brin.c

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/*
 * brin.c
 *      Implementation of BRIN indexes for Postgres
 *
 * See src/backend/access/brin/README for details.
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/brin/brin.c
 *
 * TODO
 *      * ScalarArrayOpExpr (amsearcharray -> SK_SEARCHARRAY)
 */
#include "postgres.h"

#include "access/brin.h"
#include "access/brin_page.h"
#include "access/brin_pageops.h"
#include "access/brin_xlog.h"
#include "access/relation.h"
#include "access/reloptions.h"
#include "access/relscan.h"
#include "access/table.h"
#include "access/tableam.h"
#include "access/xloginsert.h"
#include "catalog/index.h"
#include "catalog/pg_am.h"
#include "commands/vacuum.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/autovacuum.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "utils/builtins.h"
#include "utils/index_selfuncs.h"
#include "utils/memutils.h"
#include "utils/rel.h"


/*
 * We use a BrinBuildState during initial construction of a BRIN index.
 * The running state is kept in a BrinMemTuple.
 */
typedef struct BrinBuildState
{
    Relation    bs_irel;
    int         bs_numtuples;
    Buffer      bs_currentInsertBuf;
    BlockNumber bs_pagesPerRange;
    BlockNumber bs_currRangeStart;
    BrinRevmap *bs_rmAccess;
    BrinDesc   *bs_bdesc;
    BrinMemTuple *bs_dtuple;
} BrinBuildState;

/*
 * Struct used as "opaque" during index scans
 */
typedef struct BrinOpaque
{
    BlockNumber bo_pagesPerRange;
    BrinRevmap *bo_rmAccess;
    BrinDesc   *bo_bdesc;
} BrinOpaque;

#define BRIN_ALL_BLOCKRANGES    InvalidBlockNumber

static BrinBuildState *initialize_brin_buildstate(Relation idxRel,
                                                  BrinRevmap *revmap, BlockNumber pagesPerRange);
static void terminate_brin_buildstate(BrinBuildState *state);
static void brinsummarize(Relation index, Relation heapRel, BlockNumber pageRange,
                          bool include_partial, double *numSummarized, double *numExisting);
static void form_and_insert_tuple(BrinBuildState *state);
static void union_tuples(BrinDesc *bdesc, BrinMemTuple *a,
                         BrinTuple *b);
static void brin_vacuum_scan(Relation idxrel, BufferAccessStrategy strategy);


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

    amroutine->amstrategies = 0;
    amroutine->amsupport = BRIN_LAST_OPTIONAL_PROCNUM;
    amroutine->amcanorder = false;
    amroutine->amcanorderbyop = false;
    amroutine->amcanbackward = false;
    amroutine->amcanunique = false;
    amroutine->amcanmulticol = true;
    amroutine->amoptionalkey = true;
    amroutine->amsearcharray = false;
    amroutine->amsearchnulls = true;
    amroutine->amstorage = true;
    amroutine->amclusterable = false;
    amroutine->ampredlocks = false;
    amroutine->amcanparallel = false;
    amroutine->amcaninclude = false;
    amroutine->amusemaintenanceworkmem = false;
    amroutine->amparallelvacuumoptions =
        VACUUM_OPTION_PARALLEL_CLEANUP;
    amroutine->amkeytype = InvalidOid;

    amroutine->ambuild = brinbuild;
    amroutine->ambuildempty = brinbuildempty;
    amroutine->aminsert = brininsert;
    amroutine->ambulkdelete = brinbulkdelete;
    amroutine->amvacuumcleanup = brinvacuumcleanup;
    amroutine->amcanreturn = NULL;
    amroutine->amcostestimate = brincostestimate;
    amroutine->amoptions = brinoptions;
    amroutine->amproperty = NULL;
    amroutine->ambuildphasename = NULL;
    amroutine->amvalidate = brinvalidate;
    amroutine->ambeginscan = brinbeginscan;
    amroutine->amrescan = brinrescan;
    amroutine->amgettuple = NULL;
    amroutine->amgetbitmap = bringetbitmap;
    amroutine->amendscan = brinendscan;
    amroutine->ammarkpos = NULL;
    amroutine->amrestrpos = NULL;
    amroutine->amestimateparallelscan = NULL;
    amroutine->aminitparallelscan = NULL;
    amroutine->amparallelrescan = NULL;

    PG_RETURN_POINTER(amroutine);
}

/*
 * A tuple in the heap is being inserted.  To keep a brin index up to date,
 * we need to obtain the relevant index tuple and compare its stored values
 * with those of the new tuple.  If the tuple values are not consistent with
 * the summary tuple, we need to update the index tuple.
 *
 * If autosummarization is enabled, check if we need to summarize the previous
 * page range.
 *
 * If the range is not currently summarized (i.e. the revmap returns NULL for
 * it), there's nothing to do for this tuple.
 */
bool
brininsert(Relation idxRel, Datum *values, bool *nulls,
           ItemPointer heaptid, Relation heapRel,
           IndexUniqueCheck checkUnique,
           IndexInfo *indexInfo)
{
    BlockNumber pagesPerRange;
    BlockNumber origHeapBlk;
    BlockNumber heapBlk;
    BrinDesc   *bdesc = (BrinDesc *) indexInfo->ii_AmCache;
    BrinRevmap *revmap;
    Buffer      buf = InvalidBuffer;
    MemoryContext tupcxt = NULL;
    MemoryContext oldcxt = CurrentMemoryContext;
    bool        autosummarize = BrinGetAutoSummarize(idxRel);

    revmap = brinRevmapInitialize(idxRel, &pagesPerRange, NULL);

    /*
     * origHeapBlk is the block number where the insertion occurred.  heapBlk
     * is the first block in the corresponding page range.
     */
    origHeapBlk = ItemPointerGetBlockNumber(heaptid);
    heapBlk = (origHeapBlk / pagesPerRange) * pagesPerRange;

    for (;;)
    {
        bool        need_insert = false;
        OffsetNumber off;
        BrinTuple  *brtup;
        BrinMemTuple *dtup;
        int         keyno;

        CHECK_FOR_INTERRUPTS();

        /*
         * If auto-summarization is enabled and we just inserted the first
         * tuple into the first block of a new non-first page range, request a
         * summarization run of the previous range.
         */
        if (autosummarize &&
            heapBlk > 0 &&
            heapBlk == origHeapBlk &&
            ItemPointerGetOffsetNumber(heaptid) == FirstOffsetNumber)
        {
            BlockNumber lastPageRange = heapBlk - 1;
            BrinTuple  *lastPageTuple;

            lastPageTuple =
                brinGetTupleForHeapBlock(revmap, lastPageRange, &buf, &off,
                                         NULL, BUFFER_LOCK_SHARE, NULL);
            if (!lastPageTuple)
            {
                bool        recorded;

                recorded = AutoVacuumRequestWork(AVW_BRINSummarizeRange,
                                                 RelationGetRelid(idxRel),
                                                 lastPageRange);
                if (!recorded)
                    ereport(LOG,
                            (errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
                             errmsg("request for BRIN range summarization for index \"%s\" page %u was not recorded",
                                    RelationGetRelationName(idxRel),
                                    lastPageRange)));
            }
            else
                LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        }

        brtup = brinGetTupleForHeapBlock(revmap, heapBlk, &buf, &off,
                                         NULL, BUFFER_LOCK_SHARE, NULL);

        /* if range is unsummarized, there's nothing to do */
        if (!brtup)
            break;

        /* First time through in this statement? */
        if (bdesc == NULL)
        {
            MemoryContextSwitchTo(indexInfo->ii_Context);
            bdesc = brin_build_desc(idxRel);
            indexInfo->ii_AmCache = (void *) bdesc;
            MemoryContextSwitchTo(oldcxt);
        }
        /* First time through in this brininsert call? */
        if (tupcxt == NULL)
        {
            tupcxt = AllocSetContextCreate(CurrentMemoryContext,
                                           "brininsert cxt",
                                           ALLOCSET_DEFAULT_SIZES);
            MemoryContextSwitchTo(tupcxt);
        }

        dtup = brin_deform_tuple(bdesc, brtup, NULL);

        /*
         * Compare the key values of the new tuple to the stored index values;
         * our deformed tuple will get updated if the new tuple doesn't fit
         * the original range (note this means we can't break out of the loop
         * early). Make a note of whether this happens, so that we know to
         * insert the modified tuple later.
         */
        for (keyno = 0; keyno < bdesc->bd_tupdesc->natts; keyno++)
        {
            Datum       result;
            BrinValues *bval;
            FmgrInfo   *addValue;

            bval = &dtup->bt_columns[keyno];
            addValue = index_getprocinfo(idxRel, keyno + 1,
                                         BRIN_PROCNUM_ADDVALUE);
            result = FunctionCall4Coll(addValue,
                                       idxRel->rd_indcollation[keyno],
                                       PointerGetDatum(bdesc),
                                       PointerGetDatum(bval),
                                       values[keyno],
                                       nulls[keyno]);
            /* if that returned true, we need to insert the updated tuple */
            need_insert |= DatumGetBool(result);
        }

        if (!need_insert)
        {
            /*
             * The tuple is consistent with the new values, so there's nothing
             * to do.
             */
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        }
        else
        {
            Page        page = BufferGetPage(buf);
            ItemId      lp = PageGetItemId(page, off);
            Size        origsz;
            BrinTuple  *origtup;
            Size        newsz;
            BrinTuple  *newtup;
            bool        samepage;

            /*
             * Make a copy of the old tuple, so that we can compare it after
             * re-acquiring the lock.
             */
            origsz = ItemIdGetLength(lp);
            origtup = brin_copy_tuple(brtup, origsz, NULL, NULL);

            /*
             * Before releasing the lock, check if we can attempt a same-page
             * update.  Another process could insert a tuple concurrently in
             * the same page though, so downstream we must be prepared to cope
             * if this turns out to not be possible after all.
             */
            newtup = brin_form_tuple(bdesc, heapBlk, dtup, &newsz);
            samepage = brin_can_do_samepage_update(buf, origsz, newsz);
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);

            /*
             * Try to update the tuple.  If this doesn't work for whatever
             * reason, we need to restart from the top; the revmap might be
             * pointing at a different tuple for this block now, so we need to
             * recompute to ensure both our new heap tuple and the other
             * inserter's are covered by the combined tuple.  It might be that
             * we don't need to update at all.
             */
            if (!brin_doupdate(idxRel, pagesPerRange, revmap, heapBlk,
                               buf, off, origtup, origsz, newtup, newsz,
                               samepage))
            {
                /* no luck; start over */
                MemoryContextResetAndDeleteChildren(tupcxt);
                continue;
            }
        }

        /* success! */
        break;
    }

    brinRevmapTerminate(revmap);
    if (BufferIsValid(buf))
        ReleaseBuffer(buf);
    MemoryContextSwitchTo(oldcxt);
    if (tupcxt != NULL)
        MemoryContextDelete(tupcxt);

    return false;
}

/*
 * Initialize state for a BRIN index scan.
 *
 * We read the metapage here to determine the pages-per-range number that this
 * index was built with.  Note that since this cannot be changed while we're
 * holding lock on index, it's not necessary to recompute it during brinrescan.
 */
IndexScanDesc
brinbeginscan(Relation r, int nkeys, int norderbys)
{
    IndexScanDesc scan;
    BrinOpaque *opaque;

    scan = RelationGetIndexScan(r, nkeys, norderbys);

    opaque = (BrinOpaque *) palloc(sizeof(BrinOpaque));
    opaque->bo_rmAccess = brinRevmapInitialize(r, &opaque->bo_pagesPerRange,
                                               scan->xs_snapshot);
    opaque->bo_bdesc = brin_build_desc(r);
    scan->opaque = opaque;

    return scan;
}

/*
 * Execute the index scan.
 *
 * This works by reading index TIDs from the revmap, and obtaining the index
 * tuples pointed to by them; the summary values in the index tuples are
 * compared to the scan keys.  We return into the TID bitmap all the pages in
 * ranges corresponding to index tuples that match the scan keys.
 *
 * If a TID from the revmap is read as InvalidTID, we know that range is
 * unsummarized.  Pages in those ranges need to be returned regardless of scan
 * keys.
 */
int64
bringetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
{
    Relation    idxRel = scan->indexRelation;
    Buffer      buf = InvalidBuffer;
    BrinDesc   *bdesc;
    Oid         heapOid;
    Relation    heapRel;
    BrinOpaque *opaque;
    BlockNumber nblocks;
    BlockNumber heapBlk;
    int         totalpages = 0;
    FmgrInfo   *consistentFn;
    MemoryContext oldcxt;
    MemoryContext perRangeCxt;
    BrinMemTuple *dtup;
    BrinTuple  *btup = NULL;
    Size        btupsz = 0;

    opaque = (BrinOpaque *) scan->opaque;
    bdesc = opaque->bo_bdesc;
    pgstat_count_index_scan(idxRel);

    /*
     * We need to know the size of the table so that we know how long to
     * iterate on the revmap.
     */
    heapOid = IndexGetRelation(RelationGetRelid(idxRel), false);
    heapRel = table_open(heapOid, AccessShareLock);
    nblocks = RelationGetNumberOfBlocks(heapRel);
    table_close(heapRel, AccessShareLock);

    /*
     * Make room for the consistent support procedures of indexed columns.  We
     * don't look them up here; we do that lazily the first time we see a scan
     * key reference each of them.  We rely on zeroing fn_oid to InvalidOid.
     */
    consistentFn = palloc0(sizeof(FmgrInfo) * bdesc->bd_tupdesc->natts);

    /* allocate an initial in-memory tuple, out of the per-range memcxt */
    dtup = brin_new_memtuple(bdesc);

    /*
     * Setup and use a per-range memory context, which is reset every time we
     * loop below.  This avoids having to free the tuples within the loop.
     */
    perRangeCxt = AllocSetContextCreate(CurrentMemoryContext,
                                        "bringetbitmap cxt",
                                        ALLOCSET_DEFAULT_SIZES);
    oldcxt = MemoryContextSwitchTo(perRangeCxt);

    /*
     * Now scan the revmap.  We start by querying for heap page 0,
     * incrementing by the number of pages per range; this gives us a full
     * view of the table.
     */
    for (heapBlk = 0; heapBlk < nblocks; heapBlk += opaque->bo_pagesPerRange)
    {
        bool        addrange;
        bool        gottuple = false;
        BrinTuple  *tup;
        OffsetNumber off;
        Size        size;

        CHECK_FOR_INTERRUPTS();

        MemoryContextResetAndDeleteChildren(perRangeCxt);

        tup = brinGetTupleForHeapBlock(opaque->bo_rmAccess, heapBlk, &buf,
                                       &off, &size, BUFFER_LOCK_SHARE,
                                       scan->xs_snapshot);
        if (tup)
        {
            gottuple = true;
            btup = brin_copy_tuple(tup, size, btup, &btupsz);
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        }

        /*
         * For page ranges with no indexed tuple, we must return the whole
         * range; otherwise, compare it to the scan keys.
         */
        if (!gottuple)
        {
            addrange = true;
        }
        else
        {
            dtup = brin_deform_tuple(bdesc, btup, dtup);
            if (dtup->bt_placeholder)
            {
                /*
                 * Placeholder tuples are always returned, regardless of the
                 * values stored in them.
                 */
                addrange = true;
            }
            else
            {
                int         keyno;

                /*
                 * Compare scan keys with summary values stored for the range.
                 * If scan keys are matched, the page range must be added to
                 * the bitmap.  We initially assume the range needs to be
                 * added; in particular this serves the case where there are
                 * no keys.
                 */
                addrange = true;
                for (keyno = 0; keyno < scan->numberOfKeys; keyno++)
                {
                    ScanKey     key = &scan->keyData[keyno];
                    AttrNumber  keyattno = key->sk_attno;
                    BrinValues *bval = &dtup->bt_columns[keyattno - 1];
                    Datum       add;

                    /*
                     * The collation of the scan key must match the collation
                     * used in the index column (but only if the search is not
                     * IS NULL/ IS NOT NULL).  Otherwise we shouldn't be using
                     * this index ...
                     */
                    Assert((key->sk_flags & SK_ISNULL) ||
                           (key->sk_collation ==
                            TupleDescAttr(bdesc->bd_tupdesc,
                                          keyattno - 1)->attcollation));

                    /* First time this column? look up consistent function */
                    if (consistentFn[keyattno - 1].fn_oid == InvalidOid)
                    {
                        FmgrInfo   *tmp;

                        tmp = index_getprocinfo(idxRel, keyattno,
                                                BRIN_PROCNUM_CONSISTENT);
                        fmgr_info_copy(&consistentFn[keyattno - 1], tmp,
                                       CurrentMemoryContext);
                    }

                    /*
                     * Check whether the scan key is consistent with the page
                     * range values; if so, have the pages in the range added
                     * to the output bitmap.
                     *
                     * When there are multiple scan keys, failure to meet the
                     * criteria for a single one of them is enough to discard
                     * the range as a whole, so break out of the loop as soon
                     * as a false return value is obtained.
                     */
                    add = FunctionCall3Coll(&consistentFn[keyattno - 1],
                                            key->sk_collation,
                                            PointerGetDatum(bdesc),
                                            PointerGetDatum(bval),
                                            PointerGetDatum(key));
                    addrange = DatumGetBool(add);
                    if (!addrange)
                        break;
                }
            }
        }

        /* add the pages in the range to the output bitmap, if needed */
        if (addrange)
        {
            BlockNumber pageno;

            for (pageno = heapBlk;
                 pageno <= heapBlk + opaque->bo_pagesPerRange - 1;
                 pageno++)
            {
                MemoryContextSwitchTo(oldcxt);
                tbm_add_page(tbm, pageno);
                totalpages++;
                MemoryContextSwitchTo(perRangeCxt);
            }
        }
    }

    MemoryContextSwitchTo(oldcxt);
    MemoryContextDelete(perRangeCxt);

    if (buf != InvalidBuffer)
        ReleaseBuffer(buf);

    /*
     * XXX We have an approximation of the number of *pages* that our scan
     * returns, but we don't have a precise idea of the number of heap tuples
     * involved.
     */
    return totalpages * 10;
}

/*
 * Re-initialize state for a BRIN index scan
 */
void
brinrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
           ScanKey orderbys, int norderbys)
{
    /*
     * Other index AMs preprocess the scan keys at this point, or sometime
     * early during the scan; this lets them optimize by removing redundant
     * keys, or doing early returns when they are impossible to satisfy; see
     * _bt_preprocess_keys for an example.  Something like that could be added
     * here someday, too.
     */

    if (scankey && scan->numberOfKeys > 0)
        memmove(scan->keyData, scankey,
                scan->numberOfKeys * sizeof(ScanKeyData));
}

/*
 * Close down a BRIN index scan
 */
void
brinendscan(IndexScanDesc scan)
{
    BrinOpaque *opaque = (BrinOpaque *) scan->opaque;

    brinRevmapTerminate(opaque->bo_rmAccess);
    brin_free_desc(opaque->bo_bdesc);
    pfree(opaque);
}

/*
 * Per-heap-tuple callback for table_index_build_scan.
 *
 * Note we don't worry about the page range at the end of the table here; it is
 * present in the build state struct after we're called the last time, but not
 * inserted into the index.  Caller must ensure to do so, if appropriate.
 */
static void
brinbuildCallback(Relation index,
                  ItemPointer tid,
                  Datum *values,
                  bool *isnull,
                  bool tupleIsAlive,
                  void *brstate)
{
    BrinBuildState *state = (BrinBuildState *) brstate;
    BlockNumber thisblock;
    int         i;

    thisblock = ItemPointerGetBlockNumber(tid);

    /*
     * If we're in a block that belongs to a future range, summarize what
     * we've got and start afresh.  Note the scan might have skipped many
     * pages, if they were devoid of live tuples; make sure to insert index
     * tuples for those too.
     */
    while (thisblock > state->bs_currRangeStart + state->bs_pagesPerRange - 1)
    {

        BRIN_elog((DEBUG2,
                   "brinbuildCallback: completed a range: %u--%u",
                   state->bs_currRangeStart,
                   state->bs_currRangeStart + state->bs_pagesPerRange));

        /* create the index tuple and insert it */
        form_and_insert_tuple(state);

        /* set state to correspond to the next range */
        state->bs_currRangeStart += state->bs_pagesPerRange;

        /* re-initialize state for it */
        brin_memtuple_initialize(state->bs_dtuple, state->bs_bdesc);
    }

    /* Accumulate the current tuple into the running state */
    for (i = 0; i < state->bs_bdesc->bd_tupdesc->natts; i++)
    {
        FmgrInfo   *addValue;
        BrinValues *col;
        Form_pg_attribute attr = TupleDescAttr(state->bs_bdesc->bd_tupdesc, i);

        col = &state->bs_dtuple->bt_columns[i];
        addValue = index_getprocinfo(index, i + 1,
                                     BRIN_PROCNUM_ADDVALUE);

        /*
         * Update dtuple state, if and as necessary.
         */
        FunctionCall4Coll(addValue,
                          attr->attcollation,
                          PointerGetDatum(state->bs_bdesc),
                          PointerGetDatum(col),
                          values[i], isnull[i]);
    }
}

/*
 * brinbuild() -- build a new BRIN index.
 */
IndexBuildResult *
brinbuild(Relation heap, Relation index, IndexInfo *indexInfo)
{
    IndexBuildResult *result;
    double      reltuples;
    double      idxtuples;
    BrinRevmap *revmap;
    BrinBuildState *state;
    Buffer      meta;
    BlockNumber pagesPerRange;

    /*
     * We expect to be called exactly once for any index relation.
     */
    if (RelationGetNumberOfBlocks(index) != 0)
        elog(ERROR, "index \"%s\" already contains data",
             RelationGetRelationName(index));

    /*
     * Critical section not required, because on error the creation of the
     * whole relation will be rolled back.
     */

    meta = ReadBuffer(index, P_NEW);
    Assert(BufferGetBlockNumber(meta) == BRIN_METAPAGE_BLKNO);
    LockBuffer(meta, BUFFER_LOCK_EXCLUSIVE);

    brin_metapage_init(BufferGetPage(meta), BrinGetPagesPerRange(index),
                       BRIN_CURRENT_VERSION);
    MarkBufferDirty(meta);

    if (RelationNeedsWAL(index))
    {
        xl_brin_createidx xlrec;
        XLogRecPtr  recptr;
        Page        page;

        xlrec.version = BRIN_CURRENT_VERSION;
        xlrec.pagesPerRange = BrinGetPagesPerRange(index);

        XLogBeginInsert();
        XLogRegisterData((char *) &xlrec, SizeOfBrinCreateIdx);
        XLogRegisterBuffer(0, meta, REGBUF_WILL_INIT | REGBUF_STANDARD);

        recptr = XLogInsert(RM_BRIN_ID, XLOG_BRIN_CREATE_INDEX);

        page = BufferGetPage(meta);
        PageSetLSN(page, recptr);
    }

    UnlockReleaseBuffer(meta);

    /*
     * Initialize our state, including the deformed tuple state.
     */
    revmap = brinRevmapInitialize(index, &pagesPerRange, NULL);
    state = initialize_brin_buildstate(index, revmap, pagesPerRange);

    /*
     * Now scan the relation.  No syncscan allowed here because we want the
     * heap blocks in physical order.
     */
    reltuples = table_index_build_scan(heap, index, indexInfo, false, true,
                                       brinbuildCallback, (void *) state, NULL);

    /* process the final batch */
    form_and_insert_tuple(state);

    /* release resources */
    idxtuples = state->bs_numtuples;
    brinRevmapTerminate(state->bs_rmAccess);
    terminate_brin_buildstate(state);

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

    result->heap_tuples = reltuples;
    result->index_tuples = idxtuples;

    return result;
}

void
brinbuildempty(Relation index)
{
    Buffer      metabuf;

    /* An empty BRIN index has a metapage only. */
    metabuf =
        ReadBufferExtended(index, INIT_FORKNUM, P_NEW, RBM_NORMAL, NULL);
    LockBuffer(metabuf, BUFFER_LOCK_EXCLUSIVE);

    /* Initialize and xlog metabuffer. */
    START_CRIT_SECTION();
    brin_metapage_init(BufferGetPage(metabuf), BrinGetPagesPerRange(index),
                       BRIN_CURRENT_VERSION);
    MarkBufferDirty(metabuf);
    log_newpage_buffer(metabuf, true);
    END_CRIT_SECTION();

    UnlockReleaseBuffer(metabuf);
}

/*
 * brinbulkdelete
 *      Since there are no per-heap-tuple index tuples in BRIN indexes,
 *      there's not a lot we can do here.
 *
 * XXX we could mark item tuples as "dirty" (when a minimum or maximum heap
 * tuple is deleted), meaning the need to re-run summarization on the affected
 * range.  Would need to add an extra flag in brintuples for that.
 */
IndexBulkDeleteResult *
brinbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
               IndexBulkDeleteCallback callback, void *callback_state)
{
    /* allocate stats if first time through, else re-use existing struct */
    if (stats == NULL)
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));

    return stats;
}

/*
 * This routine is in charge of "vacuuming" a BRIN index: we just summarize
 * ranges that are currently unsummarized.
 */
IndexBulkDeleteResult *
brinvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
{
    Relation    heapRel;

    /* No-op in ANALYZE ONLY mode */
    if (info->analyze_only)
        return stats;

    if (!stats)
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
    stats->num_pages = RelationGetNumberOfBlocks(info->index);
    /* rest of stats is initialized by zeroing */

    heapRel = table_open(IndexGetRelation(RelationGetRelid(info->index), false),
                         AccessShareLock);

    brin_vacuum_scan(info->index, info->strategy);

    brinsummarize(info->index, heapRel, BRIN_ALL_BLOCKRANGES, false,
                  &stats->num_index_tuples, &stats->num_index_tuples);

    table_close(heapRel, AccessShareLock);

    return stats;
}

/*
 * reloptions processor for BRIN indexes
 */
bytea *
brinoptions(Datum reloptions, bool validate)
{
    static const relopt_parse_elt tab[] = {
        {"pages_per_range", RELOPT_TYPE_INT, offsetof(BrinOptions, pagesPerRange)},
        {"autosummarize", RELOPT_TYPE_BOOL, offsetof(BrinOptions, autosummarize)}
    };

    return (bytea *) build_reloptions(reloptions, validate,
                                      RELOPT_KIND_BRIN,
                                      sizeof(BrinOptions),
                                      tab, lengthof(tab));
}

/*
 * SQL-callable function to scan through an index and summarize all ranges
 * that are not currently summarized.
 */
Datum
brin_summarize_new_values(PG_FUNCTION_ARGS)
{
    Datum       relation = PG_GETARG_DATUM(0);

    return DirectFunctionCall2(brin_summarize_range,
                               relation,
                               Int64GetDatum((int64) BRIN_ALL_BLOCKRANGES));
}

/*
 * SQL-callable function to summarize the indicated page range, if not already
 * summarized.  If the second argument is BRIN_ALL_BLOCKRANGES, all
 * unsummarized ranges are summarized.
 */
Datum
brin_summarize_range(PG_FUNCTION_ARGS)
{
    Oid         indexoid = PG_GETARG_OID(0);
    int64       heapBlk64 = PG_GETARG_INT64(1);
    BlockNumber heapBlk;
    Oid         heapoid;
    Relation    indexRel;
    Relation    heapRel;
    double      numSummarized = 0;

    if (RecoveryInProgress())
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("recovery is in progress"),
                 errhint("BRIN control functions cannot be executed during recovery.")));

    if (heapBlk64 > BRIN_ALL_BLOCKRANGES || heapBlk64 < 0)
    {
        char       *blk = psprintf(INT64_FORMAT, heapBlk64);

        ereport(ERROR,
                (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
                 errmsg("block number out of range: %s", blk)));
    }
    heapBlk = (BlockNumber) heapBlk64;

    /*
     * We must lock table before index to avoid deadlocks.  However, if the
     * passed indexoid isn't an index then IndexGetRelation() will fail.
     * Rather than emitting a not-very-helpful error message, postpone
     * complaining, expecting that the is-it-an-index test below will fail.
     */
    heapoid = IndexGetRelation(indexoid, true);
    if (OidIsValid(heapoid))
        heapRel = table_open(heapoid, ShareUpdateExclusiveLock);
    else
        heapRel = NULL;

    indexRel = index_open(indexoid, ShareUpdateExclusiveLock);

    /* Must be a BRIN index */
    if (indexRel->rd_rel->relkind != RELKIND_INDEX ||
        indexRel->rd_rel->relam != BRIN_AM_OID)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is not a BRIN index",
                        RelationGetRelationName(indexRel))));

    /* User must own the index (comparable to privileges needed for VACUUM) */
    if (!pg_class_ownercheck(indexoid, GetUserId()))
        aclcheck_error(ACLCHECK_NOT_OWNER, OBJECT_INDEX,
                       RelationGetRelationName(indexRel));

    /*
     * Since we did the IndexGetRelation call above without any lock, it's
     * barely possible that a race against an index drop/recreation could have
     * netted us the wrong table.  Recheck.
     */
    if (heapRel == NULL || heapoid != IndexGetRelation(indexoid, false))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_TABLE),
                 errmsg("could not open parent table of index %s",
                        RelationGetRelationName(indexRel))));

    /* OK, do it */
    brinsummarize(indexRel, heapRel, heapBlk, true, &numSummarized, NULL);

    relation_close(indexRel, ShareUpdateExclusiveLock);
    relation_close(heapRel, ShareUpdateExclusiveLock);

    PG_RETURN_INT32((int32) numSummarized);
}

/*
 * SQL-callable interface to mark a range as no longer summarized
 */
Datum
brin_desummarize_range(PG_FUNCTION_ARGS)
{
    Oid         indexoid = PG_GETARG_OID(0);
    int64       heapBlk64 = PG_GETARG_INT64(1);
    BlockNumber heapBlk;
    Oid         heapoid;
    Relation    heapRel;
    Relation    indexRel;
    bool        done;

    if (RecoveryInProgress())
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("recovery is in progress"),
                 errhint("BRIN control functions cannot be executed during recovery.")));

    if (heapBlk64 > MaxBlockNumber || heapBlk64 < 0)
    {
        char       *blk = psprintf(INT64_FORMAT, heapBlk64);

        ereport(ERROR,
                (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
                 errmsg("block number out of range: %s", blk)));
    }
    heapBlk = (BlockNumber) heapBlk64;

    /*
     * We must lock table before index to avoid deadlocks.  However, if the
     * passed indexoid isn't an index then IndexGetRelation() will fail.
     * Rather than emitting a not-very-helpful error message, postpone
     * complaining, expecting that the is-it-an-index test below will fail.
     */
    heapoid = IndexGetRelation(indexoid, true);
    if (OidIsValid(heapoid))
        heapRel = table_open(heapoid, ShareUpdateExclusiveLock);
    else
        heapRel = NULL;

    indexRel = index_open(indexoid, ShareUpdateExclusiveLock);

    /* Must be a BRIN index */
    if (indexRel->rd_rel->relkind != RELKIND_INDEX ||
        indexRel->rd_rel->relam != BRIN_AM_OID)
        ereport(ERROR,
                (errcode(ERRCODE_WRONG_OBJECT_TYPE),
                 errmsg("\"%s\" is not a BRIN index",
                        RelationGetRelationName(indexRel))));

    /* User must own the index (comparable to privileges needed for VACUUM) */
    if (!pg_class_ownercheck(indexoid, GetUserId()))
        aclcheck_error(ACLCHECK_NOT_OWNER, OBJECT_INDEX,
                       RelationGetRelationName(indexRel));

    /*
     * Since we did the IndexGetRelation call above without any lock, it's
     * barely possible that a race against an index drop/recreation could have
     * netted us the wrong table.  Recheck.
     */
    if (heapRel == NULL || heapoid != IndexGetRelation(indexoid, false))
        ereport(ERROR,
                (errcode(ERRCODE_UNDEFINED_TABLE),
                 errmsg("could not open parent table of index %s",
                        RelationGetRelationName(indexRel))));

    /* the revmap does the hard work */
    do
    {
        done = brinRevmapDesummarizeRange(indexRel, heapBlk);
    }
    while (!done);

    relation_close(indexRel, ShareUpdateExclusiveLock);
    relation_close(heapRel, ShareUpdateExclusiveLock);

    PG_RETURN_VOID();
}

/*
 * Build a BrinDesc used to create or scan a BRIN index
 */
BrinDesc *
brin_build_desc(Relation rel)
{
    BrinOpcInfo **opcinfo;
    BrinDesc   *bdesc;
    TupleDesc   tupdesc;
    int         totalstored = 0;
    int         keyno;
    long        totalsize;
    MemoryContext cxt;
    MemoryContext oldcxt;

    cxt = AllocSetContextCreate(CurrentMemoryContext,
                                "brin desc cxt",
                                ALLOCSET_SMALL_SIZES);
    oldcxt = MemoryContextSwitchTo(cxt);
    tupdesc = RelationGetDescr(rel);

    /*
     * Obtain BrinOpcInfo for each indexed column.  While at it, accumulate
     * the number of columns stored, since the number is opclass-defined.
     */
    opcinfo = (BrinOpcInfo **) palloc(sizeof(BrinOpcInfo *) * tupdesc->natts);
    for (keyno = 0; keyno < tupdesc->natts; keyno++)
    {
        FmgrInfo   *opcInfoFn;
        Form_pg_attribute attr = TupleDescAttr(tupdesc, keyno);

        opcInfoFn = index_getprocinfo(rel, keyno + 1, BRIN_PROCNUM_OPCINFO);

        opcinfo[keyno] = (BrinOpcInfo *)
            DatumGetPointer(FunctionCall1(opcInfoFn, attr->atttypid));
        totalstored += opcinfo[keyno]->oi_nstored;
    }

    /* Allocate our result struct and fill it in */
    totalsize = offsetof(BrinDesc, bd_info) +
        sizeof(BrinOpcInfo *) * tupdesc->natts;

    bdesc = palloc(totalsize);
    bdesc->bd_context = cxt;
    bdesc->bd_index = rel;
    bdesc->bd_tupdesc = tupdesc;
    bdesc->bd_disktdesc = NULL; /* generated lazily */
    bdesc->bd_totalstored = totalstored;

    for (keyno = 0; keyno < tupdesc->natts; keyno++)
        bdesc->bd_info[keyno] = opcinfo[keyno];
    pfree(opcinfo);

    MemoryContextSwitchTo(oldcxt);

    return bdesc;
}

void
brin_free_desc(BrinDesc *bdesc)
{
    /* make sure the tupdesc is still valid */
    Assert(bdesc->bd_tupdesc->tdrefcount >= 1);
    /* no need for retail pfree */
    MemoryContextDelete(bdesc->bd_context);
}

/*
 * Fetch index's statistical data into *stats
 */
void
brinGetStats(Relation index, BrinStatsData *stats)
{
    Buffer      metabuffer;
    Page        metapage;
    BrinMetaPageData *metadata;

    metabuffer = ReadBuffer(index, BRIN_METAPAGE_BLKNO);
    LockBuffer(metabuffer, BUFFER_LOCK_SHARE);
    metapage = BufferGetPage(metabuffer);
    metadata = (BrinMetaPageData *) PageGetContents(metapage);

    stats->pagesPerRange = metadata->pagesPerRange;
    stats->revmapNumPages = metadata->lastRevmapPage - 1;

    UnlockReleaseBuffer(metabuffer);
}

/*
 * Initialize a BrinBuildState appropriate to create tuples on the given index.
 */
static BrinBuildState *
initialize_brin_buildstate(Relation idxRel, BrinRevmap *revmap,
                           BlockNumber pagesPerRange)
{
    BrinBuildState *state;

    state = palloc(sizeof(BrinBuildState));

    state->bs_irel = idxRel;
    state->bs_numtuples = 0;
    state->bs_currentInsertBuf = InvalidBuffer;
    state->bs_pagesPerRange = pagesPerRange;
    state->bs_currRangeStart = 0;
    state->bs_rmAccess = revmap;
    state->bs_bdesc = brin_build_desc(idxRel);
    state->bs_dtuple = brin_new_memtuple(state->bs_bdesc);

    brin_memtuple_initialize(state->bs_dtuple, state->bs_bdesc);

    return state;
}

/*
 * Release resources associated with a BrinBuildState.
 */
static void
terminate_brin_buildstate(BrinBuildState *state)
{
    /*
     * Release the last index buffer used.  We might as well ensure that
     * whatever free space remains in that page is available in FSM, too.
     */
    if (!BufferIsInvalid(state->bs_currentInsertBuf))
    {
        Page        page;
        Size        freespace;
        BlockNumber blk;

        page = BufferGetPage(state->bs_currentInsertBuf);
        freespace = PageGetFreeSpace(page);
        blk = BufferGetBlockNumber(state->bs_currentInsertBuf);
        ReleaseBuffer(state->bs_currentInsertBuf);
        RecordPageWithFreeSpace(state->bs_irel, blk, freespace);
        FreeSpaceMapVacuumRange(state->bs_irel, blk, blk + 1);
    }

    brin_free_desc(state->bs_bdesc);
    pfree(state->bs_dtuple);
    pfree(state);
}

/*
 * On the given BRIN index, summarize the heap page range that corresponds
 * to the heap block number given.
 *
 * This routine can run in parallel with insertions into the heap.  To avoid
 * missing those values from the summary tuple, we first insert a placeholder
 * index tuple into the index, then execute the heap scan; transactions
 * concurrent with the scan update the placeholder tuple.  After the scan, we
 * union the placeholder tuple with the one computed by this routine.  The
 * update of the index value happens in a loop, so that if somebody updates
 * the placeholder tuple after we read it, we detect the case and try again.
 * This ensures that the concurrently inserted tuples are not lost.
 *
 * A further corner case is this routine being asked to summarize the partial
 * range at the end of the table.  heapNumBlocks is the (possibly outdated)
 * table size; if we notice that the requested range lies beyond that size,
 * we re-compute the table size after inserting the placeholder tuple, to
 * avoid missing pages that were appended recently.
 */
static void
summarize_range(IndexInfo *indexInfo, BrinBuildState *state, Relation heapRel,
                BlockNumber heapBlk, BlockNumber heapNumBlks)
{
    Buffer      phbuf;
    BrinTuple  *phtup;
    Size        phsz;
    OffsetNumber offset;
    BlockNumber scanNumBlks;

    /*
     * Insert the placeholder tuple
     */
    phbuf = InvalidBuffer;
    phtup = brin_form_placeholder_tuple(state->bs_bdesc, heapBlk, &phsz);
    offset = brin_doinsert(state->bs_irel, state->bs_pagesPerRange,
                           state->bs_rmAccess, &phbuf,
                           heapBlk, phtup, phsz);

    /*
     * Compute range end.  We hold ShareUpdateExclusive lock on table, so it
     * cannot shrink concurrently (but it can grow).
     */
    Assert(heapBlk % state->bs_pagesPerRange == 0);
    if (heapBlk + state->bs_pagesPerRange > heapNumBlks)
    {
        /*
         * If we're asked to scan what we believe to be the final range on the
         * table (i.e. a range that might be partial) we need to recompute our
         * idea of what the latest page is after inserting the placeholder
         * tuple.  Anyone that grows the table later will update the
         * placeholder tuple, so it doesn't matter that we won't scan these
         * pages ourselves.  Careful: the table might have been extended
         * beyond the current range, so clamp our result.
         *
         * Fortunately, this should occur infrequently.
         */
        scanNumBlks = Min(RelationGetNumberOfBlocks(heapRel) - heapBlk,
                          state->bs_pagesPerRange);
    }
    else
    {
        /* Easy case: range is known to be complete */
        scanNumBlks = state->bs_pagesPerRange;
    }

    /*
     * Execute the partial heap scan covering the heap blocks in the specified
     * page range, summarizing the heap tuples in it.  This scan stops just
     * short of brinbuildCallback creating the new index entry.
     *
     * Note that it is critical we use the "any visible" mode of
     * table_index_build_range_scan here: otherwise, we would miss tuples
     * inserted by transactions that are still in progress, among other corner
     * cases.
     */
    state->bs_currRangeStart = heapBlk;
    table_index_build_range_scan(heapRel, state->bs_irel, indexInfo, false, true, false,
                                 heapBlk, scanNumBlks,
                                 brinbuildCallback, (void *) state, NULL);

    /*
     * Now we update the values obtained by the scan with the placeholder
     * tuple.  We do this in a loop which only terminates if we're able to
     * update the placeholder tuple successfully; if we are not, this means
     * somebody else modified the placeholder tuple after we read it.
     */
    for (;;)
    {
        BrinTuple  *newtup;
        Size        newsize;
        bool        didupdate;
        bool        samepage;

        CHECK_FOR_INTERRUPTS();

        /*
         * Update the summary tuple and try to update.
         */
        newtup = brin_form_tuple(state->bs_bdesc,
                                 heapBlk, state->bs_dtuple, &newsize);
        samepage = brin_can_do_samepage_update(phbuf, phsz, newsize);
        didupdate =
            brin_doupdate(state->bs_irel, state->bs_pagesPerRange,
                          state->bs_rmAccess, heapBlk, phbuf, offset,
                          phtup, phsz, newtup, newsize, samepage);
        brin_free_tuple(phtup);
        brin_free_tuple(newtup);

        /* If the update succeeded, we're done. */
        if (didupdate)
            break;

        /*
         * If the update didn't work, it might be because somebody updated the
         * placeholder tuple concurrently.  Extract the new version, union it
         * with the values we have from the scan, and start over.  (There are
         * other reasons for the update to fail, but it's simple to treat them
         * the same.)
         */
        phtup = brinGetTupleForHeapBlock(state->bs_rmAccess, heapBlk, &phbuf,
                                         &offset, &phsz, BUFFER_LOCK_SHARE,
                                         NULL);
        /* the placeholder tuple must exist */
        if (phtup == NULL)
            elog(ERROR, "missing placeholder tuple");
        phtup = brin_copy_tuple(phtup, phsz, NULL, NULL);
        LockBuffer(phbuf, BUFFER_LOCK_UNLOCK);

        /* merge it into the tuple from the heap scan */
        union_tuples(state->bs_bdesc, state->bs_dtuple, phtup);
    }

    ReleaseBuffer(phbuf);
}

/*
 * Summarize page ranges that are not already summarized.  If pageRange is
 * BRIN_ALL_BLOCKRANGES then the whole table is scanned; otherwise, only the
 * page range containing the given heap page number is scanned.
 * If include_partial is true, then the partial range at the end of the table
 * is summarized, otherwise not.
 *
 * For each new index tuple inserted, *numSummarized (if not NULL) is
 * incremented; for each existing tuple, *numExisting (if not NULL) is
 * incremented.
 */
static void
brinsummarize(Relation index, Relation heapRel, BlockNumber pageRange,
              bool include_partial, double *numSummarized, double *numExisting)
{
    BrinRevmap *revmap;
    BrinBuildState *state = NULL;
    IndexInfo  *indexInfo = NULL;
    BlockNumber heapNumBlocks;
    BlockNumber pagesPerRange;
    Buffer      buf;
    BlockNumber startBlk;

    revmap = brinRevmapInitialize(index, &pagesPerRange, NULL);

    /* determine range of pages to process */
    heapNumBlocks = RelationGetNumberOfBlocks(heapRel);
    if (pageRange == BRIN_ALL_BLOCKRANGES)
        startBlk = 0;
    else
    {
        startBlk = (pageRange / pagesPerRange) * pagesPerRange;
        heapNumBlocks = Min(heapNumBlocks, startBlk + pagesPerRange);
    }
    if (startBlk > heapNumBlocks)
    {
        /* Nothing to do if start point is beyond end of table */
        brinRevmapTerminate(revmap);
        return;
    }

    /*
     * Scan the revmap to find unsummarized items.
     */
    buf = InvalidBuffer;
    for (; startBlk < heapNumBlocks; startBlk += pagesPerRange)
    {
        BrinTuple  *tup;
        OffsetNumber off;

        /*
         * Unless requested to summarize even a partial range, go away now if
         * we think the next range is partial.  Caller would pass true when it
         * is typically run once bulk data loading is done
         * (brin_summarize_new_values), and false when it is typically the
         * result of arbitrarily-scheduled maintenance command (vacuuming).
         */
        if (!include_partial &&
            (startBlk + pagesPerRange > heapNumBlocks))
            break;

        CHECK_FOR_INTERRUPTS();

        tup = brinGetTupleForHeapBlock(revmap, startBlk, &buf, &off, NULL,
                                       BUFFER_LOCK_SHARE, NULL);
        if (tup == NULL)
        {
            /* no revmap entry for this heap range. Summarize it. */
            if (state == NULL)
            {
                /* first time through */
                Assert(!indexInfo);
                state = initialize_brin_buildstate(index, revmap,
                                                   pagesPerRange);
                indexInfo = BuildIndexInfo(index);
            }
            summarize_range(indexInfo, state, heapRel, startBlk, heapNumBlocks);

            /* and re-initialize state for the next range */
            brin_memtuple_initialize(state->bs_dtuple, state->bs_bdesc);

            if (numSummarized)
                *numSummarized += 1.0;
        }
        else
        {
            if (numExisting)
                *numExisting += 1.0;
            LockBuffer(buf, BUFFER_LOCK_UNLOCK);
        }
    }

    if (BufferIsValid(buf))
        ReleaseBuffer(buf);

    /* free resources */
    brinRevmapTerminate(revmap);
    if (state)
    {
        terminate_brin_buildstate(state);
        pfree(indexInfo);
    }
}

/*
 * Given a deformed tuple in the build state, convert it into the on-disk
 * format and insert it into the index, making the revmap point to it.
 */
static void
form_and_insert_tuple(BrinBuildState *state)
{
    BrinTuple  *tup;
    Size        size;

    tup = brin_form_tuple(state->bs_bdesc, state->bs_currRangeStart,
                          state->bs_dtuple, &size);
    brin_doinsert(state->bs_irel, state->bs_pagesPerRange, state->bs_rmAccess,
                  &state->bs_currentInsertBuf, state->bs_currRangeStart,
                  tup, size);
    state->bs_numtuples++;

    pfree(tup);
}

/*
 * Given two deformed tuples, adjust the first one so that it's consistent
 * with the summary values in both.
 */
static void
union_tuples(BrinDesc *bdesc, BrinMemTuple *a, BrinTuple *b)
{
    int         keyno;
    BrinMemTuple *db;
    MemoryContext cxt;
    MemoryContext oldcxt;

    /* Use our own memory context to avoid retail pfree */
    cxt = AllocSetContextCreate(CurrentMemoryContext,
                                "brin union",
                                ALLOCSET_DEFAULT_SIZES);
    oldcxt = MemoryContextSwitchTo(cxt);
    db = brin_deform_tuple(bdesc, b, NULL);
    MemoryContextSwitchTo(oldcxt);

    for (keyno = 0; keyno < bdesc->bd_tupdesc->natts; keyno++)
    {
        FmgrInfo   *unionFn;
        BrinValues *col_a = &a->bt_columns[keyno];
        BrinValues *col_b = &db->bt_columns[keyno];

        unionFn = index_getprocinfo(bdesc->bd_index, keyno + 1,
                                    BRIN_PROCNUM_UNION);
        FunctionCall3Coll(unionFn,
                          bdesc->bd_index->rd_indcollation[keyno],
                          PointerGetDatum(bdesc),
                          PointerGetDatum(col_a),
                          PointerGetDatum(col_b));
    }

    MemoryContextDelete(cxt);
}

/*
 * brin_vacuum_scan
 *      Do a complete scan of the index during VACUUM.
 *
 * This routine scans the complete index looking for uncatalogued index pages,
 * i.e. those that might have been lost due to a crash after index extension
 * and such.
 */
static void
brin_vacuum_scan(Relation idxrel, BufferAccessStrategy strategy)
{
    BlockNumber nblocks;
    BlockNumber blkno;

    /*
     * Scan the index in physical order, and clean up any possible mess in
     * each page.
     */
    nblocks = RelationGetNumberOfBlocks(idxrel);
    for (blkno = 0; blkno < nblocks; blkno++)
    {
        Buffer      buf;

        CHECK_FOR_INTERRUPTS();

        buf = ReadBufferExtended(idxrel, MAIN_FORKNUM, blkno,
                                 RBM_NORMAL, strategy);

        brin_page_cleanup(idxrel, buf);

        ReleaseBuffer(buf);
    }

    /*
     * Update all upper pages in the index's FSM, as well.  This ensures not
     * only that we propagate leaf-page FSM updates made by brin_page_cleanup,
     * but also that any pre-existing damage or out-of-dateness is repaired.
     */
    FreeSpaceMapVacuum(idxrel);
}