gistget.c

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/*-------------------------------------------------------------------------
 *
 * gistget.c
 *    fetch tuples from a GiST scan.
 *
 *
 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/gist/gistget.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "access/genam.h"
#include "access/gist_private.h"
#include "access/relscan.h"
#include "miscadmin.h"
#include "storage/lmgr.h"
#include "storage/predicate.h"
#include "pgstat.h"
#include "lib/pairingheap.h"
#include "utils/float.h"
#include "utils/memutils.h"
#include "utils/rel.h"

/*
 * gistkillitems() -- set LP_DEAD state for items an indexscan caller has
 * told us were killed.
 *
 * We re-read page here, so it's important to check page LSN. If the page
 * has been modified since the last read (as determined by LSN), we cannot
 * flag any entries because it is possible that the old entry was vacuumed
 * away and the TID was re-used by a completely different heap tuple.
 */
static void
gistkillitems(IndexScanDesc scan)
{
    GISTScanOpaque so = (GISTScanOpaque) scan->opaque;
    Buffer      buffer;
    Page        page;
    OffsetNumber offnum;
    ItemId      iid;
    int         i;
    bool        killedsomething = false;

    Assert(so->curBlkno != InvalidBlockNumber);
    Assert(!XLogRecPtrIsInvalid(so->curPageLSN));
    Assert(so->killedItems != NULL);

    buffer = ReadBuffer(scan->indexRelation, so->curBlkno);
    if (!BufferIsValid(buffer))
        return;

    LockBuffer(buffer, GIST_SHARE);
    gistcheckpage(scan->indexRelation, buffer);
    page = BufferGetPage(buffer);

    /*
     * If page LSN differs it means that the page was modified since the last
     * read. killedItems could be not valid so LP_DEAD hints applying is not
     * safe.
     */
    if (BufferGetLSNAtomic(buffer) != so->curPageLSN)
    {
        UnlockReleaseBuffer(buffer);
        so->numKilled = 0;      /* reset counter */
        return;
    }

    Assert(GistPageIsLeaf(page));

    /*
     * Mark all killedItems as dead. We need no additional recheck, because,
     * if page was modified, curPageLSN must have changed.
     */
    for (i = 0; i < so->numKilled; i++)
    {
        offnum = so->killedItems[i];
        iid = PageGetItemId(page, offnum);
        ItemIdMarkDead(iid);
        killedsomething = true;
    }

    if (killedsomething)
    {
        GistMarkPageHasGarbage(page);
        MarkBufferDirtyHint(buffer, true);
    }

    UnlockReleaseBuffer(buffer);

    /*
     * Always reset the scan state, so we don't look for same items on other
     * pages.
     */
    so->numKilled = 0;
}

/*
 * gistindex_keytest() -- does this index tuple satisfy the scan key(s)?
 *
 * The index tuple might represent either a heap tuple or a lower index page,
 * depending on whether the containing page is a leaf page or not.
 *
 * On success return for a heap tuple, *recheck_p is set to indicate whether
 * the quals need to be rechecked.  We recheck if any of the consistent()
 * functions request it.  recheck is not interesting when examining a non-leaf
 * entry, since we must visit the lower index page if there's any doubt.
 * Similarly, *recheck_distances_p is set to indicate whether the distances
 * need to be rechecked, and it is also ignored for non-leaf entries.
 *
 * If we are doing an ordered scan, so->distancesValues[] and
 * so->distancesNulls[] is filled with distance data from the distance()
 * functions before returning success.
 *
 * We must decompress the key in the IndexTuple before passing it to the
 * sk_funcs (which actually are the opclass Consistent or Distance methods).
 *
 * Note that this function is always invoked in a short-lived memory context,
 * so we don't need to worry about cleaning up allocated memory, either here
 * or in the implementation of any Consistent or Distance methods.
 */
static bool
gistindex_keytest(IndexScanDesc scan,
                  IndexTuple tuple,
                  Page page,
                  OffsetNumber offset,
                  bool *recheck_p,
                  bool *recheck_distances_p)
{
    GISTScanOpaque so = (GISTScanOpaque) scan->opaque;
    GISTSTATE  *giststate = so->giststate;
    ScanKey     key = scan->keyData;
    int         keySize = scan->numberOfKeys;
    double     *distance_value_p;
    bool       *distance_null_p;
    Relation    r = scan->indexRelation;

    *recheck_p = false;
    *recheck_distances_p = false;

    /*
     * If it's a leftover invalid tuple from pre-9.1, treat it as a match with
     * minimum possible distances.  This means we'll always follow it to the
     * referenced page.
     */
    if (GistTupleIsInvalid(tuple))
    {
        int         i;

        if (GistPageIsLeaf(page))   /* shouldn't happen */
            elog(ERROR, "invalid GiST tuple found on leaf page");
        for (i = 0; i < scan->numberOfOrderBys; i++)
        {
            so->distanceValues[i] = -get_float8_infinity();
            so->distanceNulls[i] = false;
        }
        return true;
    }

    /* Check whether it matches according to the Consistent functions */
    while (keySize > 0)
    {
        Datum       datum;
        bool        isNull;

        datum = index_getattr(tuple,
                              key->sk_attno,
                              giststate->leafTupdesc,
                              &isNull);

        if (key->sk_flags & SK_ISNULL)
        {
            /*
             * On non-leaf page we can't conclude that child hasn't NULL
             * values because of assumption in GiST: union (VAL, NULL) is VAL.
             * But if on non-leaf page key IS NULL, then all children are
             * NULL.
             */
            if (key->sk_flags & SK_SEARCHNULL)
            {
                if (GistPageIsLeaf(page) && !isNull)
                    return false;
            }
            else
            {
                Assert(key->sk_flags & SK_SEARCHNOTNULL);
                if (isNull)
                    return false;
            }
        }
        else if (isNull)
        {
            return false;
        }
        else
        {
            Datum       test;
            bool        recheck;
            GISTENTRY   de;

            gistdentryinit(giststate, key->sk_attno - 1, &de,
                           datum, r, page, offset,
                           false, isNull);

            /*
             * Call the Consistent function to evaluate the test.  The
             * arguments are the index datum (as a GISTENTRY*), the comparison
             * datum, the comparison operator's strategy number and subtype
             * from pg_amop, and the recheck flag.
             *
             * (Presently there's no need to pass the subtype since it'll
             * always be zero, but might as well pass it for possible future
             * use.)
             *
             * We initialize the recheck flag to true (the safest assumption)
             * in case the Consistent function forgets to set it.
             */
            recheck = true;

            test = FunctionCall5Coll(&key->sk_func,
                                     key->sk_collation,
                                     PointerGetDatum(&de),
                                     key->sk_argument,
                                     Int16GetDatum(key->sk_strategy),
                                     ObjectIdGetDatum(key->sk_subtype),
                                     PointerGetDatum(&recheck));

            if (!DatumGetBool(test))
                return false;
            *recheck_p |= recheck;
        }

        key++;
        keySize--;
    }

    /* OK, it passes --- now let's compute the distances */
    key = scan->orderByData;
    distance_value_p = so->distanceValues;
    distance_null_p = so->distanceNulls;
    keySize = scan->numberOfOrderBys;
    while (keySize > 0)
    {
        Datum       datum;
        bool        isNull;

        datum = index_getattr(tuple,
                              key->sk_attno,
                              giststate->leafTupdesc,
                              &isNull);

        if ((key->sk_flags & SK_ISNULL) || isNull)
        {
            /* Assume distance computes as null */
            *distance_value_p = 0.0;
            *distance_null_p = true;
        }
        else
        {
            Datum       dist;
            bool        recheck;
            GISTENTRY   de;

            gistdentryinit(giststate, key->sk_attno - 1, &de,
                           datum, r, page, offset,
                           false, isNull);

            /*
             * Call the Distance function to evaluate the distance.  The
             * arguments are the index datum (as a GISTENTRY*), the comparison
             * datum, the ordering operator's strategy number and subtype from
             * pg_amop, and the recheck flag.
             *
             * (Presently there's no need to pass the subtype since it'll
             * always be zero, but might as well pass it for possible future
             * use.)
             *
             * If the function sets the recheck flag, the returned distance is
             * a lower bound on the true distance and needs to be rechecked.
             * We initialize the flag to 'false'.  This flag was added in
             * version 9.5; distance functions written before that won't know
             * about the flag, but are expected to never be lossy.
             */
            recheck = false;
            dist = FunctionCall5Coll(&key->sk_func,
                                     key->sk_collation,
                                     PointerGetDatum(&de),
                                     key->sk_argument,
                                     Int16GetDatum(key->sk_strategy),
                                     ObjectIdGetDatum(key->sk_subtype),
                                     PointerGetDatum(&recheck));
            *recheck_distances_p |= recheck;
            *distance_value_p = DatumGetFloat8(dist);
            *distance_null_p = false;
        }

        key++;
        distance_value_p++;
        distance_null_p++;
        keySize--;
    }

    return true;
}

/*
 * Scan all items on the GiST index page identified by *pageItem, and insert
 * them into the queue (or directly to output areas)
 *
 * scan: index scan we are executing
 * pageItem: search queue item identifying an index page to scan
 * myDistanceValues: distances array associated with pageItem, or NULL at the root
 * myDistanceNulls: null flags for myDistanceValues array, or NULL at the root
 * tbm: if not NULL, gistgetbitmap's output bitmap
 * ntids: if not NULL, gistgetbitmap's output tuple counter
 *
 * If tbm/ntids aren't NULL, we are doing an amgetbitmap scan, and heap
 * tuples should be reported directly into the bitmap.  If they are NULL,
 * we're doing a plain or ordered indexscan.  For a plain indexscan, heap
 * tuple TIDs are returned into so->pageData[].  For an ordered indexscan,
 * heap tuple TIDs are pushed into individual search queue items.  In an
 * index-only scan, reconstructed index tuples are returned along with the
 * TIDs.
 *
 * If we detect that the index page has split since we saw its downlink
 * in the parent, we push its new right sibling onto the queue so the
 * sibling will be processed next.
 */
static void
gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem,
             double *myDistanceValues, bool *myDistanceNulls,
             TIDBitmap *tbm, int64 *ntids)
{
    GISTScanOpaque so = (GISTScanOpaque) scan->opaque;
    GISTSTATE  *giststate = so->giststate;
    Relation    r = scan->indexRelation;
    Buffer      buffer;
    Page        page;
    GISTPageOpaque opaque;
    OffsetNumber maxoff;
    OffsetNumber i;
    MemoryContext oldcxt;

    Assert(!GISTSearchItemIsHeap(*pageItem));

    buffer = ReadBuffer(scan->indexRelation, pageItem->blkno);
    LockBuffer(buffer, GIST_SHARE);
    PredicateLockPage(r, BufferGetBlockNumber(buffer), scan->xs_snapshot);
    gistcheckpage(scan->indexRelation, buffer);
    page = BufferGetPage(buffer);
    TestForOldSnapshot(scan->xs_snapshot, r, page);
    opaque = GistPageGetOpaque(page);

    /*
     * Check if we need to follow the rightlink. We need to follow it if the
     * page was concurrently split since we visited the parent (in which case
     * parentlsn < nsn), or if the system crashed after a page split but
     * before the downlink was inserted into the parent.
     */
    if (!XLogRecPtrIsInvalid(pageItem->data.parentlsn) &&
        (GistFollowRight(page) ||
         pageItem->data.parentlsn < GistPageGetNSN(page)) &&
        opaque->rightlink != InvalidBlockNumber /* sanity check */ )
    {
        /* There was a page split, follow right link to add pages */
        GISTSearchItem *item;

        /* This can't happen when starting at the root */
        Assert(myDistanceValues != NULL && myDistanceNulls != NULL);

        oldcxt = MemoryContextSwitchTo(so->queueCxt);

        /* Create new GISTSearchItem for the right sibling index page */
        item = palloc(SizeOfGISTSearchItem(scan->numberOfOrderBys));
        item->blkno = opaque->rightlink;
        item->data.parentlsn = pageItem->data.parentlsn;

        /* Insert it into the queue using same distances as for this page */
        memcpy(GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
               myDistanceValues, sizeof(double) * scan->numberOfOrderBys);
        memcpy(GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
               myDistanceNulls, sizeof(bool) * scan->numberOfOrderBys);

        pairingheap_add(so->queue, &item->phNode);

        MemoryContextSwitchTo(oldcxt);
    }

    /*
     * Check if the page was deleted after we saw the downlink. There's
     * nothing of interest on a deleted page. Note that we must do this after
     * checking the NSN for concurrent splits! It's possible that the page
     * originally contained some tuples that are visible to us, but was split
     * so that all the visible tuples were moved to another page, and then
     * this page was deleted.
     */
    if (GistPageIsDeleted(page))
    {
        UnlockReleaseBuffer(buffer);
        return;
    }

    so->nPageData = so->curPageData = 0;
    scan->xs_hitup = NULL;      /* might point into pageDataCxt */
    if (so->pageDataCxt)
        MemoryContextReset(so->pageDataCxt);

    /*
     * We save the LSN of the page as we read it, so that we know whether it
     * safe to apply LP_DEAD hints to the page later. This allows us to drop
     * the pin for MVCC scans, which allows vacuum to avoid blocking.
     */
    so->curPageLSN = BufferGetLSNAtomic(buffer);

    /*
     * check all tuples on page
     */
    maxoff = PageGetMaxOffsetNumber(page);
    for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
    {
        ItemId      iid = PageGetItemId(page, i);
        IndexTuple  it;
        bool        match;
        bool        recheck;
        bool        recheck_distances;

        /*
         * If the scan specifies not to return killed tuples, then we treat a
         * killed tuple as not passing the qual.
         */
        if (scan->ignore_killed_tuples && ItemIdIsDead(iid))
            continue;

        it = (IndexTuple) PageGetItem(page, iid);

        /*
         * Must call gistindex_keytest in tempCxt, and clean up any leftover
         * junk afterward.
         */
        oldcxt = MemoryContextSwitchTo(so->giststate->tempCxt);

        match = gistindex_keytest(scan, it, page, i,
                                  &recheck, &recheck_distances);

        MemoryContextSwitchTo(oldcxt);
        MemoryContextReset(so->giststate->tempCxt);

        /* Ignore tuple if it doesn't match */
        if (!match)
            continue;

        if (tbm && GistPageIsLeaf(page))
        {
            /*
             * getbitmap scan, so just push heap tuple TIDs into the bitmap
             * without worrying about ordering
             */
            tbm_add_tuples(tbm, &it->t_tid, 1, recheck);
            (*ntids)++;
        }
        else if (scan->numberOfOrderBys == 0 && GistPageIsLeaf(page))
        {
            /*
             * Non-ordered scan, so report tuples in so->pageData[]
             */
            so->pageData[so->nPageData].heapPtr = it->t_tid;
            so->pageData[so->nPageData].recheck = recheck;
            so->pageData[so->nPageData].offnum = i;

            /*
             * In an index-only scan, also fetch the data from the tuple.  The
             * reconstructed tuples are stored in pageDataCxt.
             */
            if (scan->xs_want_itup)
            {
                oldcxt = MemoryContextSwitchTo(so->pageDataCxt);
                so->pageData[so->nPageData].recontup =
                    gistFetchTuple(giststate, r, it);
                MemoryContextSwitchTo(oldcxt);
            }
            so->nPageData++;
        }
        else
        {
            /*
             * Must push item into search queue.  We get here for any lower
             * index page, and also for heap tuples if doing an ordered
             * search.
             */
            GISTSearchItem *item;
            int         nOrderBys = scan->numberOfOrderBys;

            oldcxt = MemoryContextSwitchTo(so->queueCxt);

            /* Create new GISTSearchItem for this item */
            item = palloc(SizeOfGISTSearchItem(scan->numberOfOrderBys));

            if (GistPageIsLeaf(page))
            {
                /* Creating heap-tuple GISTSearchItem */
                item->blkno = InvalidBlockNumber;
                item->data.heap.heapPtr = it->t_tid;
                item->data.heap.recheck = recheck;
                item->data.heap.recheckDistances = recheck_distances;

                /*
                 * In an index-only scan, also fetch the data from the tuple.
                 */
                if (scan->xs_want_itup)
                    item->data.heap.recontup = gistFetchTuple(giststate, r, it);
            }
            else
            {
                /* Creating index-page GISTSearchItem */
                item->blkno = ItemPointerGetBlockNumber(&it->t_tid);

                /*
                 * LSN of current page is lsn of parent page for child. We
                 * only have a shared lock, so we need to get the LSN
                 * atomically.
                 */
                item->data.parentlsn = BufferGetLSNAtomic(buffer);
            }

            /* Insert it into the queue using new distance data */
            memcpy(GISTSearchItemDistanceValues(item, nOrderBys),
                   so->distanceValues, sizeof(double) * nOrderBys);
            memcpy(GISTSearchItemDistanceNulls(item, nOrderBys),
                   so->distanceNulls, sizeof(bool) * nOrderBys);

            pairingheap_add(so->queue, &item->phNode);

            MemoryContextSwitchTo(oldcxt);
        }
    }

    UnlockReleaseBuffer(buffer);
}

/*
 * Extract next item (in order) from search queue
 *
 * Returns a GISTSearchItem or NULL.  Caller must pfree item when done with it.
 */
static GISTSearchItem *
getNextGISTSearchItem(GISTScanOpaque so)
{
    GISTSearchItem *item;

    if (!pairingheap_is_empty(so->queue))
    {
        item = (GISTSearchItem *) pairingheap_remove_first(so->queue);
    }
    else
    {
        /* Done when both heaps are empty */
        item = NULL;
    }

    /* Return item; caller is responsible to pfree it */
    return item;
}

/*
 * Fetch next heap tuple in an ordered search
 */
static bool
getNextNearest(IndexScanDesc scan)
{
    GISTScanOpaque so = (GISTScanOpaque) scan->opaque;
    bool        res = false;

    if (scan->xs_hitup)
    {
        /* free previously returned tuple */
        pfree(scan->xs_hitup);
        scan->xs_hitup = NULL;
    }

    do
    {
        GISTSearchItem *item = getNextGISTSearchItem(so);

        if (!item)
            break;

        if (GISTSearchItemIsHeap(*item))
        {
            /* found a heap item at currently minimal distance */
            scan->xs_heaptid = item->data.heap.heapPtr;
            scan->xs_recheck = item->data.heap.recheck;

            index_store_float8_orderby_distances(scan, so->orderByTypes,
                                                 GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
                                                 GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
                                                 item->data.heap.recheckDistances);

            /* in an index-only scan, also return the reconstructed tuple. */
            if (scan->xs_want_itup)
                scan->xs_hitup = item->data.heap.recontup;
            res = true;
        }
        else
        {
            /* visit an index page, extract its items into queue */
            CHECK_FOR_INTERRUPTS();

            gistScanPage(scan, item,
                         GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
                         GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
                         NULL, NULL);
        }

        pfree(item);
    } while (!res);

    return res;
}

/*
 * gistgettuple() -- Get the next tuple in the scan
 */
bool
gistgettuple(IndexScanDesc scan, ScanDirection dir)
{
    GISTScanOpaque so = (GISTScanOpaque) scan->opaque;

    if (dir != ForwardScanDirection)
        elog(ERROR, "GiST only supports forward scan direction");

    if (!so->qual_ok)
        return false;

    if (so->firstCall)
    {
        /* Begin the scan by processing the root page */
        GISTSearchItem fakeItem;

        pgstat_count_index_scan(scan->indexRelation);

        so->firstCall = false;
        so->curPageData = so->nPageData = 0;
        scan->xs_hitup = NULL;
        if (so->pageDataCxt)
            MemoryContextReset(so->pageDataCxt);

        fakeItem.blkno = GIST_ROOT_BLKNO;
        memset(&fakeItem.data.parentlsn, 0, sizeof(GistNSN));
        gistScanPage(scan, &fakeItem, NULL, NULL, NULL, NULL);
    }

    if (scan->numberOfOrderBys > 0)
    {
        /* Must fetch tuples in strict distance order */
        return getNextNearest(scan);
    }
    else
    {
        /* Fetch tuples index-page-at-a-time */
        for (;;)
        {
            if (so->curPageData < so->nPageData)
            {
                if (scan->kill_prior_tuple && so->curPageData > 0)
                {

                    if (so->killedItems == NULL)
                    {
                        MemoryContext oldCxt =
                        MemoryContextSwitchTo(so->giststate->scanCxt);

                        so->killedItems =
                            (OffsetNumber *) palloc(MaxIndexTuplesPerPage
                                                    * sizeof(OffsetNumber));

                        MemoryContextSwitchTo(oldCxt);
                    }
                    if (so->numKilled < MaxIndexTuplesPerPage)
                        so->killedItems[so->numKilled++] =
                            so->pageData[so->curPageData - 1].offnum;
                }
                /* continuing to return tuples from a leaf page */
                scan->xs_heaptid = so->pageData[so->curPageData].heapPtr;
                scan->xs_recheck = so->pageData[so->curPageData].recheck;

                /* in an index-only scan, also return the reconstructed tuple */
                if (scan->xs_want_itup)
                    scan->xs_hitup = so->pageData[so->curPageData].recontup;

                so->curPageData++;

                return true;
            }

            /*
             * Check the last returned tuple and add it to killedItems if
             * necessary
             */
            if (scan->kill_prior_tuple
                && so->curPageData > 0
                && so->curPageData == so->nPageData)
            {

                if (so->killedItems == NULL)
                {
                    MemoryContext oldCxt =
                    MemoryContextSwitchTo(so->giststate->scanCxt);

                    so->killedItems =
                        (OffsetNumber *) palloc(MaxIndexTuplesPerPage
                                                * sizeof(OffsetNumber));

                    MemoryContextSwitchTo(oldCxt);
                }
                if (so->numKilled < MaxIndexTuplesPerPage)
                    so->killedItems[so->numKilled++] =
                        so->pageData[so->curPageData - 1].offnum;
            }
            /* find and process the next index page */
            do
            {
                GISTSearchItem *item;

                if ((so->curBlkno != InvalidBlockNumber) && (so->numKilled > 0))
                    gistkillitems(scan);

                item = getNextGISTSearchItem(so);

                if (!item)
                    return false;

                CHECK_FOR_INTERRUPTS();

                /* save current item BlockNumber for next gistkillitems() call */
                so->curBlkno = item->blkno;

                /*
                 * While scanning a leaf page, ItemPointers of matching heap
                 * tuples are stored in so->pageData.  If there are any on
                 * this page, we fall out of the inner "do" and loop around to
                 * return them.
                 */
                gistScanPage(scan, item,
                             GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
                             GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
                             NULL, NULL);

                pfree(item);
            } while (so->nPageData == 0);
        }
    }
}

/*
 * gistgetbitmap() -- Get a bitmap of all heap tuple locations
 */
int64
gistgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
{
    GISTScanOpaque so = (GISTScanOpaque) scan->opaque;
    int64       ntids = 0;
    GISTSearchItem fakeItem;

    if (!so->qual_ok)
        return 0;

    pgstat_count_index_scan(scan->indexRelation);

    /* Begin the scan by processing the root page */
    so->curPageData = so->nPageData = 0;
    scan->xs_hitup = NULL;
    if (so->pageDataCxt)
        MemoryContextReset(so->pageDataCxt);

    fakeItem.blkno = GIST_ROOT_BLKNO;
    memset(&fakeItem.data.parentlsn, 0, sizeof(GistNSN));
    gistScanPage(scan, &fakeItem, NULL, NULL, tbm, &ntids);

    /*
     * While scanning a leaf page, ItemPointers of matching heap tuples will
     * be stored directly into tbm, so we don't need to deal with them here.
     */
    for (;;)
    {
        GISTSearchItem *item = getNextGISTSearchItem(so);

        if (!item)
            break;

        CHECK_FOR_INTERRUPTS();

        gistScanPage(scan, item,
                     GISTSearchItemDistanceValues(item, scan->numberOfOrderBys),
                     GISTSearchItemDistanceNulls(item, scan->numberOfOrderBys),
                     tbm, &ntids);

        pfree(item);
    }

    return ntids;
}

/*
 * Can we do index-only scans on the given index column?
 *
 * Opclasses that implement a fetch function support index-only scans.
 * Opclasses without compression functions also support index-only scans.
 * Included attributes always can be fetched for index-only scans.
 */
bool
gistcanreturn(Relation index, int attno)
{
    if (attno > IndexRelationGetNumberOfKeyAttributes(index) ||
        OidIsValid(index_getprocid(index, attno, GIST_FETCH_PROC)) ||
        !OidIsValid(index_getprocid(index, attno, GIST_COMPRESS_PROC)))
        return true;
    else
        return false;
}