);
}
 
/*
 * Process the pending-TID list between pages of the main scan
 */
static void
spgprocesspending(spgBulkDeleteState *bds)
{
    Relation    index = bds->info->index;
    spgVacPendingItem *pitem;
    spgVacPendingItem *nitem;
    BlockNumber blkno;
    Buffer      buffer;
    Page        page;
 
    for (pitem = bds->pendingList; pitem != NULL; pitem = pitem->next)
    {
        if (pitem->done)
            continue;           /* ignore already-done items */
 
        /* call vacuum_delay_point while not holding any buffer lock */
        vacuum_delay_point();
 
        /* examine the referenced page */
        blkno = ItemPointerGetBlockNumber(&pitem->tid);
        buffer = ReadBufferExtended(index, MAIN_FORKNUM, blkno,
                                    RBM_NORMAL, bds->info->strategy);
        LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
        page = (Page) BufferGetPage(buffer);
 
        if (PageIsNew(page) || SpGistPageIsDeleted(page))
        {
            /* Probably shouldn't happen, but ignore it */
        }
        else if (SpGistPageIsLeaf(page))
        {
            if (SpGistBlockIsRoot(blkno))
            {
                /* this should definitely not happen */
                elog(ERROR, "redirection leads to root page of index \"%s\"",
                     RelationGetRelationName(index));
            }
 
            /* deal with any deletable tuples */
            vacuumLeafPage(bds, index, buffer, true);
            /* might as well do this while we are here */
            vacuumRedirectAndPlaceholder(index, bds->info->heaprel, buffer);
 
            SpGistSetLastUsedPage(index, buffer);
 
            /*
             * We can mark as done not only this item, but any later ones
             * pointing at the same page, since we vacuumed the whole page.
             */
            pitem->done = true;
            for (nitem = pitem->next; nitem != NULL; nitem = nitem->next)
            {
                if (ItemPointerGetBlockNumber(&nitem->tid) == blkno)
                    nitem->done = true;
            }
        }
        else
        {
            /*
             * On an inner page, visit the referenced inner tuple and add all
             * its downlinks to the pending list.  We might have pending items
             * for more than one inner tuple on the same page (in fact this is
             * pretty likely given the way space allocation works), so get
             * them all while we are here.
             */
            for (nitem = pitem; nitem != NULL; nitem = nitem->next)
            {
                if (nitem->done)
                    continue;
                if (ItemPointerGetBlockNumber(&nitem->tid) == blkno)
                {
                    OffsetNumber offset;
                    SpGistInnerTuple innerTuple;
 
                    offset = ItemPointerGetOffsetNumber(&nitem->tid);
                    innerTuple = (SpGistInnerTuple) PageGetItem(page,
                                                                PageGetItemId(page, offset));
                    if (innerTuple->tupstate == SPGIST_LIVE)
                    {
                        SpGistNodeTuple node;
                        int         i;
 
                        SGITITERATE(innerTuple, i, node)
                        {
                            if (ItemPointerIsValid(&node->t_tid))
                                spgAddPendingTID(bds, &node->t_tid);
                        }
                    }
                    else if (innerTuple->tupstate == SPGIST_REDIRECT)
                    {
                        /* transfer attention to redirect point */
                        spgAddPendingTID(bds,
                                         &((SpGistDeadTuple) innerTuple)->pointer);
                    }
                    else
                        elog(ERROR, "unexpected SPGiST tuple state: %d",
                             innerTuple->tupstate);
 
                    nitem->done = true;
                }
            }
        }
 
        UnlockReleaseBuffer(buffer);
    }
 
    spgClearPendingList(bds);
}
 
/*
 * Perform a bulkdelete scan
 */
static void
spgvacuumscan(spgBulkDeleteState *bds)
{
    Relation    index = bds->info->index;
    bool        needLock;
    BlockNumber num_pages,
                blkno;
 
    /* Finish setting up spgBulkDeleteState */
    initSpGistState(&bds->spgstate, index);
    bds->pendingList = NULL;
    bds->myXmin = GetActiveSnapshot()->xmin;
    bds->lastFilledBlock = SPGIST_LAST_FIXED_BLKNO;
 
    /*
     * Reset counts that will be incremented during the scan; needed in case
     * of multiple scans during a single VACUUM command
     */
    bds->stats->estimated_count = false;
    bds->stats->num_index_tuples = 0;
    bds->stats->pages_deleted = 0;
 
    /* We can skip locking for new or temp relations */
    needLock = !RELATION_IS_LOCAL(index);
 
    /*
     * 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.  See more extensive comments about this
     * in btvacuumscan().
     */
    blkno = SPGIST_METAPAGE_BLKNO + 1;
    for (;;)
    {
        /* Get the current relation length */
        if (needLock)
            LockRelationForExtension(index, ExclusiveLock);
        num_pages = RelationGetNumberOfBlocks(index);
        if (needLock)
            UnlockRelationForExtension(index, ExclusiveLock);
 
        /* Quit if we've scanned the whole relation */
        if (blkno >= num_pages)
            break;
        /* Iterate over pages, then loop back to recheck length */
        for (; blkno < num_pages; blkno++)
        {
            spgvacuumpage(bds, blkno);
            /* empty the pending-list after each page */
            if (bds->pendingList != NULL)
                spgprocesspending(bds);
        }
    }
 
    /* Propagate local lastUsedPages cache to metablock */
    SpGistUpdateMetaPage(index);
 
    /*
     * If we found any empty 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 empty pages exist, we don't bother vacuuming the FSM at
     * all.
     */
    if (bds->stats->pages_deleted > 0)
        IndexFreeSpaceMapVacuum(index);
 
    /*
     * Truncate index if possible
     *
     * XXX disabled because it's unsafe due to possible concurrent inserts.
     * We'd have to rescan the pages to make sure they're still empty, and it
     * doesn't seem worth it.  Note that btree doesn't do this either.
     *
     * Another reason not to truncate is that it could invalidate the cached
     * pages-with-freespace pointers in the metapage and other backends'
     * relation caches, that is leave them pointing to nonexistent pages.
     * Adding RelationGetNumberOfBlocks calls to protect the places that use
     * those pointers would be unduly expensive.
     */
#ifdef NOT_USED
    if (num_pages > bds->lastFilledBlock + 1)
    {
        BlockNumber lastBlock = num_pages - 1;
 
        num_pages = bds->lastFilledBlock + 1;
        RelationTruncate(index, num_pages);
        bds->stats->pages_removed += lastBlock - bds->lastFilledBlock;
        bds->stats->pages_deleted -= lastBlock - bds->lastFilledBlock;
    }
#endif
 
    /* Report final stats */
    bds->stats->num_pages = num_pages;
    bds->stats->pages_newly_deleted = bds->stats->pages_deleted;
    bds->stats->pages_free = bds->stats->pages_deleted;
}
 
/*
 * 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 *
spgbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
              IndexBulkDeleteCallback callback, void *callback_state)
{
    spgBulkDeleteState bds;
 
    /* allocate stats if first time through, else re-use existing struct */
    if (stats == NULL)
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
    bds.info = info;
    bds.stats = stats;
    bds.callback = callback;
    bds.callback_state = callback_state;
 
    spgvacuumscan(&bds);
 
    return stats;
}
 
/* Dummy callback to delete no tuples during spgvacuumcleanup */
static bool
dummy_callback(ItemPointer itemptr, void *state)
{
    return false;
}
 
/*
 * Post-VACUUM cleanup.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
IndexBulkDeleteResult *
spgvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
{
    spgBulkDeleteState bds;
 
    /* No-op in ANALYZE ONLY mode */
    if (info->analyze_only)
        return stats;
 
    /*
     * We don't need to scan the index if there was a preceding bulkdelete
     * pass.  Otherwise, make a pass that won't delete any live tuples, but
     * might still accomplish useful stuff with redirect/placeholder cleanup
     * and/or FSM housekeeping, and in any case will provide stats.
     */
    if (stats == NULL)
    {
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
        bds.info = info;
        bds.stats = stats;
        bds.callback = dummy_callback;
        bds.callback_state = NULL;
 
        spgvacuumscan(&bds);
    }
 
    /*
     * It's quite possible for us to be fooled by concurrent tuple moves 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;
}