nbtree.c

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/*-------------------------------------------------------------------------
 *
 * nbtree.c
 *    Implementation of Lehman and Yao's btree management algorithm for
 *    Postgres.
 *
 * NOTES
 *    This file contains only the public interface routines.
 *
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/access/nbtree/nbtree.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "access/nbtree.h"
#include "access/relscan.h"
#include "access/xloginsert.h"
#include "commands/progress.h"
#include "commands/vacuum.h"
#include "miscadmin.h"
#include "nodes/execnodes.h"
#include "pgstat.h"
#include "storage/bulk_write.h"
#include "storage/condition_variable.h"
#include "storage/indexfsm.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/smgr.h"
#include "utils/fmgrprotos.h"
#include "utils/index_selfuncs.h"
#include "utils/memutils.h"
 
 
/*
 * BTPARALLEL_NOT_INITIALIZED indicates that the scan has not started.
 *
 * BTPARALLEL_NEED_PRIMSCAN indicates that some process must now seize the
 * scan to advance it via another call to _bt_first.
 *
 * BTPARALLEL_ADVANCING indicates that some process is advancing the scan to
 * a new page; others must wait.
 *
 * BTPARALLEL_IDLE indicates that no backend is currently advancing the scan
 * to a new page; some process can start doing that.
 *
 * BTPARALLEL_DONE indicates that the scan is complete (including error exit).
 */
typedef enum
{
    BTPARALLEL_NOT_INITIALIZED,
    BTPARALLEL_NEED_PRIMSCAN,
    BTPARALLEL_ADVANCING,
    BTPARALLEL_IDLE,
    BTPARALLEL_DONE,
} BTPS_State;
 
/*
 * BTParallelScanDescData contains btree specific shared information required
 * for parallel scan.
 */
typedef struct BTParallelScanDescData
{
    BlockNumber btps_scanPage;  /* latest or next page to be scanned */
    BTPS_State  btps_pageStatus;    /* indicates whether next page is
                                     * available for scan. see above for
                                     * possible states of parallel scan. */
    slock_t     btps_mutex;     /* protects above variables, btps_arrElems */
    ConditionVariable btps_cv;  /* used to synchronize parallel scan */
 
    /*
     * btps_arrElems is used when scans need to schedule another primitive
     * index scan.  Holds BTArrayKeyInfo.cur_elem offsets for scan keys.
     */
    int         btps_arrElems[FLEXIBLE_ARRAY_MEMBER];
}           BTParallelScanDescData;
 
typedef struct BTParallelScanDescData *BTParallelScanDesc;
 
 
static void btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
                         IndexBulkDeleteCallback callback, void *callback_state,
                         BTCycleId cycleid);
static void btvacuumpage(BTVacState *vstate, BlockNumber scanblkno);
static BTVacuumPosting btreevacuumposting(BTVacState *vstate,
                                          IndexTuple posting,
                                          OffsetNumber updatedoffset,
                                          int *nremaining);
 
 
/*
 * Btree handler function: return IndexAmRoutine with access method parameters
 * and callbacks.
 */
Datum
bthandler(PG_FUNCTION_ARGS)
{
    IndexAmRoutine *amroutine = makeNode(IndexAmRoutine);
 
    amroutine->amstrategies = BTMaxStrategyNumber;
    amroutine->amsupport = BTNProcs;
    amroutine->amoptsprocnum = BTOPTIONS_PROC;
    amroutine->amcanorder = true;
    amroutine->amcanorderbyop = false;
    amroutine->amcanbackward = true;
    amroutine->amcanunique = true;
    amroutine->amcanmulticol = true;
    amroutine->amoptionalkey = true;
    amroutine->amsearcharray = true;
    amroutine->amsearchnulls = true;
    amroutine->amstorage = false;
    amroutine->amclusterable = true;
    amroutine->ampredlocks = true;
    amroutine->amcanparallel = true;
    amroutine->amcanbuildparallel = true;
    amroutine->amcaninclude = true;
    amroutine->amusemaintenanceworkmem = false;
    amroutine->amsummarizing = false;
    amroutine->amparallelvacuumoptions =
        VACUUM_OPTION_PARALLEL_BULKDEL | VACUUM_OPTION_PARALLEL_COND_CLEANUP;
    amroutine->amkeytype = InvalidOid;
 
    amroutine->ambuild = btbuild;
    amroutine->ambuildempty = btbuildempty;
    amroutine->aminsert = btinsert;
    amroutine->aminsertcleanup = NULL;
    amroutine->ambulkdelete = btbulkdelete;
    amroutine->amvacuumcleanup = btvacuumcleanup;
    amroutine->amcanreturn = btcanreturn;
    amroutine->amcostestimate = btcostestimate;
    amroutine->amoptions = btoptions;
    amroutine->amproperty = btproperty;
    amroutine->ambuildphasename = btbuildphasename;
    amroutine->amvalidate = btvalidate;
    amroutine->amadjustmembers = btadjustmembers;
    amroutine->ambeginscan = btbeginscan;
    amroutine->amrescan = btrescan;
    amroutine->amgettuple = btgettuple;
    amroutine->amgetbitmap = btgetbitmap;
    amroutine->amendscan = btendscan;
    amroutine->ammarkpos = btmarkpos;
    amroutine->amrestrpos = btrestrpos;
    amroutine->amestimateparallelscan = btestimateparallelscan;
    amroutine->aminitparallelscan = btinitparallelscan;
    amroutine->amparallelrescan = btparallelrescan;
 
    PG_RETURN_POINTER(amroutine);
}
 
/*
 *  btbuildempty() -- build an empty btree index in the initialization fork
 */
void
btbuildempty(Relation index)
{
    bool        allequalimage = _bt_allequalimage(index, false);
    BulkWriteState *bulkstate;
    BulkWriteBuffer metabuf;
 
    bulkstate = smgr_bulk_start_rel(index, INIT_FORKNUM);
 
    /* Construct metapage. */
    metabuf = smgr_bulk_get_buf(bulkstate);
    _bt_initmetapage((Page) metabuf, P_NONE, 0, allequalimage);
    smgr_bulk_write(bulkstate, BTREE_METAPAGE, metabuf, true);
 
    smgr_bulk_finish(bulkstate);
}
 
/*
 *  btinsert() -- insert an index tuple into a btree.
 *
 *      Descend the tree recursively, find the appropriate location for our
 *      new tuple, and put it there.
 */
bool
btinsert(Relation rel, Datum *values, bool *isnull,
         ItemPointer ht_ctid, Relation heapRel,
         IndexUniqueCheck checkUnique,
         bool indexUnchanged,
         IndexInfo *indexInfo)
{
    bool        result;
    IndexTuple  itup;
 
    /* generate an index tuple */
    itup = index_form_tuple(RelationGetDescr(rel), values, isnull);
    itup->t_tid = *ht_ctid;
 
    result = _bt_doinsert(rel, itup, checkUnique, indexUnchanged, heapRel);
 
    pfree(itup);
 
    return result;
}
 
/*
 *  btgettuple() -- Get the next tuple in the scan.
 */
bool
btgettuple(IndexScanDesc scan, ScanDirection dir)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    bool        res;
 
    /* btree indexes are never lossy */
    scan->xs_recheck = false;
 
    /* Each loop iteration performs another primitive index scan */
    do
    {
        /*
         * If we've already initialized this scan, we can just advance it in
         * the appropriate direction.  If we haven't done so yet, we call
         * _bt_first() to get the first item in the scan.
         */
        if (!BTScanPosIsValid(so->currPos))
            res = _bt_first(scan, dir);
        else
        {
            /*
             * Check to see if we should kill the previously-fetched tuple.
             */
            if (scan->kill_prior_tuple)
            {
                /*
                 * Yes, remember it for later. (We'll deal with all such
                 * tuples at once right before leaving the index page.)  The
                 * test for numKilled overrun is not just paranoia: if the
                 * caller reverses direction in the indexscan then the same
                 * item might get entered multiple times. It's not worth
                 * trying to optimize that, so we don't detect it, but instead
                 * just forget any excess entries.
                 */
                if (so->killedItems == NULL)
                    so->killedItems = (int *)
                        palloc(MaxTIDsPerBTreePage * sizeof(int));
                if (so->numKilled < MaxTIDsPerBTreePage)
                    so->killedItems[so->numKilled++] = so->currPos.itemIndex;
            }
 
            /*
             * Now continue the scan.
             */
            res = _bt_next(scan, dir);
        }
 
        /* If we have a tuple, return it ... */
        if (res)
            break;
        /* ... otherwise see if we need another primitive index scan */
    } while (so->numArrayKeys && _bt_start_prim_scan(scan, dir));
 
    return res;
}
 
/*
 * btgetbitmap() -- gets all matching tuples, and adds them to a bitmap
 */
int64
btgetbitmap(IndexScanDesc scan, TIDBitmap *tbm)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    int64       ntids = 0;
    ItemPointer heapTid;
 
    /* Each loop iteration performs another primitive index scan */
    do
    {
        /* Fetch the first page & tuple */
        if (_bt_first(scan, ForwardScanDirection))
        {
            /* Save tuple ID, and continue scanning */
            heapTid = &scan->xs_heaptid;
            tbm_add_tuples(tbm, heapTid, 1, false);
            ntids++;
 
            for (;;)
            {
                /*
                 * Advance to next tuple within page.  This is the same as the
                 * easy case in _bt_next().
                 */
                if (++so->currPos.itemIndex > so->currPos.lastItem)
                {
                    /* let _bt_next do the heavy lifting */
                    if (!_bt_next(scan, ForwardScanDirection))
                        break;
                }
 
                /* Save tuple ID, and continue scanning */
                heapTid = &so->currPos.items[so->currPos.itemIndex].heapTid;
                tbm_add_tuples(tbm, heapTid, 1, false);
                ntids++;
            }
        }
        /* Now see if we need another primitive index scan */
    } while (so->numArrayKeys && _bt_start_prim_scan(scan, ForwardScanDirection));
 
    return ntids;
}
 
/*
 *  btbeginscan() -- start a scan on a btree index
 */
IndexScanDesc
btbeginscan(Relation rel, int nkeys, int norderbys)
{
    IndexScanDesc scan;
    BTScanOpaque so;
 
    /* no order by operators allowed */
    Assert(norderbys == 0);
 
    /* get the scan */
    scan = RelationGetIndexScan(rel, nkeys, norderbys);
 
    /* allocate private workspace */
    so = (BTScanOpaque) palloc(sizeof(BTScanOpaqueData));
    BTScanPosInvalidate(so->currPos);
    BTScanPosInvalidate(so->markPos);
    if (scan->numberOfKeys > 0)
        so->keyData = (ScanKey) palloc(scan->numberOfKeys * sizeof(ScanKeyData));
    else
        so->keyData = NULL;
 
    so->needPrimScan = false;
    so->scanBehind = false;
    so->arrayKeys = NULL;
    so->orderProcs = NULL;
    so->arrayContext = NULL;
 
    so->killedItems = NULL;     /* until needed */
    so->numKilled = 0;
 
    /*
     * We don't know yet whether the scan will be index-only, so we do not
     * allocate the tuple workspace arrays until btrescan.  However, we set up
     * scan->xs_itupdesc whether we'll need it or not, since that's so cheap.
     */
    so->currTuples = so->markTuples = NULL;
 
    scan->xs_itupdesc = RelationGetDescr(rel);
 
    scan->opaque = so;
 
    return scan;
}
 
/*
 *  btrescan() -- rescan an index relation
 */
void
btrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
         ScanKey orderbys, int norderbys)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
    /* we aren't holding any read locks, but gotta drop the pins */
    if (BTScanPosIsValid(so->currPos))
    {
        /* Before leaving current page, deal with any killed items */
        if (so->numKilled > 0)
            _bt_killitems(scan);
        BTScanPosUnpinIfPinned(so->currPos);
        BTScanPosInvalidate(so->currPos);
    }
 
    so->markItemIndex = -1;
    so->needPrimScan = false;
    so->scanBehind = false;
    BTScanPosUnpinIfPinned(so->markPos);
    BTScanPosInvalidate(so->markPos);
 
    /*
     * Allocate tuple workspace arrays, if needed for an index-only scan and
     * not already done in a previous rescan call.  To save on palloc
     * overhead, both workspaces are allocated as one palloc block; only this
     * function and btendscan know that.
     *
     * NOTE: this data structure also makes it safe to return data from a
     * "name" column, even though btree name_ops uses an underlying storage
     * datatype of cstring.  The risk there is that "name" is supposed to be
     * padded to NAMEDATALEN, but the actual index tuple is probably shorter.
     * However, since we only return data out of tuples sitting in the
     * currTuples array, a fetch of NAMEDATALEN bytes can at worst pull some
     * data out of the markTuples array --- running off the end of memory for
     * a SIGSEGV is not possible.  Yeah, this is ugly as sin, but it beats
     * adding special-case treatment for name_ops elsewhere.
     */
    if (scan->xs_want_itup && so->currTuples == NULL)
    {
        so->currTuples = (char *) palloc(BLCKSZ * 2);
        so->markTuples = so->currTuples + BLCKSZ;
    }
 
    /*
     * Reset the scan keys
     */
    if (scankey && scan->numberOfKeys > 0)
        memmove(scan->keyData,
                scankey,
                scan->numberOfKeys * sizeof(ScanKeyData));
    so->numberOfKeys = 0;       /* until _bt_preprocess_keys sets it */
    so->numArrayKeys = 0;       /* ditto */
}
 
/*
 *  btendscan() -- close down a scan
 */
void
btendscan(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
    /* we aren't holding any read locks, but gotta drop the pins */
    if (BTScanPosIsValid(so->currPos))
    {
        /* Before leaving current page, deal with any killed items */
        if (so->numKilled > 0)
            _bt_killitems(scan);
        BTScanPosUnpinIfPinned(so->currPos);
    }
 
    so->markItemIndex = -1;
    BTScanPosUnpinIfPinned(so->markPos);
 
    /* No need to invalidate positions, the RAM is about to be freed. */
 
    /* Release storage */
    if (so->keyData != NULL)
        pfree(so->keyData);
    /* so->arrayKeys and so->orderProcs are in arrayContext */
    if (so->arrayContext != NULL)
        MemoryContextDelete(so->arrayContext);
    if (so->killedItems != NULL)
        pfree(so->killedItems);
    if (so->currTuples != NULL)
        pfree(so->currTuples);
    /* so->markTuples should not be pfree'd, see btrescan */
    pfree(so);
}
 
/*
 *  btmarkpos() -- save current scan position
 */
void
btmarkpos(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
    /* There may be an old mark with a pin (but no lock). */
    BTScanPosUnpinIfPinned(so->markPos);
 
    /*
     * Just record the current itemIndex.  If we later step to next page
     * before releasing the marked position, _bt_steppage makes a full copy of
     * the currPos struct in markPos.  If (as often happens) the mark is moved
     * before we leave the page, we don't have to do that work.
     */
    if (BTScanPosIsValid(so->currPos))
        so->markItemIndex = so->currPos.itemIndex;
    else
    {
        BTScanPosInvalidate(so->markPos);
        so->markItemIndex = -1;
    }
}
 
/*
 *  btrestrpos() -- restore scan to last saved position
 */
void
btrestrpos(IndexScanDesc scan)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
 
    if (so->markItemIndex >= 0)
    {
        /*
         * The scan has never moved to a new page since the last mark.  Just
         * restore the itemIndex.
         *
         * NB: In this case we can't count on anything in so->markPos to be
         * accurate.
         */
        so->currPos.itemIndex = so->markItemIndex;
    }
    else
    {
        /*
         * The scan moved to a new page after last mark or restore, and we are
         * now restoring to the marked page.  We aren't holding any read
         * locks, but if we're still holding the pin for the current position,
         * we must drop it.
         */
        if (BTScanPosIsValid(so->currPos))
        {
            /* Before leaving current page, deal with any killed items */
            if (so->numKilled > 0)
                _bt_killitems(scan);
            BTScanPosUnpinIfPinned(so->currPos);
        }
 
        if (BTScanPosIsValid(so->markPos))
        {
            /* bump pin on mark buffer for assignment to current buffer */
            if (BTScanPosIsPinned(so->markPos))
                IncrBufferRefCount(so->markPos.buf);
            memcpy(&so->currPos, &so->markPos,
                   offsetof(BTScanPosData, items[1]) +
                   so->markPos.lastItem * sizeof(BTScanPosItem));
            if (so->currTuples)
                memcpy(so->currTuples, so->markTuples,
                       so->markPos.nextTupleOffset);
            /* Reset the scan's array keys (see _bt_steppage for why) */
            if (so->numArrayKeys)
            {
                _bt_start_array_keys(scan, so->currPos.dir);
                so->needPrimScan = false;
            }
        }
        else
            BTScanPosInvalidate(so->currPos);
    }
}
 
/*
 * btestimateparallelscan -- estimate storage for BTParallelScanDescData
 */
Size
btestimateparallelscan(int nkeys, int norderbys)
{
    /* Pessimistically assume all input scankeys will be output with arrays */
    return offsetof(BTParallelScanDescData, btps_arrElems) + sizeof(int) * nkeys;
}
 
/*
 * btinitparallelscan -- initialize BTParallelScanDesc for parallel btree scan
 */
void
btinitparallelscan(void *target)
{
    BTParallelScanDesc bt_target = (BTParallelScanDesc) target;
 
    SpinLockInit(&bt_target->btps_mutex);
    bt_target->btps_scanPage = InvalidBlockNumber;
    bt_target->btps_pageStatus = BTPARALLEL_NOT_INITIALIZED;
    ConditionVariableInit(&bt_target->btps_cv);
}
 
/*
 *  btparallelrescan() -- reset parallel scan
 */
void
btparallelrescan(IndexScanDesc scan)
{
    BTParallelScanDesc btscan;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
 
    Assert(parallel_scan);
 
    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);
 
    /*
     * In theory, we don't need to acquire the spinlock here, because there
     * shouldn't be any other workers running at this point, but we do so for
     * consistency.
     */
    SpinLockAcquire(&btscan->btps_mutex);
    btscan->btps_scanPage = InvalidBlockNumber;
    btscan->btps_pageStatus = BTPARALLEL_NOT_INITIALIZED;
    SpinLockRelease(&btscan->btps_mutex);
}
 
/*
 * _bt_parallel_seize() -- Begin the process of advancing the scan to a new
 *      page.  Other scans must wait until we call _bt_parallel_release()
 *      or _bt_parallel_done().
 *
 * The return value is true if we successfully seized the scan and false
 * if we did not.  The latter case occurs if no pages remain.
 *
 * If the return value is true, *pageno returns the next or current page
 * of the scan (depending on the scan direction).  An invalid block number
 * means the scan hasn't yet started, or that caller needs to start the next
 * primitive index scan (if it's the latter case we'll set so.needPrimScan).
 * The first time a participating process reaches the last page, it will return
 * true and set *pageno to P_NONE; after that, further attempts to seize the
 * scan will return false.
 *
 * Callers should ignore the value of pageno if the return value is false.
 *
 * Callers that are in a position to start a new primitive index scan must
 * pass first=true (all other callers pass first=false).  We just return false
 * for first=false callers that require another primitive index scan.
 */
bool
_bt_parallel_seize(IndexScanDesc scan, BlockNumber *pageno, bool first)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    bool        exit_loop = false;
    bool        status = true;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;
 
    *pageno = P_NONE;
 
    if (first)
    {
        /*
         * Initialize array related state when called from _bt_first, assuming
         * that this will either be the first primitive index scan for the
         * scan, or a previous explicitly scheduled primitive scan.
         *
         * Note: so->needPrimScan is only set when a scheduled primitive index
         * scan is set to be performed in caller's worker process.  It should
         * not be set here by us for the first primitive scan, nor should we
         * ever set it for a parallel scan that has no array keys.
         */
        so->needPrimScan = false;
        so->scanBehind = false;
    }
    else
    {
        /*
         * Don't attempt to seize the scan when backend requires another
         * primitive index scan unless we're in a position to start it now
         */
        if (so->needPrimScan)
            return false;
    }
 
    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);
 
    while (1)
    {
        SpinLockAcquire(&btscan->btps_mutex);
 
        if (btscan->btps_pageStatus == BTPARALLEL_DONE)
        {
            /* We're done with this parallel index scan */
            status = false;
        }
        else if (btscan->btps_pageStatus == BTPARALLEL_NEED_PRIMSCAN)
        {
            Assert(so->numArrayKeys);
 
            /*
             * If we can start another primitive scan right away, do so.
             * Otherwise just wait.
             */
            if (first)
            {
                btscan->btps_pageStatus = BTPARALLEL_ADVANCING;
                for (int i = 0; i < so->numArrayKeys; i++)
                {
                    BTArrayKeyInfo *array = &so->arrayKeys[i];
                    ScanKey     skey = &so->keyData[array->scan_key];
 
                    array->cur_elem = btscan->btps_arrElems[i];
                    skey->sk_argument = array->elem_values[array->cur_elem];
                }
                so->needPrimScan = true;
                so->scanBehind = false;
                *pageno = InvalidBlockNumber;
                exit_loop = true;
            }
        }
        else if (btscan->btps_pageStatus != BTPARALLEL_ADVANCING)
        {
            /*
             * We have successfully seized control of the scan for the purpose
             * of advancing it to a new page!
             */
            btscan->btps_pageStatus = BTPARALLEL_ADVANCING;
            *pageno = btscan->btps_scanPage;
            exit_loop = true;
        }
        SpinLockRelease(&btscan->btps_mutex);
        if (exit_loop || !status)
            break;
        ConditionVariableSleep(&btscan->btps_cv, WAIT_EVENT_BTREE_PAGE);
    }
    ConditionVariableCancelSleep();
 
    return status;
}
 
/*
 * _bt_parallel_release() -- Complete the process of advancing the scan to a
 *      new page.  We now have the new value btps_scanPage; some other backend
 *      can now begin advancing the scan.
 *
 * Callers whose scan uses array keys must save their scan_page argument so
 * that it can be passed to _bt_parallel_primscan_schedule, should caller
 * determine that another primitive index scan is required.  If that happens,
 * scan_page won't be scanned by any backend (unless the next primitive index
 * scan lands on scan_page).
 */
void
_bt_parallel_release(IndexScanDesc scan, BlockNumber scan_page)
{
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;
 
    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);
 
    SpinLockAcquire(&btscan->btps_mutex);
    btscan->btps_scanPage = scan_page;
    btscan->btps_pageStatus = BTPARALLEL_IDLE;
    SpinLockRelease(&btscan->btps_mutex);
    ConditionVariableSignal(&btscan->btps_cv);
}
 
/*
 * _bt_parallel_done() -- Mark the parallel scan as complete.
 *
 * When there are no pages left to scan, this function should be called to
 * notify other workers.  Otherwise, they might wait forever for the scan to
 * advance to the next page.
 */
void
_bt_parallel_done(IndexScanDesc scan)
{
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;
    bool        status_changed = false;
 
    /* Do nothing, for non-parallel scans */
    if (parallel_scan == NULL)
        return;
 
    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);
 
    /*
     * Mark the parallel scan as done, unless some other process did so
     * already
     */
    SpinLockAcquire(&btscan->btps_mutex);
    if (btscan->btps_pageStatus != BTPARALLEL_DONE)
    {
        btscan->btps_pageStatus = BTPARALLEL_DONE;
        status_changed = true;
    }
    SpinLockRelease(&btscan->btps_mutex);
 
    /* wake up all the workers associated with this parallel scan */
    if (status_changed)
        ConditionVariableBroadcast(&btscan->btps_cv);
}
 
/*
 * _bt_parallel_primscan_schedule() -- Schedule another primitive index scan.
 *
 * Caller passes the block number most recently passed to _bt_parallel_release
 * by its backend.  Caller successfully schedules the next primitive index scan
 * if the shared parallel state hasn't been seized since caller's backend last
 * advanced the scan.
 */
void
_bt_parallel_primscan_schedule(IndexScanDesc scan, BlockNumber prev_scan_page)
{
    BTScanOpaque so = (BTScanOpaque) scan->opaque;
    ParallelIndexScanDesc parallel_scan = scan->parallel_scan;
    BTParallelScanDesc btscan;
 
    Assert(so->numArrayKeys);
 
    btscan = (BTParallelScanDesc) OffsetToPointer((void *) parallel_scan,
                                                  parallel_scan->ps_offset);
 
    SpinLockAcquire(&btscan->btps_mutex);
    if (btscan->btps_scanPage == prev_scan_page &&
        btscan->btps_pageStatus == BTPARALLEL_IDLE)
    {
        btscan->btps_scanPage = InvalidBlockNumber;
        btscan->btps_pageStatus = BTPARALLEL_NEED_PRIMSCAN;
 
        /* Serialize scan's current array keys */
        for (int i = 0; i < so->numArrayKeys; i++)
        {
            BTArrayKeyInfo *array = &so->arrayKeys[i];
 
            btscan->btps_arrElems[i] = array->cur_elem;
        }
    }
    SpinLockRelease(&btscan->btps_mutex);
}
 
/*
 * Bulk deletion of all index entries pointing to a set of heap tuples.
 * The set of target tuples is specified via a callback routine that tells
 * whether any given heap tuple (identified by ItemPointer) is being deleted.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
IndexBulkDeleteResult *
btbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
             IndexBulkDeleteCallback callback, void *callback_state)
{
    Relation    rel = info->index;
    BTCycleId   cycleid;
 
    /* allocate stats if first time through, else re-use existing struct */
    if (stats == NULL)
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
 
    /* Establish the vacuum cycle ID to use for this scan */
    /* The ENSURE stuff ensures we clean up shared memory on failure */
    PG_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
    {
        cycleid = _bt_start_vacuum(rel);
 
        btvacuumscan(info, stats, callback, callback_state, cycleid);
    }
    PG_END_ENSURE_ERROR_CLEANUP(_bt_end_vacuum_callback, PointerGetDatum(rel));
    _bt_end_vacuum(rel);
 
    return stats;
}
 
/*
 * Post-VACUUM cleanup.
 *
 * Result: a palloc'd struct containing statistical info for VACUUM displays.
 */
IndexBulkDeleteResult *
btvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
{
    BlockNumber num_delpages;
 
    /* No-op in ANALYZE ONLY mode */
    if (info->analyze_only)
        return stats;
 
    /*
     * If btbulkdelete was called, we need not do anything (we just maintain
     * the information used within _bt_vacuum_needs_cleanup() by calling
     * _bt_set_cleanup_info() below).
     *
     * If btbulkdelete was _not_ called, then we have a choice to make: we
     * must decide whether or not a btvacuumscan() call is needed now (i.e.
     * whether the ongoing VACUUM operation can entirely avoid a physical scan
     * of the index).  A call to _bt_vacuum_needs_cleanup() decides it for us
     * now.
     */
    if (stats == NULL)
    {
        /* Check if VACUUM operation can entirely avoid btvacuumscan() call */
        if (!_bt_vacuum_needs_cleanup(info->index))
            return NULL;
 
        /*
         * Since we aren't going to actually delete any leaf items, there's no
         * need to go through all the vacuum-cycle-ID pushups here.
         *
         * Posting list tuples are a source of inaccuracy for cleanup-only
         * scans.  btvacuumscan() will assume that the number of index tuples
         * from each page can be used as num_index_tuples, even though
         * num_index_tuples is supposed to represent the number of TIDs in the
         * index.  This naive approach can underestimate the number of tuples
         * in the index significantly.
         *
         * We handle the problem by making num_index_tuples an estimate in
         * cleanup-only case.
         */
        stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
        btvacuumscan(info, stats, NULL, NULL, 0);
        stats->estimated_count = true;
    }
 
    /*
     * Maintain num_delpages value in metapage for _bt_vacuum_needs_cleanup().
     *
     * num_delpages is the number of deleted pages now in the index that were
     * not safe to place in the FSM to be recycled just yet.  num_delpages is
     * greater than 0 only when _bt_pagedel() actually deleted pages during
     * our call to btvacuumscan().  Even then, _bt_pendingfsm_finalize() must
     * have failed to place any newly deleted pages in the FSM just moments
     * ago.  (Actually, there are edge cases where recycling of the current
     * VACUUM's newly deleted pages does not even become safe by the time the
     * next VACUUM comes around.  See nbtree/README.)
     */
    Assert(stats->pages_deleted >= stats->pages_free);
    num_delpages = stats->pages_deleted - stats->pages_free;
    _bt_set_cleanup_info(info->index, num_delpages);
 
    /*
     * It's quite possible for us to be fooled by concurrent page splits into
     * double-counting some index tuples, so disbelieve any total that exceeds
     * the underlying heap's count ... if we know that accurately.  Otherwise
     * this might just make matters worse.
     */
    if (!info->estimated_count)
    {
        if (stats->num_index_tuples > info->num_heap_tuples)
            stats->num_index_tuples = info->num_heap_tuples;
    }
 
    return stats;
}
 
/*
 * btvacuumscan --- scan the index for VACUUMing purposes
 *
 * This combines the functions of looking for leaf tuples that are deletable
 * according to the vacuum callback, looking for empty pages that can be
 * deleted, and looking for old deleted pages that can be recycled.  Both
 * btbulkdelete and btvacuumcleanup invoke this (the latter only if no
 * btbulkdelete call occurred and _bt_vacuum_needs_cleanup returned true).
 *
 * The caller is responsible for initially allocating/zeroing a stats struct
 * and for obtaining a vacuum cycle ID if necessary.
 */
static void
btvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
             IndexBulkDeleteCallback callback, void *callback_state,
             BTCycleId cycleid)
{
    Relation    rel = info->index;
    BTVacState  vstate;
    BlockNumber num_pages;
    BlockNumber scanblkno;
    bool        needLock;
 
    /*
     * Reset fields that track information about the entire index now.  This
     * avoids double-counting in the case where a single VACUUM command
     * requires multiple scans of the index.
     *
     * Avoid resetting the tuples_removed and pages_newly_deleted fields here,
     * since they track information about the VACUUM command, and so must last
     * across each call to btvacuumscan().
     *
     * (Note that pages_free is treated as state about the whole index, not
     * the current VACUUM.  This is appropriate because RecordFreeIndexPage()
     * calls are idempotent, and get repeated for the same deleted pages in
     * some scenarios.  The point for us is to track the number of recyclable
     * pages in the index at the end of the VACUUM command.)
     */
    stats->num_pages = 0;
    stats->num_index_tuples = 0;
    stats->pages_deleted = 0;
    stats->pages_free = 0;
 
    /* Set up info to pass down to btvacuumpage */
    vstate.info = info;
    vstate.stats = stats;
    vstate.callback = callback;
    vstate.callback_state = callback_state;
    vstate.cycleid = cycleid;
 
    /* Create a temporary memory context to run _bt_pagedel in */
    vstate.pagedelcontext = AllocSetContextCreate(CurrentMemoryContext,
                                                  "_bt_pagedel",
                                                  ALLOCSET_DEFAULT_SIZES);
 
    /* Initialize vstate fields used by _bt_pendingfsm_finalize */
    vstate.bufsize = 0;
    vstate.maxbufsize = 0;
    vstate.pendingpages = NULL;
    vstate.npendingpages = 0;
    /* Consider applying _bt_pendingfsm_finalize optimization */
    _bt_pendingfsm_init(rel, &vstate, (callback == NULL));
 
    /*
     * The outer loop iterates over all index pages except the metapage, in
     * physical order (we hope the kernel will cooperate in providing
     * read-ahead for speed).  It is critical that we visit all leaf pages,
     * including ones added after we start the scan, else we might fail to
     * delete some deletable tuples.  Hence, we must repeatedly check the
     * relation length.  We must acquire the relation-extension lock while
     * doing so to avoid a race condition: if someone else is extending the
     * relation, there is a window where bufmgr/smgr have created a new
     * all-zero page but it hasn't yet been write-locked by _bt_getbuf(). If
     * we manage to scan such a page here, we'll improperly assume it can be
     * recycled.  Taking the lock synchronizes things enough to prevent a
     * problem: either num_pages won't include the new page, or _bt_getbuf
     * already has write lock on the buffer and it will be fully initialized
     * before we can examine it.  Also, we need not worry if a page is added
     * immediately after we look; the page splitting code already has
     * write-lock on the left page before it adds a right page, so we must
     * already have processed any tuples due to be moved into such a page.
     *
     * XXX: Now that new pages are locked with RBM_ZERO_AND_LOCK, I don't
     * think the use of the extension lock is still required.
     *
     * We can skip locking for new or temp relations, however, since no one
     * else could be accessing them.
     */
    needLock = !RELATION_IS_LOCAL(rel);
 
    scanblkno = BTREE_METAPAGE + 1;
    for (;;)
    {
        /* Get the current relation length */
        if (needLock)
            LockRelationForExtension(rel, ExclusiveLock);
        num_pages = RelationGetNumberOfBlocks(rel);
        if (needLock)
            UnlockRelationForExtension(rel, ExclusiveLock);
 
        if (info->report_progress)
            pgstat_progress_update_param(PROGRESS_SCAN_BLOCKS_TOTAL,
                                         num_pages);
 
        /* Quit if we've scanned the whole relation */
        if (scanblkno >= num_pages)
            break;
        /* Iterate over pages, then loop back to recheck length */
        for (; scanblkno < num_pages; scanblkno++)
        {
            btvacuumpage(&vstate, scanblkno);
            if (info->report_progress)
                pgstat_progress_update_param(PROGRESS_SCAN_BLOCKS_DONE,
                                             scanblkno);
        }
    }
 
    /* Set statistics num_pages field to final size of index */
    stats->num_pages = num_pages;
 
    MemoryContextDelete(vstate.pagedelcontext);
 
    /*
     * If there were any calls to _bt_pagedel() during scan of the index then
     * see if any of the resulting pages can be placed in the FSM now.  When
     * it's not safe we'll have to leave it up to a future VACUUM operation.
     *
     * Finally, if we placed any pages in the FSM (either just now or during
     * the scan), forcibly update the upper-level FSM pages to ensure that
     * searchers can find them.
     */
    _bt_pendingfsm_finalize(rel, &vstate);
    if (stats->pages_free > 0)
        IndexFreeSpaceMapVacuum(rel);
}
 
/*
 * btvacuumpage --- VACUUM one page
 *
 * This processes a single page for btvacuumscan().  In some cases we must
 * backtrack to re-examine and VACUUM pages that were the scanblkno during
 * a previous call here.  This is how we handle page splits (that happened
 * after our cycleid was acquired) whose right half page happened to reuse
 * a block that we might have processed at some point before it was
 * recycled (i.e. before the page split).
 */
static void
btvacuumpage(BTVacState *vstate, BlockNumber scanblkno)
{
    IndexVacuumInfo *info = vstate->info;
    IndexBulkDeleteResult *stats = vstate->stats;
    IndexBulkDeleteCallback callback = vstate->callback;
    void       *callback_state = vstate->callback_state;
    Relation    rel = info->index;
    Relation    heaprel = info->heaprel;
    bool        attempt_pagedel;
    BlockNumber blkno,
                backtrack_to;
    Buffer      buf;
    Page        page;
    BTPageOpaque opaque;
 
    blkno = scanblkno;
 
backtrack:
 
    attempt_pagedel = false;
    backtrack_to = P_NONE;
 
    /* call vacuum_delay_point while not holding any buffer lock */
    vacuum_delay_point();
 
    /*
     * We can't use _bt_getbuf() here because it always applies
     * _bt_checkpage(), which will barf on an all-zero page. We want to
     * recycle all-zero pages, not fail.  Also, we want to use a nondefault
     * buffer access strategy.
     */
    buf = ReadBufferExtended(rel, MAIN_FORKNUM, blkno, RBM_NORMAL,
                             info->strategy);
    _bt_lockbuf(rel, buf, BT_READ);
    page = BufferGetPage(buf);
    opaque = NULL;
    if (!PageIsNew(page))
    {
        _bt_checkpage(rel, buf);
        opaque = BTPageGetOpaque(page);
    }
 
    Assert(blkno <= scanblkno);
    if (blkno != scanblkno)
    {
        /*
         * We're backtracking.
         *
         * We followed a right link to a sibling leaf page (a page that
         * happens to be from a block located before scanblkno).  The only
         * case we want to do anything with is a live leaf page having the
         * current vacuum cycle ID.
         *
         * The page had better be in a state that's consistent with what we
         * expect.  Check for conditions that imply corruption in passing.  It
         * can't be half-dead because only an interrupted VACUUM process can
         * leave pages in that state, so we'd definitely have dealt with it
         * back when the page was the scanblkno page (half-dead pages are
         * always marked fully deleted by _bt_pagedel(), barring corruption).
         */
        if (!opaque || !P_ISLEAF(opaque) || P_ISHALFDEAD(opaque))
        {
            Assert(false);
            ereport(LOG,
                    (errcode(ERRCODE_INDEX_CORRUPTED),
                     errmsg_internal("right sibling %u of scanblkno %u unexpectedly in an inconsistent state in index \"%s\"",
                                     blkno, scanblkno, RelationGetRelationName(rel))));
            _bt_relbuf(rel, buf);
            return;
        }
 
        /*
         * We may have already processed the page in an earlier call, when the
         * page was scanblkno.  This happens when the leaf page split occurred
         * after the scan began, but before the right sibling page became the
         * scanblkno.
         *
         * Page may also have been deleted by current btvacuumpage() call,
         * since _bt_pagedel() sometimes deletes the right sibling page of
         * scanblkno in passing (it does so after we decided where to
         * backtrack to).  We don't need to process this page as a deleted
         * page a second time now (in fact, it would be wrong to count it as a
         * deleted page in the bulk delete statistics a second time).
         */
        if (opaque->btpo_cycleid != vstate->cycleid || P_ISDELETED(opaque))
        {
            /* Done with current scanblkno (and all lower split pages) */
            _bt_relbuf(rel, buf);
            return;
        }
    }
 
    if (!opaque || BTPageIsRecyclable(page, heaprel))
    {
        /* Okay to recycle this page (which could be leaf or internal) */
        RecordFreeIndexPage(rel, blkno);
        stats->pages_deleted++;
        stats->pages_free++;
    }
    else if (P_ISDELETED(opaque))
    {
        /*
         * Already deleted page (which could be leaf or internal).  Can't
         * recycle yet.
         */
        stats->pages_deleted++;
    }
    else if (P_ISHALFDEAD(opaque))
    {
        /* Half-dead leaf page (from interrupted VACUUM) -- finish deleting */
        attempt_pagedel = true;
 
        /*
         * _bt_pagedel() will increment both pages_newly_deleted and
         * pages_deleted stats in all cases (barring corruption)
         */
    }
    else if (P_ISLEAF(opaque))
    {
        OffsetNumber deletable[MaxIndexTuplesPerPage];
        int         ndeletable;
        BTVacuumPosting updatable[MaxIndexTuplesPerPage];
        int         nupdatable;
        OffsetNumber offnum,
                    minoff,
                    maxoff;
        int         nhtidsdead,
                    nhtidslive;
 
        /*
         * Trade in the initial read lock for a full cleanup lock on this
         * page.  We must get such a lock on every leaf page over the course
         * of the vacuum scan, whether or not it actually contains any
         * deletable tuples --- see nbtree/README.
         */
        _bt_upgradelockbufcleanup(rel, buf);
 
        /*
         * Check whether we need to backtrack to earlier pages.  What we are
         * concerned about is a page split that happened since we started the
         * vacuum scan.  If the split moved tuples on the right half of the
         * split (i.e. the tuples that sort high) to a block that we already
         * passed over, then we might have missed the tuples.  We need to
         * backtrack now.  (Must do this before possibly clearing btpo_cycleid
         * or deleting scanblkno page below!)
         */
        if (vstate->cycleid != 0 &&
            opaque->btpo_cycleid == vstate->cycleid &&
            !(opaque->btpo_flags & BTP_SPLIT_END) &&
            !P_RIGHTMOST(opaque) &&
            opaque->btpo_next < scanblkno)
            backtrack_to = opaque->btpo_next;
 
        ndeletable = 0;
        nupdatable = 0;
        minoff = P_FIRSTDATAKEY(opaque);
        maxoff = PageGetMaxOffsetNumber(page);
        nhtidsdead = 0;
        nhtidslive = 0;
        if (callback)
        {
            /* btbulkdelete callback tells us what to delete (or update) */
            for (offnum = minoff;
                 offnum <= maxoff;
                 offnum = OffsetNumberNext(offnum))
            {
                IndexTuple  itup;
 
                itup = (IndexTuple) PageGetItem(page,
                                                PageGetItemId(page, offnum));
 
                Assert(!BTreeTupleIsPivot(itup));
                if (!BTreeTupleIsPosting(itup))
                {
                    /* Regular tuple, standard table TID representation */
                    if (callback(&itup->t_tid, callback_state))
                    {
                        deletable[ndeletable++] = offnum;
                        nhtidsdead++;
                    }
                    else
                        nhtidslive++;
                }
                else
                {
                    BTVacuumPosting vacposting;
                    int         nremaining;
 
                    /* Posting list tuple */
                    vacposting = btreevacuumposting(vstate, itup, offnum,
                                                    &nremaining);
                    if (vacposting == NULL)
                    {
                        /*
                         * All table TIDs from the posting tuple remain, so no
                         * delete or update required
                         */
                        Assert(nremaining == BTreeTupleGetNPosting(itup));
                    }
                    else if (nremaining > 0)
                    {
 
                        /*
                         * Store metadata about posting list tuple in
                         * updatable array for entire page.  Existing tuple
                         * will be updated during the later call to
                         * _bt_delitems_vacuum().
                         */
                        Assert(nremaining < BTreeTupleGetNPosting(itup));
                        updatable[nupdatable++] = vacposting;
                        nhtidsdead += BTreeTupleGetNPosting(itup) - nremaining;
                    }
                    else
                    {
                        /*
                         * All table TIDs from the posting list must be
                         * deleted.  We'll delete the index tuple completely
                         * (no update required).
                         */
                        Assert(nremaining == 0);
                        deletable[ndeletable++] = offnum;
                        nhtidsdead += BTreeTupleGetNPosting(itup);
                        pfree(vacposting);
                    }
 
                    nhtidslive += nremaining;
                }
            }
        }
 
        /*
         * Apply any needed deletes or updates.  We issue just one
         * _bt_delitems_vacuum() call per page, so as to minimize WAL traffic.
         */
        if (ndeletable > 0 || nupdatable > 0)
        {
            Assert(nhtidsdead >= ndeletable + nupdatable);
            _bt_delitems_vacuum(rel, buf, deletable, ndeletable, updatable,
                                nupdatable);
 
            stats->tuples_removed += nhtidsdead;
            /* must recompute maxoff */
            maxoff = PageGetMaxOffsetNumber(page);
 
            /* can't leak memory here */
            for (int i = 0; i < nupdatable; i++)
                pfree(updatable[i]);
        }
        else
        {
            /*
             * If the leaf page has been split during this vacuum cycle, it
             * seems worth expending a write to clear btpo_cycleid even if we
             * don't have any deletions to do.  (If we do, _bt_delitems_vacuum
             * takes care of this.)  This ensures we won't process the page
             * again.
             *
             * We treat this like a hint-bit update because there's no need to
             * WAL-log it.
             */
            Assert(nhtidsdead == 0);
            if (vstate->cycleid != 0 &&
                opaque->btpo_cycleid == vstate->cycleid)
            {
                opaque->btpo_cycleid = 0;
                MarkBufferDirtyHint(buf, true);
            }
        }
 
        /*
         * If the leaf page is now empty, try to delete it; else count the
         * live tuples (live table TIDs in posting lists are counted as
         * separate live tuples).  We don't delete when backtracking, though,
         * since that would require teaching _bt_pagedel() about backtracking
         * (doesn't seem worth adding more complexity to deal with that).
         *
         * We don't count the number of live TIDs during cleanup-only calls to
         * btvacuumscan (i.e. when callback is not set).  We count the number
         * of index tuples directly instead.  This avoids the expense of
         * directly examining all of the tuples on each page.  VACUUM will
         * treat num_index_tuples as an estimate in cleanup-only case, so it
         * doesn't matter that this underestimates num_index_tuples
         * significantly in some cases.
         */
        if (minoff > maxoff)
            attempt_pagedel = (blkno == scanblkno);
        else if (callback)
            stats->num_index_tuples += nhtidslive;
        else
            stats->num_index_tuples += maxoff - minoff + 1;
 
        Assert(!attempt_pagedel || nhtidslive == 0);
    }
 
    if (attempt_pagedel)
    {
        MemoryContext oldcontext;
 
        /* Run pagedel in a temp context to avoid memory leakage */
        MemoryContextReset(vstate->pagedelcontext);
        oldcontext = MemoryContextSwitchTo(vstate->pagedelcontext);
 
        /*
         * _bt_pagedel maintains the bulk delete stats on our behalf;
         * pages_newly_deleted and pages_deleted are likely to be incremented
         * during call
         */
        Assert(blkno == scanblkno);
        _bt_pagedel(rel, buf, vstate);
 
        MemoryContextSwitchTo(oldcontext);
        /* pagedel released buffer, so we shouldn't */
    }
    else
        _bt_relbuf(rel, buf);
 
    if (backtrack_to != P_NONE)
    {
        blkno = backtrack_to;
        goto backtrack;
    }
}
 
/*
 * btreevacuumposting --- determine TIDs still needed in posting list
 *
 * Returns metadata describing how to build replacement tuple without the TIDs
 * that VACUUM needs to delete.  Returned value is NULL in the common case
 * where no changes are needed to caller's posting list tuple (we avoid
 * allocating memory here as an optimization).
 *
 * The number of TIDs that should remain in the posting list tuple is set for
 * caller in *nremaining.
 */
static BTVacuumPosting
btreevacuumposting(BTVacState *vstate, IndexTuple posting,
                   OffsetNumber updatedoffset, int *nremaining)
{
    int         live = 0;
    int         nitem = BTreeTupleGetNPosting(posting);
    ItemPointer items = BTreeTupleGetPosting(posting);
    BTVacuumPosting vacposting = NULL;
 
    for (int i = 0; i < nitem; i++)
    {
        if (!vstate->callback(items + i, vstate->callback_state))
        {
            /* Live table TID */
            live++;
        }
        else if (vacposting == NULL)
        {
            /*
             * First dead table TID encountered.
             *
             * It's now clear that we need to delete one or more dead table
             * TIDs, so start maintaining metadata describing how to update
             * existing posting list tuple.
             */
            vacposting = palloc(offsetof(BTVacuumPostingData, deletetids) +
                                nitem * sizeof(uint16));
 
            vacposting->itup = posting;
            vacposting->updatedoffset = updatedoffset;
            vacposting->ndeletedtids = 0;
            vacposting->deletetids[vacposting->ndeletedtids++] = i;
        }
        else
        {
            /* Second or subsequent dead table TID */
            vacposting->deletetids[vacposting->ndeletedtids++] = i;
        }
    }
 
    *nremaining = live;
    return vacposting;
}
 
/*
 *  btcanreturn() -- Check whether btree indexes support index-only scans.
 *
 * btrees always do, so this is trivial.
 */
bool
btcanreturn(Relation index, int attno)
{
    return true;
}