nodeBitmapHeapscan.c

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/*-------------------------------------------------------------------------
 *
 * nodeBitmapHeapscan.c
 *    Routines to support bitmapped scans of relations
 *
 * NOTE: it is critical that this plan type only be used with MVCC-compliant
 * snapshots (ie, regular snapshots, not SnapshotAny or one of the other
 * special snapshots).  The reason is that since index and heap scans are
 * decoupled, there can be no assurance that the index tuple prompting a
 * visit to a particular heap TID still exists when the visit is made.
 * Therefore the tuple might not exist anymore either (which is OK because
 * heap_fetch will cope) --- but worse, the tuple slot could have been
 * re-used for a newer tuple.  With an MVCC snapshot the newer tuple is
 * certain to fail the time qual and so it will not be mistakenly returned,
 * but with anything else we might return a tuple that doesn't meet the
 * required index qual conditions.
 *
 *
 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeBitmapHeapscan.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * INTERFACE ROUTINES
 *      ExecBitmapHeapScan          scans a relation using bitmap info
 *      ExecBitmapHeapNext          workhorse for above
 *      ExecInitBitmapHeapScan      creates and initializes state info.
 *      ExecReScanBitmapHeapScan    prepares to rescan the plan.
 *      ExecEndBitmapHeapScan       releases all storage.
 */
#include "postgres.h"

#include <math.h>

#include "access/relscan.h"
#include "access/tableam.h"
#include "access/transam.h"
#include "access/visibilitymap.h"
#include "executor/execdebug.h"
#include "executor/nodeBitmapHeapscan.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/bufmgr.h"
#include "storage/predicate.h"
#include "utils/memutils.h"
#include "utils/rel.h"
#include "utils/spccache.h"
#include "utils/snapmgr.h"


static TupleTableSlot *BitmapHeapNext(BitmapHeapScanState *node);
static inline void BitmapDoneInitializingSharedState(ParallelBitmapHeapState *pstate);
static inline void BitmapAdjustPrefetchIterator(BitmapHeapScanState *node,
                                                TBMIterateResult *tbmres);
static inline void BitmapAdjustPrefetchTarget(BitmapHeapScanState *node);
static inline void BitmapPrefetch(BitmapHeapScanState *node,
                                  TableScanDesc scan);
static bool BitmapShouldInitializeSharedState(ParallelBitmapHeapState *pstate);


/* ----------------------------------------------------------------
 *      BitmapHeapNext
 *
 *      Retrieve next tuple from the BitmapHeapScan node's currentRelation
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
BitmapHeapNext(BitmapHeapScanState *node)
{
    ExprContext *econtext;
    TableScanDesc scan;
    TIDBitmap  *tbm;
    TBMIterator *tbmiterator = NULL;
    TBMSharedIterator *shared_tbmiterator = NULL;
    TBMIterateResult *tbmres;
    TupleTableSlot *slot;
    ParallelBitmapHeapState *pstate = node->pstate;
    dsa_area   *dsa = node->ss.ps.state->es_query_dsa;

    /*
     * extract necessary information from index scan node
     */
    econtext = node->ss.ps.ps_ExprContext;
    slot = node->ss.ss_ScanTupleSlot;
    scan = node->ss.ss_currentScanDesc;
    tbm = node->tbm;
    if (pstate == NULL)
        tbmiterator = node->tbmiterator;
    else
        shared_tbmiterator = node->shared_tbmiterator;
    tbmres = node->tbmres;

    /*
     * If we haven't yet performed the underlying index scan, do it, and begin
     * the iteration over the bitmap.
     *
     * For prefetching, we use *two* iterators, one for the pages we are
     * actually scanning and another that runs ahead of the first for
     * prefetching.  node->prefetch_pages tracks exactly how many pages ahead
     * the prefetch iterator is.  Also, node->prefetch_target tracks the
     * desired prefetch distance, which starts small and increases up to the
     * node->prefetch_maximum.  This is to avoid doing a lot of prefetching in
     * a scan that stops after a few tuples because of a LIMIT.
     */
    if (!node->initialized)
    {
        if (!pstate)
        {
            tbm = (TIDBitmap *) MultiExecProcNode(outerPlanState(node));

            if (!tbm || !IsA(tbm, TIDBitmap))
                elog(ERROR, "unrecognized result from subplan");

            node->tbm = tbm;
            node->tbmiterator = tbmiterator = tbm_begin_iterate(tbm);
            node->tbmres = tbmres = NULL;

#ifdef USE_PREFETCH
            if (node->prefetch_maximum > 0)
            {
                node->prefetch_iterator = tbm_begin_iterate(tbm);
                node->prefetch_pages = 0;
                node->prefetch_target = -1;
            }
#endif                          /* USE_PREFETCH */
        }
        else
        {
            /*
             * The leader will immediately come out of the function, but
             * others will be blocked until leader populates the TBM and wakes
             * them up.
             */
            if (BitmapShouldInitializeSharedState(pstate))
            {
                tbm = (TIDBitmap *) MultiExecProcNode(outerPlanState(node));
                if (!tbm || !IsA(tbm, TIDBitmap))
                    elog(ERROR, "unrecognized result from subplan");

                node->tbm = tbm;

                /*
                 * Prepare to iterate over the TBM. This will return the
                 * dsa_pointer of the iterator state which will be used by
                 * multiple processes to iterate jointly.
                 */
                pstate->tbmiterator = tbm_prepare_shared_iterate(tbm);
#ifdef USE_PREFETCH
                if (node->prefetch_maximum > 0)
                {
                    pstate->prefetch_iterator =
                        tbm_prepare_shared_iterate(tbm);

                    /*
                     * We don't need the mutex here as we haven't yet woke up
                     * others.
                     */
                    pstate->prefetch_pages = 0;
                    pstate->prefetch_target = -1;
                }
#endif

                /* We have initialized the shared state so wake up others. */
                BitmapDoneInitializingSharedState(pstate);
            }

            /* Allocate a private iterator and attach the shared state to it */
            node->shared_tbmiterator = shared_tbmiterator =
                tbm_attach_shared_iterate(dsa, pstate->tbmiterator);
            node->tbmres = tbmres = NULL;

#ifdef USE_PREFETCH
            if (node->prefetch_maximum > 0)
            {
                node->shared_prefetch_iterator =
                    tbm_attach_shared_iterate(dsa, pstate->prefetch_iterator);
            }
#endif                          /* USE_PREFETCH */
        }
        node->initialized = true;
    }

    for (;;)
    {
        bool        skip_fetch;

        CHECK_FOR_INTERRUPTS();

        /*
         * Get next page of results if needed
         */
        if (tbmres == NULL)
        {
            if (!pstate)
                node->tbmres = tbmres = tbm_iterate(tbmiterator);
            else
                node->tbmres = tbmres = tbm_shared_iterate(shared_tbmiterator);
            if (tbmres == NULL)
            {
                /* no more entries in the bitmap */
                break;
            }

            BitmapAdjustPrefetchIterator(node, tbmres);

            /*
             * We can skip fetching the heap page if we don't need any fields
             * from the heap, and the bitmap entries don't need rechecking,
             * and all tuples on the page are visible to our transaction.
             *
             * XXX: It's a layering violation that we do these checks above
             * tableam, they should probably moved below it at some point.
             */
            skip_fetch = (node->can_skip_fetch &&
                          !tbmres->recheck &&
                          VM_ALL_VISIBLE(node->ss.ss_currentRelation,
                                         tbmres->blockno,
                                         &node->vmbuffer));

            if (skip_fetch)
            {
                /* can't be lossy in the skip_fetch case */
                Assert(tbmres->ntuples >= 0);

                /*
                 * The number of tuples on this page is put into
                 * node->return_empty_tuples.
                 */
                node->return_empty_tuples = tbmres->ntuples;
            }
            else if (!table_scan_bitmap_next_block(scan, tbmres))
            {
                /* AM doesn't think this block is valid, skip */
                continue;
            }

            if (tbmres->ntuples >= 0)
                node->exact_pages++;
            else
                node->lossy_pages++;

            /* Adjust the prefetch target */
            BitmapAdjustPrefetchTarget(node);
        }
        else
        {
            /*
             * Continuing in previously obtained page.
             */

#ifdef USE_PREFETCH

            /*
             * Try to prefetch at least a few pages even before we get to the
             * second page if we don't stop reading after the first tuple.
             */
            if (!pstate)
            {
                if (node->prefetch_target < node->prefetch_maximum)
                    node->prefetch_target++;
            }
            else if (pstate->prefetch_target < node->prefetch_maximum)
            {
                /* take spinlock while updating shared state */
                SpinLockAcquire(&pstate->mutex);
                if (pstate->prefetch_target < node->prefetch_maximum)
                    pstate->prefetch_target++;
                SpinLockRelease(&pstate->mutex);
            }
#endif                          /* USE_PREFETCH */
        }

        /*
         * We issue prefetch requests *after* fetching the current page to try
         * to avoid having prefetching interfere with the main I/O. Also, this
         * should happen only when we have determined there is still something
         * to do on the current page, else we may uselessly prefetch the same
         * page we are just about to request for real.
         *
         * XXX: It's a layering violation that we do these checks above
         * tableam, they should probably moved below it at some point.
         */
        BitmapPrefetch(node, scan);

        if (node->return_empty_tuples > 0)
        {
            /*
             * If we don't have to fetch the tuple, just return nulls.
             */
            ExecStoreAllNullTuple(slot);

            if (--node->return_empty_tuples == 0)
            {
                /* no more tuples to return in the next round */
                node->tbmres = tbmres = NULL;
            }
        }
        else
        {
            /*
             * Attempt to fetch tuple from AM.
             */
            if (!table_scan_bitmap_next_tuple(scan, tbmres, slot))
            {
                /* nothing more to look at on this page */
                node->tbmres = tbmres = NULL;
                continue;
            }

            /*
             * If we are using lossy info, we have to recheck the qual
             * conditions at every tuple.
             */
            if (tbmres->recheck)
            {
                econtext->ecxt_scantuple = slot;
                if (!ExecQualAndReset(node->bitmapqualorig, econtext))
                {
                    /* Fails recheck, so drop it and loop back for another */
                    InstrCountFiltered2(node, 1);
                    ExecClearTuple(slot);
                    continue;
                }
            }
        }

        /* OK to return this tuple */
        return slot;
    }

    /*
     * if we get here it means we are at the end of the scan..
     */
    return ExecClearTuple(slot);
}

/*
 *  BitmapDoneInitializingSharedState - Shared state is initialized
 *
 *  By this time the leader has already populated the TBM and initialized the
 *  shared state so wake up other processes.
 */
static inline void
BitmapDoneInitializingSharedState(ParallelBitmapHeapState *pstate)
{
    SpinLockAcquire(&pstate->mutex);
    pstate->state = BM_FINISHED;
    SpinLockRelease(&pstate->mutex);
    ConditionVariableBroadcast(&pstate->cv);
}

/*
 *  BitmapAdjustPrefetchIterator - Adjust the prefetch iterator
 */
static inline void
BitmapAdjustPrefetchIterator(BitmapHeapScanState *node,
                             TBMIterateResult *tbmres)
{
#ifdef USE_PREFETCH
    ParallelBitmapHeapState *pstate = node->pstate;

    if (pstate == NULL)
    {
        TBMIterator *prefetch_iterator = node->prefetch_iterator;

        if (node->prefetch_pages > 0)
        {
            /* The main iterator has closed the distance by one page */
            node->prefetch_pages--;
        }
        else if (prefetch_iterator)
        {
            /* Do not let the prefetch iterator get behind the main one */
            TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);

            if (tbmpre == NULL || tbmpre->blockno != tbmres->blockno)
                elog(ERROR, "prefetch and main iterators are out of sync");
        }
        return;
    }

    if (node->prefetch_maximum > 0)
    {
        TBMSharedIterator *prefetch_iterator = node->shared_prefetch_iterator;

        SpinLockAcquire(&pstate->mutex);
        if (pstate->prefetch_pages > 0)
        {
            pstate->prefetch_pages--;
            SpinLockRelease(&pstate->mutex);
        }
        else
        {
            /* Release the mutex before iterating */
            SpinLockRelease(&pstate->mutex);

            /*
             * In case of shared mode, we can not ensure that the current
             * blockno of the main iterator and that of the prefetch iterator
             * are same.  It's possible that whatever blockno we are
             * prefetching will be processed by another process.  Therefore,
             * we don't validate the blockno here as we do in non-parallel
             * case.
             */
            if (prefetch_iterator)
                tbm_shared_iterate(prefetch_iterator);
        }
    }
#endif                          /* USE_PREFETCH */
}

/*
 * BitmapAdjustPrefetchTarget - Adjust the prefetch target
 *
 * Increase prefetch target if it's not yet at the max.  Note that
 * we will increase it to zero after fetching the very first
 * page/tuple, then to one after the second tuple is fetched, then
 * it doubles as later pages are fetched.
 */
static inline void
BitmapAdjustPrefetchTarget(BitmapHeapScanState *node)
{
#ifdef USE_PREFETCH
    ParallelBitmapHeapState *pstate = node->pstate;

    if (pstate == NULL)
    {
        if (node->prefetch_target >= node->prefetch_maximum)
             /* don't increase any further */ ;
        else if (node->prefetch_target >= node->prefetch_maximum / 2)
            node->prefetch_target = node->prefetch_maximum;
        else if (node->prefetch_target > 0)
            node->prefetch_target *= 2;
        else
            node->prefetch_target++;
        return;
    }

    /* Do an unlocked check first to save spinlock acquisitions. */
    if (pstate->prefetch_target < node->prefetch_maximum)
    {
        SpinLockAcquire(&pstate->mutex);
        if (pstate->prefetch_target >= node->prefetch_maximum)
             /* don't increase any further */ ;
        else if (pstate->prefetch_target >= node->prefetch_maximum / 2)
            pstate->prefetch_target = node->prefetch_maximum;
        else if (pstate->prefetch_target > 0)
            pstate->prefetch_target *= 2;
        else
            pstate->prefetch_target++;
        SpinLockRelease(&pstate->mutex);
    }
#endif                          /* USE_PREFETCH */
}

/*
 * BitmapPrefetch - Prefetch, if prefetch_pages are behind prefetch_target
 */
static inline void
BitmapPrefetch(BitmapHeapScanState *node, TableScanDesc scan)
{
#ifdef USE_PREFETCH
    ParallelBitmapHeapState *pstate = node->pstate;

    if (pstate == NULL)
    {
        TBMIterator *prefetch_iterator = node->prefetch_iterator;

        if (prefetch_iterator)
        {
            while (node->prefetch_pages < node->prefetch_target)
            {
                TBMIterateResult *tbmpre = tbm_iterate(prefetch_iterator);
                bool        skip_fetch;

                if (tbmpre == NULL)
                {
                    /* No more pages to prefetch */
                    tbm_end_iterate(prefetch_iterator);
                    node->prefetch_iterator = NULL;
                    break;
                }
                node->prefetch_pages++;

                /*
                 * If we expect not to have to actually read this heap page,
                 * skip this prefetch call, but continue to run the prefetch
                 * logic normally.  (Would it be better not to increment
                 * prefetch_pages?)
                 *
                 * This depends on the assumption that the index AM will
                 * report the same recheck flag for this future heap page as
                 * it did for the current heap page; which is not a certainty
                 * but is true in many cases.
                 */
                skip_fetch = (node->can_skip_fetch &&
                              (node->tbmres ? !node->tbmres->recheck : false) &&
                              VM_ALL_VISIBLE(node->ss.ss_currentRelation,
                                             tbmpre->blockno,
                                             &node->pvmbuffer));

                if (!skip_fetch)
                    PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno);
            }
        }

        return;
    }

    if (pstate->prefetch_pages < pstate->prefetch_target)
    {
        TBMSharedIterator *prefetch_iterator = node->shared_prefetch_iterator;

        if (prefetch_iterator)
        {
            while (1)
            {
                TBMIterateResult *tbmpre;
                bool        do_prefetch = false;
                bool        skip_fetch;

                /*
                 * Recheck under the mutex. If some other process has already
                 * done enough prefetching then we need not to do anything.
                 */
                SpinLockAcquire(&pstate->mutex);
                if (pstate->prefetch_pages < pstate->prefetch_target)
                {
                    pstate->prefetch_pages++;
                    do_prefetch = true;
                }
                SpinLockRelease(&pstate->mutex);

                if (!do_prefetch)
                    return;

                tbmpre = tbm_shared_iterate(prefetch_iterator);
                if (tbmpre == NULL)
                {
                    /* No more pages to prefetch */
                    tbm_end_shared_iterate(prefetch_iterator);
                    node->shared_prefetch_iterator = NULL;
                    break;
                }

                /* As above, skip prefetch if we expect not to need page */
                skip_fetch = (node->can_skip_fetch &&
                              (node->tbmres ? !node->tbmres->recheck : false) &&
                              VM_ALL_VISIBLE(node->ss.ss_currentRelation,
                                             tbmpre->blockno,
                                             &node->pvmbuffer));

                if (!skip_fetch)
                    PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, tbmpre->blockno);
            }
        }
    }
#endif                          /* USE_PREFETCH */
}

/*
 * BitmapHeapRecheck -- access method routine to recheck a tuple in EvalPlanQual
 */
static bool
BitmapHeapRecheck(BitmapHeapScanState *node, TupleTableSlot *slot)
{
    ExprContext *econtext;

    /*
     * extract necessary information from index scan node
     */
    econtext = node->ss.ps.ps_ExprContext;

    /* Does the tuple meet the original qual conditions? */
    econtext->ecxt_scantuple = slot;
    return ExecQualAndReset(node->bitmapqualorig, econtext);
}

/* ----------------------------------------------------------------
 *      ExecBitmapHeapScan(node)
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecBitmapHeapScan(PlanState *pstate)
{
    BitmapHeapScanState *node = castNode(BitmapHeapScanState, pstate);

    return ExecScan(&node->ss,
                    (ExecScanAccessMtd) BitmapHeapNext,
                    (ExecScanRecheckMtd) BitmapHeapRecheck);
}

/* ----------------------------------------------------------------
 *      ExecReScanBitmapHeapScan(node)
 * ----------------------------------------------------------------
 */
void
ExecReScanBitmapHeapScan(BitmapHeapScanState *node)
{
    PlanState  *outerPlan = outerPlanState(node);

    /* rescan to release any page pin */
    table_rescan(node->ss.ss_currentScanDesc, NULL);

    /* release bitmaps and buffers if any */
    if (node->tbmiterator)
        tbm_end_iterate(node->tbmiterator);
    if (node->prefetch_iterator)
        tbm_end_iterate(node->prefetch_iterator);
    if (node->shared_tbmiterator)
        tbm_end_shared_iterate(node->shared_tbmiterator);
    if (node->shared_prefetch_iterator)
        tbm_end_shared_iterate(node->shared_prefetch_iterator);
    if (node->tbm)
        tbm_free(node->tbm);
    if (node->vmbuffer != InvalidBuffer)
        ReleaseBuffer(node->vmbuffer);
    if (node->pvmbuffer != InvalidBuffer)
        ReleaseBuffer(node->pvmbuffer);
    node->tbm = NULL;
    node->tbmiterator = NULL;
    node->tbmres = NULL;
    node->prefetch_iterator = NULL;
    node->initialized = false;
    node->shared_tbmiterator = NULL;
    node->shared_prefetch_iterator = NULL;
    node->vmbuffer = InvalidBuffer;
    node->pvmbuffer = InvalidBuffer;

    ExecScanReScan(&node->ss);

    /*
     * if chgParam of subnode is not null then plan will be re-scanned by
     * first ExecProcNode.
     */
    if (outerPlan->chgParam == NULL)
        ExecReScan(outerPlan);
}

/* ----------------------------------------------------------------
 *      ExecEndBitmapHeapScan
 * ----------------------------------------------------------------
 */
void
ExecEndBitmapHeapScan(BitmapHeapScanState *node)
{
    TableScanDesc scanDesc;

    /*
     * extract information from the node
     */
    scanDesc = node->ss.ss_currentScanDesc;

    /*
     * Free the exprcontext
     */
    ExecFreeExprContext(&node->ss.ps);

    /*
     * clear out tuple table slots
     */
    if (node->ss.ps.ps_ResultTupleSlot)
        ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
    ExecClearTuple(node->ss.ss_ScanTupleSlot);

    /*
     * close down subplans
     */
    ExecEndNode(outerPlanState(node));

    /*
     * release bitmaps and buffers if any
     */
    if (node->tbmiterator)
        tbm_end_iterate(node->tbmiterator);
    if (node->prefetch_iterator)
        tbm_end_iterate(node->prefetch_iterator);
    if (node->tbm)
        tbm_free(node->tbm);
    if (node->shared_tbmiterator)
        tbm_end_shared_iterate(node->shared_tbmiterator);
    if (node->shared_prefetch_iterator)
        tbm_end_shared_iterate(node->shared_prefetch_iterator);
    if (node->vmbuffer != InvalidBuffer)
        ReleaseBuffer(node->vmbuffer);
    if (node->pvmbuffer != InvalidBuffer)
        ReleaseBuffer(node->pvmbuffer);

    /*
     * close heap scan
     */
    table_endscan(scanDesc);
}

/* ----------------------------------------------------------------
 *      ExecInitBitmapHeapScan
 *
 *      Initializes the scan's state information.
 * ----------------------------------------------------------------
 */
BitmapHeapScanState *
ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
{
    BitmapHeapScanState *scanstate;
    Relation    currentRelation;
    int         io_concurrency;

    /* check for unsupported flags */
    Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));

    /*
     * Assert caller didn't ask for an unsafe snapshot --- see comments at
     * head of file.
     */
    Assert(IsMVCCSnapshot(estate->es_snapshot));

    /*
     * create state structure
     */
    scanstate = makeNode(BitmapHeapScanState);
    scanstate->ss.ps.plan = (Plan *) node;
    scanstate->ss.ps.state = estate;
    scanstate->ss.ps.ExecProcNode = ExecBitmapHeapScan;

    scanstate->tbm = NULL;
    scanstate->tbmiterator = NULL;
    scanstate->tbmres = NULL;
    scanstate->return_empty_tuples = 0;
    scanstate->vmbuffer = InvalidBuffer;
    scanstate->pvmbuffer = InvalidBuffer;
    scanstate->exact_pages = 0;
    scanstate->lossy_pages = 0;
    scanstate->prefetch_iterator = NULL;
    scanstate->prefetch_pages = 0;
    scanstate->prefetch_target = 0;
    /* may be updated below */
    scanstate->prefetch_maximum = target_prefetch_pages;
    scanstate->pscan_len = 0;
    scanstate->initialized = false;
    scanstate->shared_tbmiterator = NULL;
    scanstate->shared_prefetch_iterator = NULL;
    scanstate->pstate = NULL;

    /*
     * We can potentially skip fetching heap pages if we do not need any
     * columns of the table, either for checking non-indexable quals or for
     * returning data.  This test is a bit simplistic, as it checks the
     * stronger condition that there's no qual or return tlist at all.  But in
     * most cases it's probably not worth working harder than that.
     */
    scanstate->can_skip_fetch = (node->scan.plan.qual == NIL &&
                                 node->scan.plan.targetlist == NIL);

    /*
     * Miscellaneous initialization
     *
     * create expression context for node
     */
    ExecAssignExprContext(estate, &scanstate->ss.ps);

    /*
     * open the scan relation
     */
    currentRelation = ExecOpenScanRelation(estate, node->scan.scanrelid, eflags);

    /*
     * initialize child nodes
     */
    outerPlanState(scanstate) = ExecInitNode(outerPlan(node), estate, eflags);

    /*
     * get the scan type from the relation descriptor.
     */
    ExecInitScanTupleSlot(estate, &scanstate->ss,
                          RelationGetDescr(currentRelation),
                          table_slot_callbacks(currentRelation));

    /*
     * Initialize result type and projection.
     */
    ExecInitResultTypeTL(&scanstate->ss.ps);
    ExecAssignScanProjectionInfo(&scanstate->ss);

    /*
     * initialize child expressions
     */
    scanstate->ss.ps.qual =
        ExecInitQual(node->scan.plan.qual, (PlanState *) scanstate);
    scanstate->bitmapqualorig =
        ExecInitQual(node->bitmapqualorig, (PlanState *) scanstate);

    /*
     * Determine the maximum for prefetch_target.  If the tablespace has a
     * specific IO concurrency set, use that to compute the corresponding
     * maximum value; otherwise, we already initialized to the value computed
     * by the GUC machinery.
     */
    io_concurrency =
        get_tablespace_io_concurrency(currentRelation->rd_rel->reltablespace);
    if (io_concurrency != effective_io_concurrency)
    {
        double      maximum;

        if (ComputeIoConcurrency(io_concurrency, &maximum))
            scanstate->prefetch_maximum = rint(maximum);
    }

    scanstate->ss.ss_currentRelation = currentRelation;

    scanstate->ss.ss_currentScanDesc = table_beginscan_bm(currentRelation,
                                                          estate->es_snapshot,
                                                          0,
                                                          NULL);

    /*
     * all done.
     */
    return scanstate;
}

/*----------------
 *      BitmapShouldInitializeSharedState
 *
 *      The first process to come here and see the state to the BM_INITIAL
 *      will become the leader for the parallel bitmap scan and will be
 *      responsible for populating the TIDBitmap.  The other processes will
 *      be blocked by the condition variable until the leader wakes them up.
 * ---------------
 */
static bool
BitmapShouldInitializeSharedState(ParallelBitmapHeapState *pstate)
{
    SharedBitmapState state;

    while (1)
    {
        SpinLockAcquire(&pstate->mutex);
        state = pstate->state;
        if (pstate->state == BM_INITIAL)
            pstate->state = BM_INPROGRESS;
        SpinLockRelease(&pstate->mutex);

        /* Exit if bitmap is done, or if we're the leader. */
        if (state != BM_INPROGRESS)
            break;

        /* Wait for the leader to wake us up. */
        ConditionVariableSleep(&pstate->cv, WAIT_EVENT_PARALLEL_BITMAP_SCAN);
    }

    ConditionVariableCancelSleep();

    return (state == BM_INITIAL);
}

/* ----------------------------------------------------------------
 *      ExecBitmapHeapEstimate
 *
 *      Compute the amount of space we'll need in the parallel
 *      query DSM, and inform pcxt->estimator about our needs.
 * ----------------------------------------------------------------
 */
void
ExecBitmapHeapEstimate(BitmapHeapScanState *node,
                       ParallelContext *pcxt)
{
    EState     *estate = node->ss.ps.state;

    node->pscan_len = add_size(offsetof(ParallelBitmapHeapState,
                                        phs_snapshot_data),
                               EstimateSnapshotSpace(estate->es_snapshot));

    shm_toc_estimate_chunk(&pcxt->estimator, node->pscan_len);
    shm_toc_estimate_keys(&pcxt->estimator, 1);
}

/* ----------------------------------------------------------------
 *      ExecBitmapHeapInitializeDSM
 *
 *      Set up a parallel bitmap heap scan descriptor.
 * ----------------------------------------------------------------
 */
void
ExecBitmapHeapInitializeDSM(BitmapHeapScanState *node,
                            ParallelContext *pcxt)
{
    ParallelBitmapHeapState *pstate;
    EState     *estate = node->ss.ps.state;
    dsa_area   *dsa = node->ss.ps.state->es_query_dsa;

    /* If there's no DSA, there are no workers; initialize nothing. */
    if (dsa == NULL)
        return;

    pstate = shm_toc_allocate(pcxt->toc, node->pscan_len);

    pstate->tbmiterator = 0;
    pstate->prefetch_iterator = 0;

    /* Initialize the mutex */
    SpinLockInit(&pstate->mutex);
    pstate->prefetch_pages = 0;
    pstate->prefetch_target = 0;
    pstate->state = BM_INITIAL;

    ConditionVariableInit(&pstate->cv);
    SerializeSnapshot(estate->es_snapshot, pstate->phs_snapshot_data);

    shm_toc_insert(pcxt->toc, node->ss.ps.plan->plan_node_id, pstate);
    node->pstate = pstate;
}

/* ----------------------------------------------------------------
 *      ExecBitmapHeapReInitializeDSM
 *
 *      Reset shared state before beginning a fresh scan.
 * ----------------------------------------------------------------
 */
void
ExecBitmapHeapReInitializeDSM(BitmapHeapScanState *node,
                              ParallelContext *pcxt)
{
    ParallelBitmapHeapState *pstate = node->pstate;
    dsa_area   *dsa = node->ss.ps.state->es_query_dsa;

    /* If there's no DSA, there are no workers; do nothing. */
    if (dsa == NULL)
        return;

    pstate->state = BM_INITIAL;

    if (DsaPointerIsValid(pstate->tbmiterator))
        tbm_free_shared_area(dsa, pstate->tbmiterator);

    if (DsaPointerIsValid(pstate->prefetch_iterator))
        tbm_free_shared_area(dsa, pstate->prefetch_iterator);

    pstate->tbmiterator = InvalidDsaPointer;
    pstate->prefetch_iterator = InvalidDsaPointer;
}

/* ----------------------------------------------------------------
 *      ExecBitmapHeapInitializeWorker
 *
 *      Copy relevant information from TOC into planstate.
 * ----------------------------------------------------------------
 */
void
ExecBitmapHeapInitializeWorker(BitmapHeapScanState *node,
                               ParallelWorkerContext *pwcxt)
{
    ParallelBitmapHeapState *pstate;
    Snapshot    snapshot;

    Assert(node->ss.ps.state->es_query_dsa != NULL);

    pstate = shm_toc_lookup(pwcxt->toc, node->ss.ps.plan->plan_node_id, false);
    node->pstate = pstate;

    snapshot = RestoreSnapshot(pstate->phs_snapshot_data);
    table_scan_update_snapshot(node->ss.ss_currentScanDesc, snapshot);
}