gist.c

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/*-------------------------------------------------------------------------
 *
 * gist.c
 *    interface routines for the postgres GiST index access method.
 *
 *
 * Portions Copyright (c) 1996-2015, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/gist/gist.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/genam.h"
#include "access/gist_private.h"
#include "access/xloginsert.h"
#include "catalog/index.h"
#include "catalog/pg_collation.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/indexfsm.h"
#include "utils/memutils.h"
#include "utils/rel.h"

/* non-export function prototypes */
static void gistfixsplit(GISTInsertState *state, GISTSTATE *giststate);
static bool gistinserttuple(GISTInsertState *state, GISTInsertStack *stack,
             GISTSTATE *giststate, IndexTuple tuple, OffsetNumber oldoffnum);
static bool gistinserttuples(GISTInsertState *state, GISTInsertStack *stack,
                 GISTSTATE *giststate,
                 IndexTuple *tuples, int ntup, OffsetNumber oldoffnum,
                 Buffer leftchild, Buffer rightchild,
                 bool unlockbuf, bool unlockleftchild);
static void gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
                GISTSTATE *giststate, List *splitinfo, bool releasebuf);
static void gistvacuumpage(Relation rel, Page page, Buffer buffer);


#define ROTATEDIST(d) do { \
    SplitedPageLayout *tmp=(SplitedPageLayout*)palloc(sizeof(SplitedPageLayout)); \
    memset(tmp,0,sizeof(SplitedPageLayout)); \
    tmp->block.blkno = InvalidBlockNumber;  \
    tmp->buffer = InvalidBuffer;    \
    tmp->next = (d); \
    (d)=tmp; \
} while(0)


/*
 * Create and return a temporary memory context for use by GiST. We
 * _always_ invoke user-provided methods in a temporary memory
 * context, so that memory leaks in those functions cannot cause
 * problems. Also, we use some additional temporary contexts in the
 * GiST code itself, to avoid the need to do some awkward manual
 * memory management.
 */
MemoryContext
createTempGistContext(void)
{
    return AllocSetContextCreate(CurrentMemoryContext,
                                 "GiST temporary context",
                                 ALLOCSET_DEFAULT_MINSIZE,
                                 ALLOCSET_DEFAULT_INITSIZE,
                                 ALLOCSET_DEFAULT_MAXSIZE);
}

/*
 *  gistbuildempty() -- build an empty gist index in the initialization fork
 */
Datum
gistbuildempty(PG_FUNCTION_ARGS)
{
    Relation    index = (Relation) PG_GETARG_POINTER(0);
    Buffer      buffer;

    /* Initialize the root page */
    buffer = ReadBufferExtended(index, INIT_FORKNUM, P_NEW, RBM_NORMAL, NULL);
    LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

    /* Initialize and xlog buffer */
    START_CRIT_SECTION();
    GISTInitBuffer(buffer, F_LEAF);
    MarkBufferDirty(buffer);
    log_newpage_buffer(buffer, true);
    END_CRIT_SECTION();

    /* Unlock and release the buffer */
    UnlockReleaseBuffer(buffer);

    PG_RETURN_VOID();
}

/*
 *  gistinsert -- wrapper for GiST tuple insertion.
 *
 *    This is the public interface routine for tuple insertion in GiSTs.
 *    It doesn't do any work; just locks the relation and passes the buck.
 */
Datum
gistinsert(PG_FUNCTION_ARGS)
{
    Relation    r = (Relation) PG_GETARG_POINTER(0);
    Datum      *values = (Datum *) PG_GETARG_POINTER(1);
    bool       *isnull = (bool *) PG_GETARG_POINTER(2);
    ItemPointer ht_ctid = (ItemPointer) PG_GETARG_POINTER(3);

#ifdef NOT_USED
    Relation    heapRel = (Relation) PG_GETARG_POINTER(4);
    IndexUniqueCheck checkUnique = (IndexUniqueCheck) PG_GETARG_INT32(5);
#endif
    IndexTuple  itup;
    GISTSTATE  *giststate;
    MemoryContext oldCxt;

    giststate = initGISTstate(r);

    /*
     * We use the giststate's scan context as temp context too.  This means
     * that any memory leaked by the support functions is not reclaimed until
     * end of insert.  In most cases, we aren't going to call the support
     * functions very many times before finishing the insert, so this seems
     * cheaper than resetting a temp context for each function call.
     */
    oldCxt = MemoryContextSwitchTo(giststate->tempCxt);

    itup = gistFormTuple(giststate, r,
                         values, isnull, true /* size is currently bogus */ );
    itup->t_tid = *ht_ctid;

    gistdoinsert(r, itup, 0, giststate);

    /* cleanup */
    MemoryContextSwitchTo(oldCxt);
    freeGISTstate(giststate);

    PG_RETURN_BOOL(false);
}


/*
 * Place tuples from 'itup' to 'buffer'. If 'oldoffnum' is valid, the tuple
 * at that offset is atomically removed along with inserting the new tuples.
 * This is used to replace a tuple with a new one.
 *
 * If 'leftchildbuf' is valid, we're inserting the downlink for the page
 * to the right of 'leftchildbuf', or updating the downlink for 'leftchildbuf'.
 * F_FOLLOW_RIGHT flag on 'leftchildbuf' is cleared and NSN is set.
 *
 * If 'markfollowright' is true and the page is split, the left child is
 * marked with F_FOLLOW_RIGHT flag. That is the normal case. During buffered
 * index build, however, there is no concurrent access and the page splitting
 * is done in a slightly simpler fashion, and false is passed.
 *
 * If there is not enough room on the page, it is split. All the split
 * pages are kept pinned and locked and returned in *splitinfo, the caller
 * is responsible for inserting the downlinks for them. However, if
 * 'buffer' is the root page and it needs to be split, gistplacetopage()
 * performs the split as one atomic operation, and *splitinfo is set to NIL.
 * In that case, we continue to hold the root page locked, and the child
 * pages are released; note that new tuple(s) are *not* on the root page
 * but in one of the new child pages.
 *
 * If 'newblkno' is not NULL, returns the block number of page the first
 * new/updated tuple was inserted to. Usually it's the given page, but could
 * be its right sibling if the page was split.
 *
 * Returns 'true' if the page was split, 'false' otherwise.
 */
bool
gistplacetopage(Relation rel, Size freespace, GISTSTATE *giststate,
                Buffer buffer,
                IndexTuple *itup, int ntup, OffsetNumber oldoffnum,
                BlockNumber *newblkno,
                Buffer leftchildbuf,
                List **splitinfo,
                bool markfollowright)
{
    BlockNumber blkno = BufferGetBlockNumber(buffer);
    Page        page = BufferGetPage(buffer);
    bool        is_leaf = (GistPageIsLeaf(page)) ? true : false;
    XLogRecPtr  recptr;
    int         i;
    bool        is_split;

    /*
     * Refuse to modify a page that's incompletely split. This should not
     * happen because we finish any incomplete splits while we walk down the
     * tree. However, it's remotely possible that another concurrent inserter
     * splits a parent page, and errors out before completing the split. We
     * will just throw an error in that case, and leave any split we had in
     * progress unfinished too. The next insert that comes along will clean up
     * the mess.
     */
    if (GistFollowRight(page))
        elog(ERROR, "concurrent GiST page split was incomplete");

    *splitinfo = NIL;

    /*
     * if isupdate, remove old key: This node's key has been modified, either
     * because a child split occurred or because we needed to adjust our key
     * for an insert in a child node. Therefore, remove the old version of
     * this node's key.
     *
     * for WAL replay, in the non-split case we handle this by setting up a
     * one-element todelete array; in the split case, it's handled implicitly
     * because the tuple vector passed to gistSplit won't include this tuple.
     */
    is_split = gistnospace(page, itup, ntup, oldoffnum, freespace);

    /*
     * If leaf page is full, try at first to delete dead tuples. And then
     * check again.
     */
    if (is_split && GistPageIsLeaf(page) && GistPageHasGarbage(page))
    {
        gistvacuumpage(rel, page, buffer);
        is_split = gistnospace(page, itup, ntup, oldoffnum, freespace);
    }

    if (is_split)
    {
        /* no space for insertion */
        IndexTuple *itvec;
        int         tlen;
        SplitedPageLayout *dist = NULL,
                   *ptr;
        BlockNumber oldrlink = InvalidBlockNumber;
        GistNSN     oldnsn = 0;
        SplitedPageLayout rootpg;
        bool        is_rootsplit;
        int         npage;

        is_rootsplit = (blkno == GIST_ROOT_BLKNO);

        /*
         * Form index tuples vector to split. If we're replacing an old tuple,
         * remove the old version from the vector.
         */
        itvec = gistextractpage(page, &tlen);
        if (OffsetNumberIsValid(oldoffnum))
        {
            /* on inner page we should remove old tuple */
            int         pos = oldoffnum - FirstOffsetNumber;

            tlen--;
            if (pos != tlen)
                memmove(itvec + pos, itvec + pos + 1, sizeof(IndexTuple) * (tlen - pos));
        }
        itvec = gistjoinvector(itvec, &tlen, itup, ntup);
        dist = gistSplit(rel, page, itvec, tlen, giststate);

        /*
         * Check that split didn't produce too many pages.
         */
        npage = 0;
        for (ptr = dist; ptr; ptr = ptr->next)
            npage++;
        /* in a root split, we'll add one more page to the list below */
        if (is_rootsplit)
            npage++;
        if (npage > GIST_MAX_SPLIT_PAGES)
            elog(ERROR, "GiST page split into too many halves (%d, maximum %d)",
                 npage, GIST_MAX_SPLIT_PAGES);

        /*
         * Set up pages to work with. Allocate new buffers for all but the
         * leftmost page. The original page becomes the new leftmost page, and
         * is just replaced with the new contents.
         *
         * For a root-split, allocate new buffers for all child pages, the
         * original page is overwritten with new root page containing
         * downlinks to the new child pages.
         */
        ptr = dist;
        if (!is_rootsplit)
        {
            /* save old rightlink and NSN */
            oldrlink = GistPageGetOpaque(page)->rightlink;
            oldnsn = GistPageGetNSN(page);

            dist->buffer = buffer;
            dist->block.blkno = BufferGetBlockNumber(buffer);
            dist->page = PageGetTempPageCopySpecial(BufferGetPage(buffer));

            /* clean all flags except F_LEAF */
            GistPageGetOpaque(dist->page)->flags = (is_leaf) ? F_LEAF : 0;

            ptr = ptr->next;
        }
        for (; ptr; ptr = ptr->next)
        {
            /* Allocate new page */
            ptr->buffer = gistNewBuffer(rel);
            GISTInitBuffer(ptr->buffer, (is_leaf) ? F_LEAF : 0);
            ptr->page = BufferGetPage(ptr->buffer);
            ptr->block.blkno = BufferGetBlockNumber(ptr->buffer);
        }

        /*
         * Now that we know which blocks the new pages go to, set up downlink
         * tuples to point to them.
         */
        for (ptr = dist; ptr; ptr = ptr->next)
        {
            ItemPointerSetBlockNumber(&(ptr->itup->t_tid), ptr->block.blkno);
            GistTupleSetValid(ptr->itup);
        }

        /*
         * If this is a root split, we construct the new root page with the
         * downlinks here directly, instead of requiring the caller to insert
         * them. Add the new root page to the list along with the child pages.
         */
        if (is_rootsplit)
        {
            IndexTuple *downlinks;
            int         ndownlinks = 0;
            int         i;

            rootpg.buffer = buffer;
            rootpg.page = PageGetTempPageCopySpecial(BufferGetPage(rootpg.buffer));
            GistPageGetOpaque(rootpg.page)->flags = 0;

            /* Prepare a vector of all the downlinks */
            for (ptr = dist; ptr; ptr = ptr->next)
                ndownlinks++;
            downlinks = palloc(sizeof(IndexTuple) * ndownlinks);
            for (i = 0, ptr = dist; ptr; ptr = ptr->next)
                downlinks[i++] = ptr->itup;

            rootpg.block.blkno = GIST_ROOT_BLKNO;
            rootpg.block.num = ndownlinks;
            rootpg.list = gistfillitupvec(downlinks, ndownlinks,
                                          &(rootpg.lenlist));
            rootpg.itup = NULL;

            rootpg.next = dist;
            dist = &rootpg;
        }
        else
        {
            /* Prepare split-info to be returned to caller */
            for (ptr = dist; ptr; ptr = ptr->next)
            {
                GISTPageSplitInfo *si = palloc(sizeof(GISTPageSplitInfo));

                si->buf = ptr->buffer;
                si->downlink = ptr->itup;
                *splitinfo = lappend(*splitinfo, si);
            }
        }

        /*
         * Fill all pages. All the pages are new, ie. freshly allocated empty
         * pages, or a temporary copy of the old page.
         */
        for (ptr = dist; ptr; ptr = ptr->next)
        {
            char       *data = (char *) (ptr->list);

            for (i = 0; i < ptr->block.num; i++)
            {
                IndexTuple  thistup = (IndexTuple) data;

                if (PageAddItem(ptr->page, (Item) data, IndexTupleSize(thistup), i + FirstOffsetNumber, false, false) == InvalidOffsetNumber)
                    elog(ERROR, "failed to add item to index page in \"%s\"", RelationGetRelationName(rel));

                /*
                 * If this is the first inserted/updated tuple, let the caller
                 * know which page it landed on.
                 */
                if (newblkno && ItemPointerEquals(&thistup->t_tid, &(*itup)->t_tid))
                    *newblkno = ptr->block.blkno;

                data += IndexTupleSize(thistup);
            }

            /* Set up rightlinks */
            if (ptr->next && ptr->block.blkno != GIST_ROOT_BLKNO)
                GistPageGetOpaque(ptr->page)->rightlink =
                    ptr->next->block.blkno;
            else
                GistPageGetOpaque(ptr->page)->rightlink = oldrlink;

            /*
             * Mark the all but the right-most page with the follow-right
             * flag. It will be cleared as soon as the downlink is inserted
             * into the parent, but this ensures that if we error out before
             * that, the index is still consistent. (in buffering build mode,
             * any error will abort the index build anyway, so this is not
             * needed.)
             */
            if (ptr->next && !is_rootsplit && markfollowright)
                GistMarkFollowRight(ptr->page);
            else
                GistClearFollowRight(ptr->page);

            /*
             * Copy the NSN of the original page to all pages. The
             * F_FOLLOW_RIGHT flags ensure that scans will follow the
             * rightlinks until the downlinks are inserted.
             */
            GistPageSetNSN(ptr->page, oldnsn);
        }

        /*
         * gistXLogSplit() needs to WAL log a lot of pages, prepare WAL
         * insertion for that. NB: The number of pages and data segments
         * specified here must match the calculations in gistXLogSplit()!
         */
        if (RelationNeedsWAL(rel))
            XLogEnsureRecordSpace(npage, 1 + npage * 2);

        START_CRIT_SECTION();

        /*
         * Must mark buffers dirty before XLogInsert, even though we'll still
         * be changing their opaque fields below.
         */
        for (ptr = dist; ptr; ptr = ptr->next)
            MarkBufferDirty(ptr->buffer);
        if (BufferIsValid(leftchildbuf))
            MarkBufferDirty(leftchildbuf);

        /*
         * The first page in the chain was a temporary working copy meant to
         * replace the old page. Copy it over the old page.
         */
        PageRestoreTempPage(dist->page, BufferGetPage(dist->buffer));
        dist->page = BufferGetPage(dist->buffer);

        /* Write the WAL record */
        if (RelationNeedsWAL(rel))
            recptr = gistXLogSplit(rel->rd_node, blkno, is_leaf,
                                   dist, oldrlink, oldnsn, leftchildbuf,
                                   markfollowright);
        else
            recptr = gistGetFakeLSN(rel);

        for (ptr = dist; ptr; ptr = ptr->next)
        {
            PageSetLSN(ptr->page, recptr);
        }

        /*
         * Return the new child buffers to the caller.
         *
         * If this was a root split, we've already inserted the downlink
         * pointers, in the form of a new root page. Therefore we can release
         * all the new buffers, and keep just the root page locked.
         */
        if (is_rootsplit)
        {
            for (ptr = dist->next; ptr; ptr = ptr->next)
                UnlockReleaseBuffer(ptr->buffer);
        }
    }
    else
    {
        /*
         * Enough space. We also get here if ntuples==0.
         */
        START_CRIT_SECTION();

        /*
         * While we delete only one tuple at once we could mix calls
         * PageIndexTupleDelete() here and PageIndexMultiDelete() in
         * gistRedoPageUpdateRecord()
         */
        if (OffsetNumberIsValid(oldoffnum))
            PageIndexTupleDelete(page, oldoffnum);
        gistfillbuffer(page, itup, ntup, InvalidOffsetNumber);

        MarkBufferDirty(buffer);

        if (BufferIsValid(leftchildbuf))
            MarkBufferDirty(leftchildbuf);

        if (RelationNeedsWAL(rel))
        {
            OffsetNumber ndeloffs = 0,
                        deloffs[1];

            if (OffsetNumberIsValid(oldoffnum))
            {
                deloffs[0] = oldoffnum;
                ndeloffs = 1;
            }

            recptr = gistXLogUpdate(rel->rd_node, buffer,
                                    deloffs, ndeloffs, itup, ntup,
                                    leftchildbuf);

            PageSetLSN(page, recptr);
        }
        else
        {
            recptr = gistGetFakeLSN(rel);
            PageSetLSN(page, recptr);
        }

        if (newblkno)
            *newblkno = blkno;
    }

    /*
     * If we inserted the downlink for a child page, set NSN and clear
     * F_FOLLOW_RIGHT flag on the left child, so that concurrent scans know to
     * follow the rightlink if and only if they looked at the parent page
     * before we inserted the downlink.
     *
     * Note that we do this *after* writing the WAL record. That means that
     * the possible full page image in the WAL record does not include these
     * changes, and they must be replayed even if the page is restored from
     * the full page image. There's a chicken-and-egg problem: if we updated
     * the child pages first, we wouldn't know the recptr of the WAL record
     * we're about to write.
     */
    if (BufferIsValid(leftchildbuf))
    {
        Page        leftpg = BufferGetPage(leftchildbuf);

        GistPageSetNSN(leftpg, recptr);
        GistClearFollowRight(leftpg);

        PageSetLSN(leftpg, recptr);
    }

    END_CRIT_SECTION();

    return is_split;
}

/*
 * Workhouse routine for doing insertion into a GiST index. Note that
 * this routine assumes it is invoked in a short-lived memory context,
 * so it does not bother releasing palloc'd allocations.
 */
void
gistdoinsert(Relation r, IndexTuple itup, Size freespace, GISTSTATE *giststate)
{
    ItemId      iid;
    IndexTuple  idxtuple;
    GISTInsertStack firststack;
    GISTInsertStack *stack;
    GISTInsertState state;
    bool        xlocked = false;

    memset(&state, 0, sizeof(GISTInsertState));
    state.freespace = freespace;
    state.r = r;

    /* Start from the root */
    firststack.blkno = GIST_ROOT_BLKNO;
    firststack.lsn = 0;
    firststack.parent = NULL;
    firststack.downlinkoffnum = InvalidOffsetNumber;
    state.stack = stack = &firststack;

    /*
     * Walk down along the path of smallest penalty, updating the parent
     * pointers with the key we're inserting as we go. If we crash in the
     * middle, the tree is consistent, although the possible parent updates
     * were a waste.
     */
    for (;;)
    {
        if (XLogRecPtrIsInvalid(stack->lsn))
            stack->buffer = ReadBuffer(state.r, stack->blkno);

        /*
         * Be optimistic and grab shared lock first. Swap it for an exclusive
         * lock later if we need to update the page.
         */
        if (!xlocked)
        {
            LockBuffer(stack->buffer, GIST_SHARE);
            gistcheckpage(state.r, stack->buffer);
        }

        stack->page = (Page) BufferGetPage(stack->buffer);
        stack->lsn = PageGetLSN(stack->page);
        Assert(!RelationNeedsWAL(state.r) || !XLogRecPtrIsInvalid(stack->lsn));

        /*
         * If this page was split but the downlink was never inserted to the
         * parent because the inserting backend crashed before doing that, fix
         * that now.
         */
        if (GistFollowRight(stack->page))
        {
            if (!xlocked)
            {
                LockBuffer(stack->buffer, GIST_UNLOCK);
                LockBuffer(stack->buffer, GIST_EXCLUSIVE);
                xlocked = true;
                /* someone might've completed the split when we unlocked */
                if (!GistFollowRight(stack->page))
                    continue;
            }
            gistfixsplit(&state, giststate);

            UnlockReleaseBuffer(stack->buffer);
            xlocked = false;
            state.stack = stack = stack->parent;
            continue;
        }

        if (stack->blkno != GIST_ROOT_BLKNO &&
            stack->parent->lsn < GistPageGetNSN(stack->page))
        {
            /*
             * Concurrent split detected. There's no guarantee that the
             * downlink for this page is consistent with the tuple we're
             * inserting anymore, so go back to parent and rechoose the best
             * child.
             */
            UnlockReleaseBuffer(stack->buffer);
            xlocked = false;
            state.stack = stack = stack->parent;
            continue;
        }

        if (!GistPageIsLeaf(stack->page))
        {
            /*
             * This is an internal page so continue to walk down the tree.
             * Find the child node that has the minimum insertion penalty.
             */
            BlockNumber childblkno;
            IndexTuple  newtup;
            GISTInsertStack *item;
            OffsetNumber downlinkoffnum;

            downlinkoffnum = gistchoose(state.r, stack->page, itup, giststate);
            iid = PageGetItemId(stack->page, downlinkoffnum);
            idxtuple = (IndexTuple) PageGetItem(stack->page, iid);
            childblkno = ItemPointerGetBlockNumber(&(idxtuple->t_tid));

            /*
             * Check that it's not a leftover invalid tuple from pre-9.1
             */
            if (GistTupleIsInvalid(idxtuple))
                ereport(ERROR,
                        (errmsg("index \"%s\" contains an inner tuple marked as invalid",
                                RelationGetRelationName(r)),
                         errdetail("This is caused by an incomplete page split at crash recovery before upgrading to PostgreSQL 9.1."),
                         errhint("Please REINDEX it.")));

            /*
             * Check that the key representing the target child node is
             * consistent with the key we're inserting. Update it if it's not.
             */
            newtup = gistgetadjusted(state.r, idxtuple, itup, giststate);
            if (newtup)
            {
                /*
                 * Swap shared lock for an exclusive one. Beware, the page may
                 * change while we unlock/lock the page...
                 */
                if (!xlocked)
                {
                    LockBuffer(stack->buffer, GIST_UNLOCK);
                    LockBuffer(stack->buffer, GIST_EXCLUSIVE);
                    xlocked = true;
                    stack->page = (Page) BufferGetPage(stack->buffer);

                    if (PageGetLSN(stack->page) != stack->lsn)
                    {
                        /* the page was changed while we unlocked it, retry */
                        continue;
                    }
                }

                /*
                 * Update the tuple.
                 *
                 * We still hold the lock after gistinserttuple(), but it
                 * might have to split the page to make the updated tuple fit.
                 * In that case the updated tuple might migrate to the other
                 * half of the split, so we have to go back to the parent and
                 * descend back to the half that's a better fit for the new
                 * tuple.
                 */
                if (gistinserttuple(&state, stack, giststate, newtup,
                                    downlinkoffnum))
                {
                    /*
                     * If this was a root split, the root page continues to be
                     * the parent and the updated tuple went to one of the
                     * child pages, so we just need to retry from the root
                     * page.
                     */
                    if (stack->blkno != GIST_ROOT_BLKNO)
                    {
                        UnlockReleaseBuffer(stack->buffer);
                        xlocked = false;
                        state.stack = stack = stack->parent;
                    }
                    continue;
                }
            }
            LockBuffer(stack->buffer, GIST_UNLOCK);
            xlocked = false;

            /* descend to the chosen child */
            item = (GISTInsertStack *) palloc0(sizeof(GISTInsertStack));
            item->blkno = childblkno;
            item->parent = stack;
            item->downlinkoffnum = downlinkoffnum;
            state.stack = stack = item;
        }
        else
        {
            /*
             * Leaf page. Insert the new key. We've already updated all the
             * parents on the way down, but we might have to split the page if
             * it doesn't fit. gistinserthere() will take care of that.
             */

            /*
             * Swap shared lock for an exclusive one. Be careful, the page may
             * change while we unlock/lock the page...
             */
            if (!xlocked)
            {
                LockBuffer(stack->buffer, GIST_UNLOCK);
                LockBuffer(stack->buffer, GIST_EXCLUSIVE);
                xlocked = true;
                stack->page = (Page) BufferGetPage(stack->buffer);
                stack->lsn = PageGetLSN(stack->page);

                if (stack->blkno == GIST_ROOT_BLKNO)
                {
                    /*
                     * the only page that can become inner instead of leaf is
                     * the root page, so for root we should recheck it
                     */
                    if (!GistPageIsLeaf(stack->page))
                    {
                        /*
                         * very rare situation: during unlock/lock index with
                         * number of pages = 1 was increased
                         */
                        LockBuffer(stack->buffer, GIST_UNLOCK);
                        xlocked = false;
                        continue;
                    }

                    /*
                     * we don't need to check root split, because checking
                     * leaf/inner is enough to recognize split for root
                     */
                }
                else if (GistFollowRight(stack->page) ||
                         stack->parent->lsn < GistPageGetNSN(stack->page))
                {
                    /*
                     * The page was split while we momentarily unlocked the
                     * page. Go back to parent.
                     */
                    UnlockReleaseBuffer(stack->buffer);
                    xlocked = false;
                    state.stack = stack = stack->parent;
                    continue;
                }
            }

            /* now state.stack->(page, buffer and blkno) points to leaf page */

            gistinserttuple(&state, stack, giststate, itup,
                            InvalidOffsetNumber);
            LockBuffer(stack->buffer, GIST_UNLOCK);

            /* Release any pins we might still hold before exiting */
            for (; stack; stack = stack->parent)
                ReleaseBuffer(stack->buffer);
            break;
        }
    }
}

/*
 * Traverse the tree to find path from root page to specified "child" block.
 *
 * returns a new insertion stack, starting from the parent of "child", up
 * to the root. *downlinkoffnum is set to the offset of the downlink in the
 * direct parent of child.
 *
 * To prevent deadlocks, this should lock only one page at a time.
 */
static GISTInsertStack *
gistFindPath(Relation r, BlockNumber child, OffsetNumber *downlinkoffnum)
{
    Page        page;
    Buffer      buffer;
    OffsetNumber i,
                maxoff;
    ItemId      iid;
    IndexTuple  idxtuple;
    List       *fifo;
    GISTInsertStack *top,
               *ptr;
    BlockNumber blkno;

    top = (GISTInsertStack *) palloc0(sizeof(GISTInsertStack));
    top->blkno = GIST_ROOT_BLKNO;
    top->downlinkoffnum = InvalidOffsetNumber;

    fifo = list_make1(top);
    while (fifo != NIL)
    {
        /* Get next page to visit */
        top = linitial(fifo);
        fifo = list_delete_first(fifo);

        buffer = ReadBuffer(r, top->blkno);
        LockBuffer(buffer, GIST_SHARE);
        gistcheckpage(r, buffer);
        page = (Page) BufferGetPage(buffer);

        if (GistPageIsLeaf(page))
        {
            /*
             * Because we scan the index top-down, all the rest of the pages
             * in the queue must be leaf pages as well.
             */
            UnlockReleaseBuffer(buffer);
            break;
        }

        top->lsn = PageGetLSN(page);

        /*
         * If F_FOLLOW_RIGHT is set, the page to the right doesn't have a
         * downlink. This should not normally happen..
         */
        if (GistFollowRight(page))
            elog(ERROR, "concurrent GiST page split was incomplete");

        if (top->parent && top->parent->lsn < GistPageGetNSN(page) &&
            GistPageGetOpaque(page)->rightlink != InvalidBlockNumber /* sanity check */ )
        {
            /*
             * Page was split while we looked elsewhere. We didn't see the
             * downlink to the right page when we scanned the parent, so add
             * it to the queue now.
             *
             * Put the right page ahead of the queue, so that we visit it
             * next. That's important, because if this is the lowest internal
             * level, just above leaves, we might already have queued up some
             * leaf pages, and we assume that there can't be any non-leaf
             * pages behind leaf pages.
             */
            ptr = (GISTInsertStack *) palloc0(sizeof(GISTInsertStack));
            ptr->blkno = GistPageGetOpaque(page)->rightlink;
            ptr->downlinkoffnum = InvalidOffsetNumber;
            ptr->parent = top->parent;

            fifo = lcons(ptr, fifo);
        }

        maxoff = PageGetMaxOffsetNumber(page);

        for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
        {
            iid = PageGetItemId(page, i);
            idxtuple = (IndexTuple) PageGetItem(page, iid);
            blkno = ItemPointerGetBlockNumber(&(idxtuple->t_tid));
            if (blkno == child)
            {
                /* Found it! */
                UnlockReleaseBuffer(buffer);
                *downlinkoffnum = i;
                return top;
            }
            else
            {
                /* Append this child to the list of pages to visit later */
                ptr = (GISTInsertStack *) palloc0(sizeof(GISTInsertStack));
                ptr->blkno = blkno;
                ptr->downlinkoffnum = i;
                ptr->parent = top;

                fifo = lappend(fifo, ptr);
            }
        }

        UnlockReleaseBuffer(buffer);
    }

    elog(ERROR, "failed to re-find parent of a page in index \"%s\", block %u",
         RelationGetRelationName(r), child);
    return NULL;                /* keep compiler quiet */
}

/*
 * Updates the stack so that child->parent is the correct parent of the
 * child. child->parent must be exclusively locked on entry, and will
 * remain so at exit, but it might not be the same page anymore.
 */
static void
gistFindCorrectParent(Relation r, GISTInsertStack *child)
{
    GISTInsertStack *parent = child->parent;

    gistcheckpage(r, parent->buffer);
    parent->page = (Page) BufferGetPage(parent->buffer);

    /* here we don't need to distinguish between split and page update */
    if (child->downlinkoffnum == InvalidOffsetNumber ||
        parent->lsn != PageGetLSN(parent->page))
    {
        /* parent is changed, look child in right links until found */
        OffsetNumber i,
                    maxoff;
        ItemId      iid;
        IndexTuple  idxtuple;
        GISTInsertStack *ptr;

        while (true)
        {
            maxoff = PageGetMaxOffsetNumber(parent->page);
            for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
            {
                iid = PageGetItemId(parent->page, i);
                idxtuple = (IndexTuple) PageGetItem(parent->page, iid);
                if (ItemPointerGetBlockNumber(&(idxtuple->t_tid)) == child->blkno)
                {
                    /* yes!!, found */
                    child->downlinkoffnum = i;
                    return;
                }
            }

            parent->blkno = GistPageGetOpaque(parent->page)->rightlink;
            UnlockReleaseBuffer(parent->buffer);
            if (parent->blkno == InvalidBlockNumber)
            {
                /*
                 * End of chain and still didn't find parent. It's a very-very
                 * rare situation when root splited.
                 */
                break;
            }
            parent->buffer = ReadBuffer(r, parent->blkno);
            LockBuffer(parent->buffer, GIST_EXCLUSIVE);
            gistcheckpage(r, parent->buffer);
            parent->page = (Page) BufferGetPage(parent->buffer);
        }

        /*
         * awful!!, we need search tree to find parent ... , but before we
         * should release all old parent
         */

        ptr = child->parent->parent;    /* child->parent already released
                                         * above */
        while (ptr)
        {
            ReleaseBuffer(ptr->buffer);
            ptr = ptr->parent;
        }

        /* ok, find new path */
        ptr = parent = gistFindPath(r, child->blkno, &child->downlinkoffnum);

        /* read all buffers as expected by caller */
        /* note we don't lock them or gistcheckpage them here! */
        while (ptr)
        {
            ptr->buffer = ReadBuffer(r, ptr->blkno);
            ptr->page = (Page) BufferGetPage(ptr->buffer);
            ptr = ptr->parent;
        }

        /* install new chain of parents to stack */
        child->parent = parent;

        /* make recursive call to normal processing */
        LockBuffer(child->parent->buffer, GIST_EXCLUSIVE);
        gistFindCorrectParent(r, child);
    }

    return;
}

/*
 * Form a downlink pointer for the page in 'buf'.
 */
static IndexTuple
gistformdownlink(Relation rel, Buffer buf, GISTSTATE *giststate,
                 GISTInsertStack *stack)
{
    Page        page = BufferGetPage(buf);
    OffsetNumber maxoff;
    OffsetNumber offset;
    IndexTuple  downlink = NULL;

    maxoff = PageGetMaxOffsetNumber(page);
    for (offset = FirstOffsetNumber; offset <= maxoff; offset = OffsetNumberNext(offset))
    {
        IndexTuple  ituple = (IndexTuple)
        PageGetItem(page, PageGetItemId(page, offset));

        if (downlink == NULL)
            downlink = CopyIndexTuple(ituple);
        else
        {
            IndexTuple  newdownlink;

            newdownlink = gistgetadjusted(rel, downlink, ituple,
                                          giststate);
            if (newdownlink)
                downlink = newdownlink;
        }
    }

    /*
     * If the page is completely empty, we can't form a meaningful downlink
     * for it. But we have to insert a downlink for the page. Any key will do,
     * as long as its consistent with the downlink of parent page, so that we
     * can legally insert it to the parent. A minimal one that matches as few
     * scans as possible would be best, to keep scans from doing useless work,
     * but we don't know how to construct that. So we just use the downlink of
     * the original page that was split - that's as far from optimal as it can
     * get but will do..
     */
    if (!downlink)
    {
        ItemId      iid;

        LockBuffer(stack->parent->buffer, GIST_EXCLUSIVE);
        gistFindCorrectParent(rel, stack);
        iid = PageGetItemId(stack->parent->page, stack->downlinkoffnum);
        downlink = (IndexTuple) PageGetItem(stack->parent->page, iid);
        downlink = CopyIndexTuple(downlink);
        LockBuffer(stack->parent->buffer, GIST_UNLOCK);
    }

    ItemPointerSetBlockNumber(&(downlink->t_tid), BufferGetBlockNumber(buf));
    GistTupleSetValid(downlink);

    return downlink;
}


/*
 * Complete the incomplete split of state->stack->page.
 */
static void
gistfixsplit(GISTInsertState *state, GISTSTATE *giststate)
{
    GISTInsertStack *stack = state->stack;
    Buffer      buf;
    Page        page;
    List       *splitinfo = NIL;

    elog(LOG, "fixing incomplete split in index \"%s\", block %u",
         RelationGetRelationName(state->r), stack->blkno);

    Assert(GistFollowRight(stack->page));
    Assert(OffsetNumberIsValid(stack->downlinkoffnum));

    buf = stack->buffer;

    /*
     * Read the chain of split pages, following the rightlinks. Construct a
     * downlink tuple for each page.
     */
    for (;;)
    {
        GISTPageSplitInfo *si = palloc(sizeof(GISTPageSplitInfo));
        IndexTuple  downlink;

        page = BufferGetPage(buf);

        /* Form the new downlink tuples to insert to parent */
        downlink = gistformdownlink(state->r, buf, giststate, stack);

        si->buf = buf;
        si->downlink = downlink;

        splitinfo = lappend(splitinfo, si);

        if (GistFollowRight(page))
        {
            /* lock next page */
            buf = ReadBuffer(state->r, GistPageGetOpaque(page)->rightlink);
            LockBuffer(buf, GIST_EXCLUSIVE);
        }
        else
            break;
    }

    /* Insert the downlinks */
    gistfinishsplit(state, stack, giststate, splitinfo, false);
}

/*
 * Insert or replace a tuple in stack->buffer. If 'oldoffnum' is valid, the
 * tuple at 'oldoffnum' is replaced, otherwise the tuple is inserted as new.
 * 'stack' represents the path from the root to the page being updated.
 *
 * The caller must hold an exclusive lock on stack->buffer.  The lock is still
 * held on return, but the page might not contain the inserted tuple if the
 * page was split. The function returns true if the page was split, false
 * otherwise.
 */
static bool
gistinserttuple(GISTInsertState *state, GISTInsertStack *stack,
              GISTSTATE *giststate, IndexTuple tuple, OffsetNumber oldoffnum)
{
    return gistinserttuples(state, stack, giststate, &tuple, 1, oldoffnum,
                            InvalidBuffer, InvalidBuffer, false, false);
}

/* ----------------
 * An extended workhorse version of gistinserttuple(). This version allows
 * inserting multiple tuples, or replacing a single tuple with multiple tuples.
 * This is used to recursively update the downlinks in the parent when a page
 * is split.
 *
 * If leftchild and rightchild are valid, we're inserting/replacing the
 * downlink for rightchild, and leftchild is its left sibling. We clear the
 * F_FOLLOW_RIGHT flag and update NSN on leftchild, atomically with the
 * insertion of the downlink.
 *
 * To avoid holding locks for longer than necessary, when recursing up the
 * tree to update the parents, the locking is a bit peculiar here. On entry,
 * the caller must hold an exclusive lock on stack->buffer, as well as
 * leftchild and rightchild if given. On return:
 *
 *  - Lock on stack->buffer is released, if 'unlockbuf' is true. The page is
 *    always kept pinned, however.
 *  - Lock on 'leftchild' is released, if 'unlockleftchild' is true. The page
 *    is kept pinned.
 *  - Lock and pin on 'rightchild' are always released.
 *
 * Returns 'true' if the page had to be split. Note that if the page was
 * split, the inserted/updated tuples might've been inserted to a right
 * sibling of stack->buffer instead of stack->buffer itself.
 */
static bool
gistinserttuples(GISTInsertState *state, GISTInsertStack *stack,
                 GISTSTATE *giststate,
                 IndexTuple *tuples, int ntup, OffsetNumber oldoffnum,
                 Buffer leftchild, Buffer rightchild,
                 bool unlockbuf, bool unlockleftchild)
{
    List       *splitinfo;
    bool        is_split;

    /* Insert the tuple(s) to the page, splitting the page if necessary */
    is_split = gistplacetopage(state->r, state->freespace, giststate,
                               stack->buffer,
                               tuples, ntup,
                               oldoffnum, NULL,
                               leftchild,
                               &splitinfo,
                               true);

    /*
     * Before recursing up in case the page was split, release locks on the
     * child pages. We don't need to keep them locked when updating the
     * parent.
     */
    if (BufferIsValid(rightchild))
        UnlockReleaseBuffer(rightchild);
    if (BufferIsValid(leftchild) && unlockleftchild)
        LockBuffer(leftchild, GIST_UNLOCK);

    /*
     * If we had to split, insert/update the downlinks in the parent. If the
     * caller requested us to release the lock on stack->buffer, tell
     * gistfinishsplit() to do that as soon as it's safe to do so. If we
     * didn't have to split, release it ourselves.
     */
    if (splitinfo)
        gistfinishsplit(state, stack, giststate, splitinfo, unlockbuf);
    else if (unlockbuf)
        LockBuffer(stack->buffer, GIST_UNLOCK);

    return is_split;
}

/*
 * Finish an incomplete split by inserting/updating the downlinks in parent
 * page. 'splitinfo' contains all the child pages involved in the split,
 * from left-to-right.
 *
 * On entry, the caller must hold a lock on stack->buffer and all the child
 * pages in 'splitinfo'. If 'unlockbuf' is true, the lock on stack->buffer is
 * released on return. The child pages are always unlocked and unpinned.
 */
static void
gistfinishsplit(GISTInsertState *state, GISTInsertStack *stack,
                GISTSTATE *giststate, List *splitinfo, bool unlockbuf)
{
    ListCell   *lc;
    List       *reversed;
    GISTPageSplitInfo *right;
    GISTPageSplitInfo *left;
    IndexTuple  tuples[2];

    /* A split always contains at least two halves */
    Assert(list_length(splitinfo) >= 2);

    /*
     * We need to insert downlinks for each new page, and update the downlink
     * for the original (leftmost) page in the split. Begin at the rightmost
     * page, inserting one downlink at a time until there's only two pages
     * left. Finally insert the downlink for the last new page and update the
     * downlink for the original page as one operation.
     */

    /* for convenience, create a copy of the list in reverse order */
    reversed = NIL;
    foreach(lc, splitinfo)
    {
        reversed = lcons(lfirst(lc), reversed);
    }

    LockBuffer(stack->parent->buffer, GIST_EXCLUSIVE);
    gistFindCorrectParent(state->r, stack);

    /*
     * insert downlinks for the siblings from right to left, until there are
     * only two siblings left.
     */
    while (list_length(reversed) > 2)
    {
        right = (GISTPageSplitInfo *) linitial(reversed);
        left = (GISTPageSplitInfo *) lsecond(reversed);

        if (gistinserttuples(state, stack->parent, giststate,
                             &right->downlink, 1,
                             InvalidOffsetNumber,
                             left->buf, right->buf, false, false))
        {
            /*
             * If the parent page was split, need to relocate the original
             * parent pointer.
             */
            gistFindCorrectParent(state->r, stack);
        }
        /* gistinserttuples() released the lock on right->buf. */
        reversed = list_delete_first(reversed);
    }

    right = (GISTPageSplitInfo *) linitial(reversed);
    left = (GISTPageSplitInfo *) lsecond(reversed);

    /*
     * Finally insert downlink for the remaining right page and update the
     * downlink for the original page to not contain the tuples that were
     * moved to the new pages.
     */
    tuples[0] = left->downlink;
    tuples[1] = right->downlink;
    gistinserttuples(state, stack->parent, giststate,
                     tuples, 2,
                     stack->downlinkoffnum,
                     left->buf, right->buf,
                     true,      /* Unlock parent */
                     unlockbuf  /* Unlock stack->buffer if caller wants that */
        );
    Assert(left->buf == stack->buffer);
}

/*
 * gistSplit -- split a page in the tree and fill struct
 * used for XLOG and real writes buffers. Function is recursive, ie
 * it will split page until keys will fit in every page.
 */
SplitedPageLayout *
gistSplit(Relation r,
          Page page,
          IndexTuple *itup,     /* contains compressed entry */
          int len,
          GISTSTATE *giststate)
{
    IndexTuple *lvectup,
               *rvectup;
    GistSplitVector v;
    int         i;
    SplitedPageLayout *res = NULL;

    /* this should never recurse very deeply, but better safe than sorry */
    check_stack_depth();

    /* there's no point in splitting an empty page */
    Assert(len > 0);

    /*
     * If a single tuple doesn't fit on a page, no amount of splitting will
     * help.
     */
    if (len == 1)
        ereport(ERROR,
                (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
            errmsg("index row size %zu exceeds maximum %zu for index \"%s\"",
                   IndexTupleSize(itup[0]), GiSTPageSize,
                   RelationGetRelationName(r))));

    memset(v.spl_lisnull, TRUE, sizeof(bool) * giststate->tupdesc->natts);
    memset(v.spl_risnull, TRUE, sizeof(bool) * giststate->tupdesc->natts);
    gistSplitByKey(r, page, itup, len, giststate, &v, 0);

    /* form left and right vector */
    lvectup = (IndexTuple *) palloc(sizeof(IndexTuple) * (len + 1));
    rvectup = (IndexTuple *) palloc(sizeof(IndexTuple) * (len + 1));

    for (i = 0; i < v.splitVector.spl_nleft; i++)
        lvectup[i] = itup[v.splitVector.spl_left[i] - 1];

    for (i = 0; i < v.splitVector.spl_nright; i++)
        rvectup[i] = itup[v.splitVector.spl_right[i] - 1];

    /* finalize splitting (may need another split) */
    if (!gistfitpage(rvectup, v.splitVector.spl_nright))
    {
        res = gistSplit(r, page, rvectup, v.splitVector.spl_nright, giststate);
    }
    else
    {
        ROTATEDIST(res);
        res->block.num = v.splitVector.spl_nright;
        res->list = gistfillitupvec(rvectup, v.splitVector.spl_nright, &(res->lenlist));
        res->itup = gistFormTuple(giststate, r, v.spl_rattr, v.spl_risnull, false);
    }

    if (!gistfitpage(lvectup, v.splitVector.spl_nleft))
    {
        SplitedPageLayout *resptr,
                   *subres;

        resptr = subres = gistSplit(r, page, lvectup, v.splitVector.spl_nleft, giststate);

        /* install on list's tail */
        while (resptr->next)
            resptr = resptr->next;

        resptr->next = res;
        res = subres;
    }
    else
    {
        ROTATEDIST(res);
        res->block.num = v.splitVector.spl_nleft;
        res->list = gistfillitupvec(lvectup, v.splitVector.spl_nleft, &(res->lenlist));
        res->itup = gistFormTuple(giststate, r, v.spl_lattr, v.spl_lisnull, false);
    }

    return res;
}

/*
 * Create a GISTSTATE and fill it with information about the index
 */
GISTSTATE *
initGISTstate(Relation index)
{
    GISTSTATE  *giststate;
    MemoryContext scanCxt;
    MemoryContext oldCxt;
    int         i;

    /* safety check to protect fixed-size arrays in GISTSTATE */
    if (index->rd_att->natts > INDEX_MAX_KEYS)
        elog(ERROR, "numberOfAttributes %d > %d",
             index->rd_att->natts, INDEX_MAX_KEYS);

    /* Create the memory context that will hold the GISTSTATE */
    scanCxt = AllocSetContextCreate(CurrentMemoryContext,
                                    "GiST scan context",
                                    ALLOCSET_DEFAULT_MINSIZE,
                                    ALLOCSET_DEFAULT_INITSIZE,
                                    ALLOCSET_DEFAULT_MAXSIZE);
    oldCxt = MemoryContextSwitchTo(scanCxt);

    /* Create and fill in the GISTSTATE */
    giststate = (GISTSTATE *) palloc(sizeof(GISTSTATE));

    giststate->scanCxt = scanCxt;
    giststate->tempCxt = scanCxt;       /* caller must change this if needed */
    giststate->tupdesc = index->rd_att;

    for (i = 0; i < index->rd_att->natts; i++)
    {
        fmgr_info_copy(&(giststate->consistentFn[i]),
                       index_getprocinfo(index, i + 1, GIST_CONSISTENT_PROC),
                       scanCxt);
        fmgr_info_copy(&(giststate->unionFn[i]),
                       index_getprocinfo(index, i + 1, GIST_UNION_PROC),
                       scanCxt);
        fmgr_info_copy(&(giststate->compressFn[i]),
                       index_getprocinfo(index, i + 1, GIST_COMPRESS_PROC),
                       scanCxt);
        fmgr_info_copy(&(giststate->decompressFn[i]),
                       index_getprocinfo(index, i + 1, GIST_DECOMPRESS_PROC),
                       scanCxt);
        fmgr_info_copy(&(giststate->penaltyFn[i]),
                       index_getprocinfo(index, i + 1, GIST_PENALTY_PROC),
                       scanCxt);
        fmgr_info_copy(&(giststate->picksplitFn[i]),
                       index_getprocinfo(index, i + 1, GIST_PICKSPLIT_PROC),
                       scanCxt);
        fmgr_info_copy(&(giststate->equalFn[i]),
                       index_getprocinfo(index, i + 1, GIST_EQUAL_PROC),
                       scanCxt);
        /* opclasses are not required to provide a Distance method */
        if (OidIsValid(index_getprocid(index, i + 1, GIST_DISTANCE_PROC)))
            fmgr_info_copy(&(giststate->distanceFn[i]),
                         index_getprocinfo(index, i + 1, GIST_DISTANCE_PROC),
                           scanCxt);
        else
            giststate->distanceFn[i].fn_oid = InvalidOid;

        /* opclasses are not required to provide a Fetch method */
        if (OidIsValid(index_getprocid(index, i + 1, GIST_FETCH_PROC)))
            fmgr_info_copy(&(giststate->fetchFn[i]),
                           index_getprocinfo(index, i + 1, GIST_FETCH_PROC),
                           scanCxt);
        else
            giststate->fetchFn[i].fn_oid = InvalidOid;

        /*
         * If the index column has a specified collation, we should honor that
         * while doing comparisons.  However, we may have a collatable storage
         * type for a noncollatable indexed data type.  If there's no index
         * collation then specify default collation in case the support
         * functions need collation.  This is harmless if the support
         * functions don't care about collation, so we just do it
         * unconditionally.  (We could alternatively call get_typcollation,
         * but that seems like expensive overkill --- there aren't going to be
         * any cases where a GiST storage type has a nondefault collation.)
         */
        if (OidIsValid(index->rd_indcollation[i]))
            giststate->supportCollation[i] = index->rd_indcollation[i];
        else
            giststate->supportCollation[i] = DEFAULT_COLLATION_OID;
    }

    MemoryContextSwitchTo(oldCxt);

    return giststate;
}

void
freeGISTstate(GISTSTATE *giststate)
{
    /* It's sufficient to delete the scanCxt */
    MemoryContextDelete(giststate->scanCxt);
}

/*
 * gistvacuumpage() -- try to remove LP_DEAD items from the given page.
 * Function assumes that buffer is exclusively locked.
 */
static void
gistvacuumpage(Relation rel, Page page, Buffer buffer)
{
    OffsetNumber deletable[MaxIndexTuplesPerPage];
    int          ndeletable = 0;
    OffsetNumber offnum, maxoff;

    Assert(GistPageIsLeaf(page));

    /*
     * Scan over all items to see which ones need to be deleted according to
     * LP_DEAD flags.
     */
    maxoff = PageGetMaxOffsetNumber(page);
    for (offnum = FirstOffsetNumber;
         offnum <= maxoff;
         offnum = OffsetNumberNext(offnum))
    {
        ItemId      itemId = PageGetItemId(page, offnum);

        if (ItemIdIsDead(itemId))
            deletable[ndeletable++] = offnum;
    }

    if (ndeletable > 0)
    {
        START_CRIT_SECTION();

        PageIndexMultiDelete(page, deletable, ndeletable);

        /*
         * Mark the page as not containing any LP_DEAD items.  This is not
         * certainly true (there might be some that have recently been marked,
         * but weren't included in our target-item list), but it will almost
         * always be true and it doesn't seem worth an additional page scan to
         * check it. Remember that F_HAS_GARBAGE is only a hint anyway.
         */
        GistClearPageHasGarbage(page);

        MarkBufferDirty(buffer);

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

            recptr = gistXLogUpdate(rel->rd_node, buffer,
                                    deletable, ndeletable,
                                    NULL, 0, InvalidBuffer);

            PageSetLSN(page, recptr);
        }
        else
            PageSetLSN(page, gistGetFakeLSN(rel));

        END_CRIT_SECTION();
    }

    /*
     * Note: if we didn't find any LP_DEAD items, then the page's
     * F_HAS_GARBAGE hint bit is falsely set.  We do not bother expending a
     * separate write to clear it, however.  We will clear it when we split
     * the page.
     */
}