gistutil.c

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/*-------------------------------------------------------------------------
 *
 * gistutil.c
 *    utilities routines for the postgres GiST index access method.
 *
 *
 * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *          src/backend/access/gist/gistutil.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <math.h>

#include "access/gist_private.h"
#include "access/reloptions.h"
#include "storage/indexfsm.h"
#include "storage/lmgr.h"
#include "utils/builtins.h"


/*
 * Write itup vector to page, has no control of free space.
 */
void
gistfillbuffer(Page page, IndexTuple *itup, int len, OffsetNumber off)
{
    OffsetNumber l = InvalidOffsetNumber;
    int         i;

    if (off == InvalidOffsetNumber)
        off = (PageIsEmpty(page)) ? FirstOffsetNumber :
            OffsetNumberNext(PageGetMaxOffsetNumber(page));

    for (i = 0; i < len; i++)
    {
        Size        sz = IndexTupleSize(itup[i]);

        l = PageAddItem(page, (Item) itup[i], sz, off, false, false);
        if (l == InvalidOffsetNumber)
            elog(ERROR, "failed to add item to GiST index page, item %d out of %d, size %d bytes",
                 i, len, (int) sz);
        off++;
    }
}

/*
 * Check space for itup vector on page
 */
bool
gistnospace(Page page, IndexTuple *itvec, int len, OffsetNumber todelete, Size freespace)
{
    unsigned int size = freespace,
                deleted = 0;
    int         i;

    for (i = 0; i < len; i++)
        size += IndexTupleSize(itvec[i]) + sizeof(ItemIdData);

    if (todelete != InvalidOffsetNumber)
    {
        IndexTuple  itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, todelete));

        deleted = IndexTupleSize(itup) + sizeof(ItemIdData);
    }

    return (PageGetFreeSpace(page) + deleted < size);
}

bool
gistfitpage(IndexTuple *itvec, int len)
{
    int         i;
    Size        size = 0;

    for (i = 0; i < len; i++)
        size += IndexTupleSize(itvec[i]) + sizeof(ItemIdData);

    /* TODO: Consider fillfactor */
    return (size <= GiSTPageSize);
}

/*
 * Read buffer into itup vector
 */
IndexTuple *
gistextractpage(Page page, int *len /* out */ )
{
    OffsetNumber i,
                maxoff;
    IndexTuple *itvec;

    maxoff = PageGetMaxOffsetNumber(page);
    *len = maxoff;
    itvec = palloc(sizeof(IndexTuple) * maxoff);
    for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
        itvec[i - FirstOffsetNumber] = (IndexTuple) PageGetItem(page, PageGetItemId(page, i));

    return itvec;
}

/*
 * join two vectors into one
 */
IndexTuple *
gistjoinvector(IndexTuple *itvec, int *len, IndexTuple *additvec, int addlen)
{
    itvec = (IndexTuple *) repalloc((void *) itvec, sizeof(IndexTuple) * ((*len) + addlen));
    memmove(&itvec[*len], additvec, sizeof(IndexTuple) * addlen);
    *len += addlen;
    return itvec;
}

/*
 * make plain IndexTupleVector
 */

IndexTupleData *
gistfillitupvec(IndexTuple *vec, int veclen, int *memlen)
{
    char       *ptr,
               *ret;
    int         i;

    *memlen = 0;

    for (i = 0; i < veclen; i++)
        *memlen += IndexTupleSize(vec[i]);

    ptr = ret = palloc(*memlen);

    for (i = 0; i < veclen; i++)
    {
        memcpy(ptr, vec[i], IndexTupleSize(vec[i]));
        ptr += IndexTupleSize(vec[i]);
    }

    return (IndexTupleData *) ret;
}

/*
 * Make unions of keys in IndexTuple vector (one union datum per index column).
 * Union Datums are returned into the attr/isnull arrays.
 * Resulting Datums aren't compressed.
 */
void
gistMakeUnionItVec(GISTSTATE *giststate, IndexTuple *itvec, int len,
                   Datum *attr, bool *isnull)
{
    int         i;
    GistEntryVector *evec;
    int         attrsize;

    evec = (GistEntryVector *) palloc((len + 2) * sizeof(GISTENTRY) + GEVHDRSZ);

    for (i = 0; i < giststate->tupdesc->natts; i++)
    {
        int         j;

        /* Collect non-null datums for this column */
        evec->n = 0;
        for (j = 0; j < len; j++)
        {
            Datum       datum;
            bool        IsNull;

            datum = index_getattr(itvec[j], i + 1, giststate->tupdesc, &IsNull);
            if (IsNull)
                continue;

            gistdentryinit(giststate, i,
                           evec->vector + evec->n,
                           datum,
                           NULL, NULL, (OffsetNumber) 0,
                           FALSE, IsNull);
            evec->n++;
        }

        /* If this column was all NULLs, the union is NULL */
        if (evec->n == 0)
        {
            attr[i] = (Datum) 0;
            isnull[i] = TRUE;
        }
        else
        {
            if (evec->n == 1)
            {
                /* unionFn may expect at least two inputs */
                evec->n = 2;
                evec->vector[1] = evec->vector[0];
            }

            /* Make union and store in attr array */
            attr[i] = FunctionCall2Coll(&giststate->unionFn[i],
                                        giststate->supportCollation[i],
                                        PointerGetDatum(evec),
                                        PointerGetDatum(&attrsize));

            isnull[i] = FALSE;
        }
    }
}

/*
 * Return an IndexTuple containing the result of applying the "union"
 * method to the specified IndexTuple vector.
 */
IndexTuple
gistunion(Relation r, IndexTuple *itvec, int len, GISTSTATE *giststate)
{
    Datum       attr[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];

    gistMakeUnionItVec(giststate, itvec, len, attr, isnull);

    return gistFormTuple(giststate, r, attr, isnull, false);
}

/*
 * makes union of two key
 */
void
gistMakeUnionKey(GISTSTATE *giststate, int attno,
                 GISTENTRY *entry1, bool isnull1,
                 GISTENTRY *entry2, bool isnull2,
                 Datum *dst, bool *dstisnull)
{
    /* we need a GistEntryVector with room for exactly 2 elements */
    union
    {
        GistEntryVector gev;
        char        padding[2 * sizeof(GISTENTRY) + GEVHDRSZ];
    }           storage;
    GistEntryVector *evec = &storage.gev;
    int         dstsize;

    evec->n = 2;

    if (isnull1 && isnull2)
    {
        *dstisnull = TRUE;
        *dst = (Datum) 0;
    }
    else
    {
        if (isnull1 == FALSE && isnull2 == FALSE)
        {
            evec->vector[0] = *entry1;
            evec->vector[1] = *entry2;
        }
        else if (isnull1 == FALSE)
        {
            evec->vector[0] = *entry1;
            evec->vector[1] = *entry1;
        }
        else
        {
            evec->vector[0] = *entry2;
            evec->vector[1] = *entry2;
        }

        *dstisnull = FALSE;
        *dst = FunctionCall2Coll(&giststate->unionFn[attno],
                                 giststate->supportCollation[attno],
                                 PointerGetDatum(evec),
                                 PointerGetDatum(&dstsize));
    }
}

bool
gistKeyIsEQ(GISTSTATE *giststate, int attno, Datum a, Datum b)
{
    bool        result;

    FunctionCall3Coll(&giststate->equalFn[attno],
                      giststate->supportCollation[attno],
                      a, b,
                      PointerGetDatum(&result));
    return result;
}

/*
 * Decompress all keys in tuple
 */
void
gistDeCompressAtt(GISTSTATE *giststate, Relation r, IndexTuple tuple, Page p,
                  OffsetNumber o, GISTENTRY *attdata, bool *isnull)
{
    int         i;

    for (i = 0; i < r->rd_att->natts; i++)
    {
        Datum       datum;

        datum = index_getattr(tuple, i + 1, giststate->tupdesc, &isnull[i]);
        gistdentryinit(giststate, i, &attdata[i],
                       datum, r, p, o,
                       FALSE, isnull[i]);
    }
}

/*
 * Forms union of oldtup and addtup, if union == oldtup then return NULL
 */
IndexTuple
gistgetadjusted(Relation r, IndexTuple oldtup, IndexTuple addtup, GISTSTATE *giststate)
{
    bool        neednew = FALSE;
    GISTENTRY   oldentries[INDEX_MAX_KEYS],
                addentries[INDEX_MAX_KEYS];
    bool        oldisnull[INDEX_MAX_KEYS],
                addisnull[INDEX_MAX_KEYS];
    Datum       attr[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];
    IndexTuple  newtup = NULL;
    int         i;

    gistDeCompressAtt(giststate, r, oldtup, NULL,
                      (OffsetNumber) 0, oldentries, oldisnull);

    gistDeCompressAtt(giststate, r, addtup, NULL,
                      (OffsetNumber) 0, addentries, addisnull);

    for (i = 0; i < r->rd_att->natts; i++)
    {
        gistMakeUnionKey(giststate, i,
                         oldentries + i, oldisnull[i],
                         addentries + i, addisnull[i],
                         attr + i, isnull + i);

        if (neednew)
            /* we already need new key, so we can skip check */
            continue;

        if (isnull[i])
            /* union of key may be NULL if and only if both keys are NULL */
            continue;

        if (!addisnull[i])
        {
            if (oldisnull[i] ||
                !gistKeyIsEQ(giststate, i, oldentries[i].key, attr[i]))
                neednew = true;
        }
    }

    if (neednew)
    {
        /* need to update key */
        newtup = gistFormTuple(giststate, r, attr, isnull, false);
        newtup->t_tid = oldtup->t_tid;
    }

    return newtup;
}

/*
 * Search an upper index page for the entry with lowest penalty for insertion
 * of the new index key contained in "it".
 *
 * Returns the index of the page entry to insert into.
 */
OffsetNumber
gistchoose(Relation r, Page p, IndexTuple it,   /* it has compressed entry */
           GISTSTATE *giststate)
{
    OffsetNumber result;
    OffsetNumber maxoff;
    OffsetNumber i;
    float       best_penalty[INDEX_MAX_KEYS];
    GISTENTRY   entry,
                identry[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];
    int         keep_current_best;

    Assert(!GistPageIsLeaf(p));

    gistDeCompressAtt(giststate, r,
                      it, NULL, (OffsetNumber) 0,
                      identry, isnull);

    /* we'll return FirstOffsetNumber if page is empty (shouldn't happen) */
    result = FirstOffsetNumber;

    /*
     * The index may have multiple columns, and there's a penalty value for
     * each column.  The penalty associated with a column that appears earlier
     * in the index definition is strictly more important than the penalty of
     * a column that appears later in the index definition.
     *
     * best_penalty[j] is the best penalty we have seen so far for column j,
     * or -1 when we haven't yet examined column j.  Array entries to the
     * right of the first -1 are undefined.
     */
    best_penalty[0] = -1;

    /*
     * If we find a tuple that's exactly as good as the currently best one, we
     * could use either one.  When inserting a lot of tuples with the same or
     * similar keys, it's preferable to descend down the same path when
     * possible, as that's more cache-friendly.  On the other hand, if all
     * inserts land on the same leaf page after a split, we're never going to
     * insert anything to the other half of the split, and will end up using
     * only 50% of the available space.  Distributing the inserts evenly would
     * lead to better space usage, but that hurts cache-locality during
     * insertion.  To get the best of both worlds, when we find a tuple that's
     * exactly as good as the previous best, choose randomly whether to stick
     * to the old best, or use the new one.  Once we decide to stick to the
     * old best, we keep sticking to it for any subsequent equally good tuples
     * we might find.  This favors tuples with low offsets, but still allows
     * some inserts to go to other equally-good subtrees.
     *
     * keep_current_best is -1 if we haven't yet had to make a random choice
     * whether to keep the current best tuple.  If we have done so, and
     * decided to keep it, keep_current_best is 1; if we've decided to
     * replace, keep_current_best is 0.  (This state will be reset to -1 as
     * soon as we've made the replacement, but sometimes we make the choice in
     * advance of actually finding a replacement best tuple.)
     */
    keep_current_best = -1;

    /*
     * Loop over tuples on page.
     */
    maxoff = PageGetMaxOffsetNumber(p);
    Assert(maxoff >= FirstOffsetNumber);

    for (i = FirstOffsetNumber; i <= maxoff; i = OffsetNumberNext(i))
    {
        IndexTuple  itup = (IndexTuple) PageGetItem(p, PageGetItemId(p, i));
        bool        zero_penalty;
        int         j;

        zero_penalty = true;

        /* Loop over index attributes. */
        for (j = 0; j < r->rd_att->natts; j++)
        {
            Datum       datum;
            float       usize;
            bool        IsNull;

            /* Compute penalty for this column. */
            datum = index_getattr(itup, j + 1, giststate->tupdesc, &IsNull);
            gistdentryinit(giststate, j, &entry, datum, r, p, i,
                           FALSE, IsNull);
            usize = gistpenalty(giststate, j, &entry, IsNull,
                                &identry[j], isnull[j]);
            if (usize > 0)
                zero_penalty = false;

            if (best_penalty[j] < 0 || usize < best_penalty[j])
            {
                /*
                 * New best penalty for column.  Tentatively select this tuple
                 * as the target, and record the best penalty.  Then reset the
                 * next column's penalty to "unknown" (and indirectly, the
                 * same for all the ones to its right).  This will force us to
                 * adopt this tuple's penalty values as the best for all the
                 * remaining columns during subsequent loop iterations.
                 */
                result = i;
                best_penalty[j] = usize;

                if (j < r->rd_att->natts - 1)
                    best_penalty[j + 1] = -1;

                /* we have new best, so reset keep-it decision */
                keep_current_best = -1;
            }
            else if (best_penalty[j] == usize)
            {
                /*
                 * The current tuple is exactly as good for this column as the
                 * best tuple seen so far.  The next iteration of this loop
                 * will compare the next column.
                 */
            }
            else
            {
                /*
                 * The current tuple is worse for this column than the best
                 * tuple seen so far.  Skip the remaining columns and move on
                 * to the next tuple, if any.
                 */
                zero_penalty = false;   /* so outer loop won't exit */
                break;
            }
        }

        /*
         * If we looped past the last column, and did not update "result",
         * then this tuple is exactly as good as the prior best tuple.
         */
        if (j == r->rd_att->natts && result != i)
        {
            if (keep_current_best == -1)
            {
                /* we didn't make the random choice yet for this old best */
                keep_current_best = (random() <= (MAX_RANDOM_VALUE / 2)) ? 1 : 0;
            }
            if (keep_current_best == 0)
            {
                /* we choose to use the new tuple */
                result = i;
                /* choose again if there are even more exactly-as-good ones */
                keep_current_best = -1;
            }
        }

        /*
         * If we find a tuple with zero penalty for all columns, and we've
         * decided we don't want to search for another tuple with equal
         * penalty, there's no need to examine remaining tuples; just break
         * out of the loop and return it.
         */
        if (zero_penalty)
        {
            if (keep_current_best == -1)
            {
                /* we didn't make the random choice yet for this old best */
                keep_current_best = (random() <= (MAX_RANDOM_VALUE / 2)) ? 1 : 0;
            }
            if (keep_current_best == 1)
                break;
        }
    }

    return result;
}

/*
 * initialize a GiST entry with a decompressed version of key
 */
void
gistdentryinit(GISTSTATE *giststate, int nkey, GISTENTRY *e,
               Datum k, Relation r, Page pg, OffsetNumber o,
               bool l, bool isNull)
{
    if (!isNull)
    {
        GISTENTRY  *dep;

        gistentryinit(*e, k, r, pg, o, l);
        dep = (GISTENTRY *)
            DatumGetPointer(FunctionCall1Coll(&giststate->decompressFn[nkey],
                                           giststate->supportCollation[nkey],
                                              PointerGetDatum(e)));
        /* decompressFn may just return the given pointer */
        if (dep != e)
            gistentryinit(*e, dep->key, dep->rel, dep->page, dep->offset,
                          dep->leafkey);
    }
    else
        gistentryinit(*e, (Datum) 0, r, pg, o, l);
}

IndexTuple
gistFormTuple(GISTSTATE *giststate, Relation r,
              Datum attdata[], bool isnull[], bool isleaf)
{
    Datum       compatt[INDEX_MAX_KEYS];
    int         i;
    IndexTuple  res;

    /*
     * Call the compress method on each attribute.
     */
    for (i = 0; i < r->rd_att->natts; i++)
    {
        if (isnull[i])
            compatt[i] = (Datum) 0;
        else
        {
            GISTENTRY   centry;
            GISTENTRY  *cep;

            gistentryinit(centry, attdata[i], r, NULL, (OffsetNumber) 0,
                          isleaf);
            cep = (GISTENTRY *)
                DatumGetPointer(FunctionCall1Coll(&giststate->compressFn[i],
                                              giststate->supportCollation[i],
                                                  PointerGetDatum(&centry)));
            compatt[i] = cep->key;
        }
    }

    res = index_form_tuple(giststate->tupdesc, compatt, isnull);

    /*
     * The offset number on tuples on internal pages is unused. For historical
     * reasons, it is set to 0xffff.
     */
    ItemPointerSetOffsetNumber(&(res->t_tid), 0xffff);
    return res;
}

/*
 * initialize a GiST entry with fetched value in key field
 */
static Datum
gistFetchAtt(GISTSTATE *giststate, int nkey, Datum k, Relation r)
{
    GISTENTRY   fentry;
    GISTENTRY  *fep;

    gistentryinit(fentry, k, r, NULL, (OffsetNumber) 0, false);

    fep = (GISTENTRY *)
        DatumGetPointer(FunctionCall1Coll(&giststate->fetchFn[nkey],
                                          giststate->supportCollation[nkey],
                                          PointerGetDatum(&fentry)));

    /* fetchFn set 'key', return it to the caller */
    return fep->key;
}

/*
 * Fetch all keys in tuple.
 * returns new IndexTuple that contains GISTENTRY with fetched data
 */
IndexTuple
gistFetchTuple(GISTSTATE *giststate, Relation r, IndexTuple tuple)
{
    MemoryContext oldcxt = MemoryContextSwitchTo(giststate->tempCxt);
    Datum       fetchatt[INDEX_MAX_KEYS];
    bool        isnull[INDEX_MAX_KEYS];
    int         i;

    for (i = 0; i < r->rd_att->natts; i++)
    {
        Datum       datum;

        datum = index_getattr(tuple, i + 1, giststate->tupdesc, &isnull[i]);

        if (giststate->fetchFn[i].fn_oid != InvalidOid)
        {
            if (!isnull[i])
                fetchatt[i] = gistFetchAtt(giststate, i, datum, r);
            else
                fetchatt[i] = (Datum) 0;
        }
        else
        {
            /*
             * Index-only scans not supported for this column. Since the
             * planner chose an index-only scan anyway, it is not interested
             * in this column, and we can replace it with a NULL.
             */
            isnull[i] = true;
            fetchatt[i] = (Datum) 0;
        }
    }
    MemoryContextSwitchTo(oldcxt);

    return index_form_tuple(giststate->fetchTupdesc, fetchatt, isnull);
}

float
gistpenalty(GISTSTATE *giststate, int attno,
            GISTENTRY *orig, bool isNullOrig,
            GISTENTRY *add, bool isNullAdd)
{
    float       penalty = 0.0;

    if (giststate->penaltyFn[attno].fn_strict == FALSE ||
        (isNullOrig == FALSE && isNullAdd == FALSE))
    {
        FunctionCall3Coll(&giststate->penaltyFn[attno],
                          giststate->supportCollation[attno],
                          PointerGetDatum(orig),
                          PointerGetDatum(add),
                          PointerGetDatum(&penalty));
        /* disallow negative or NaN penalty */
        if (isnan(penalty) || penalty < 0.0)
            penalty = 0.0;
    }
    else if (isNullOrig && isNullAdd)
        penalty = 0.0;
    else
    {
        /* try to prevent mixing null and non-null values */
        penalty = get_float4_infinity();
    }

    return penalty;
}

/*
 * Initialize a new index page
 */
void
GISTInitBuffer(Buffer b, uint32 f)
{
    GISTPageOpaque opaque;
    Page        page;
    Size        pageSize;

    pageSize = BufferGetPageSize(b);
    page = BufferGetPage(b);
    PageInit(page, pageSize, sizeof(GISTPageOpaqueData));

    opaque = GistPageGetOpaque(page);
    /* page was already zeroed by PageInit, so this is not needed: */
    /* memset(&(opaque->nsn), 0, sizeof(GistNSN)); */
    opaque->rightlink = InvalidBlockNumber;
    opaque->flags = f;
    opaque->gist_page_id = GIST_PAGE_ID;
}

/*
 * Verify that a freshly-read page looks sane.
 */
void
gistcheckpage(Relation rel, Buffer buf)
{
    Page        page = BufferGetPage(buf);

    /*
     * ReadBuffer verifies that every newly-read page passes
     * PageHeaderIsValid, which means it either contains a reasonably sane
     * page header or is all-zero.  We have to defend against the all-zero
     * case, however.
     */
    if (PageIsNew(page))
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
             errmsg("index \"%s\" contains unexpected zero page at block %u",
                    RelationGetRelationName(rel),
                    BufferGetBlockNumber(buf)),
                 errhint("Please REINDEX it.")));

    /*
     * Additionally check that the special area looks sane.
     */
    if (PageGetSpecialSize(page) != MAXALIGN(sizeof(GISTPageOpaqueData)))
        ereport(ERROR,
                (errcode(ERRCODE_INDEX_CORRUPTED),
                 errmsg("index \"%s\" contains corrupted page at block %u",
                        RelationGetRelationName(rel),
                        BufferGetBlockNumber(buf)),
                 errhint("Please REINDEX it.")));
}


/*
 * Allocate a new page (either by recycling, or by extending the index file)
 *
 * The returned buffer is already pinned and exclusive-locked
 *
 * Caller is responsible for initializing the page by calling GISTInitBuffer
 */
Buffer
gistNewBuffer(Relation r)
{
    Buffer      buffer;
    bool        needLock;

    /* First, try to get a page from FSM */
    for (;;)
    {
        BlockNumber blkno = GetFreeIndexPage(r);

        if (blkno == InvalidBlockNumber)
            break;              /* nothing left in FSM */

        buffer = ReadBuffer(r, blkno);

        /*
         * We have to guard against the possibility that someone else already
         * recycled this page; the buffer may be locked if so.
         */
        if (ConditionalLockBuffer(buffer))
        {
            Page        page = BufferGetPage(buffer);

            if (PageIsNew(page))
                return buffer;  /* OK to use, if never initialized */

            gistcheckpage(r, buffer);

            if (GistPageIsDeleted(page))
                return buffer;  /* OK to use */

            LockBuffer(buffer, GIST_UNLOCK);
        }

        /* Can't use it, so release buffer and try again */
        ReleaseBuffer(buffer);
    }

    /* Must extend the file */
    needLock = !RELATION_IS_LOCAL(r);

    if (needLock)
        LockRelationForExtension(r, ExclusiveLock);

    buffer = ReadBuffer(r, P_NEW);
    LockBuffer(buffer, GIST_EXCLUSIVE);

    if (needLock)
        UnlockRelationForExtension(r, ExclusiveLock);

    return buffer;
}

bytea *
gistoptions(Datum reloptions, bool validate)
{
    relopt_value *options;
    GiSTOptions *rdopts;
    int         numoptions;
    static const relopt_parse_elt tab[] = {
        {"fillfactor", RELOPT_TYPE_INT, offsetof(GiSTOptions, fillfactor)},
        {"buffering", RELOPT_TYPE_STRING, offsetof(GiSTOptions, bufferingModeOffset)}
    };

    options = parseRelOptions(reloptions, validate, RELOPT_KIND_GIST,
                              &numoptions);

    /* if none set, we're done */
    if (numoptions == 0)
        return NULL;

    rdopts = allocateReloptStruct(sizeof(GiSTOptions), options, numoptions);

    fillRelOptions((void *) rdopts, sizeof(GiSTOptions), options, numoptions,
                   validate, tab, lengthof(tab));

    pfree(options);

    return (bytea *) rdopts;
}

/*
 * Temporary and unlogged GiST indexes are not WAL-logged, but we need LSNs
 * to detect concurrent page splits anyway. This function provides a fake
 * sequence of LSNs for that purpose.
 */
XLogRecPtr
gistGetFakeLSN(Relation rel)
{
    static XLogRecPtr counter = 1;

    if (rel->rd_rel->relpersistence == RELPERSISTENCE_TEMP)
    {
        /*
         * Temporary relations are only accessible in our session, so a simple
         * backend-local counter will do.
         */
        return counter++;
    }
    else
    {
        /*
         * Unlogged relations are accessible from other backends, and survive
         * (clean) restarts. GetFakeLSNForUnloggedRel() handles that for us.
         */
        Assert(rel->rd_rel->relpersistence == RELPERSISTENCE_UNLOGGED);
        return GetFakeLSNForUnloggedRel();
    }
}