nodeIndexonlyscan.c

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/*-------------------------------------------------------------------------
 *
 * nodeIndexonlyscan.c
 *    Routines to support index-only scans
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeIndexonlyscan.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * INTERFACE ROUTINES
 *      ExecIndexOnlyScan           scans an index
 *      IndexOnlyNext               retrieve next tuple
 *      ExecInitIndexOnlyScan       creates and initializes state info.
 *      ExecReScanIndexOnlyScan     rescans the indexed relation.
 *      ExecEndIndexOnlyScan        releases all storage.
 *      ExecIndexOnlyMarkPos        marks scan position.
 *      ExecIndexOnlyRestrPos       restores scan position.
 *      ExecIndexOnlyScanEstimate   estimates DSM space needed for
 *                      parallel index-only scan
 *      ExecIndexOnlyScanInitializeDSM  initialize DSM for parallel
 *                      index-only scan
 *      ExecIndexOnlyScanReInitializeDSM    reinitialize DSM for fresh scan
 *      ExecIndexOnlyScanInitializeWorker attach to DSM info in parallel worker
 */
#include "postgres.h"
 
#include "access/genam.h"
#include "access/relscan.h"
#include "access/tableam.h"
#include "access/tupdesc.h"
#include "access/visibilitymap.h"
#include "executor/executor.h"
#include "executor/nodeIndexonlyscan.h"
#include "executor/nodeIndexscan.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/predicate.h"
#include "utils/rel.h"
 
 
static TupleTableSlot *IndexOnlyNext(IndexOnlyScanState *node);
static void StoreIndexTuple(TupleTableSlot *slot, IndexTuple itup,
                            TupleDesc itupdesc);
 
 
/* ----------------------------------------------------------------
 *      IndexOnlyNext
 *
 *      Retrieve a tuple from the IndexOnlyScan node's index.
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
IndexOnlyNext(IndexOnlyScanState *node)
{
    EState     *estate;
    ExprContext *econtext;
    ScanDirection direction;
    IndexScanDesc scandesc;
    TupleTableSlot *slot;
    ItemPointer tid;
 
    /*
     * extract necessary information from index scan node
     */
    estate = node->ss.ps.state;
 
    /*
     * Determine which direction to scan the index in based on the plan's scan
     * direction and the current direction of execution.
     */
    direction = ScanDirectionCombine(estate->es_direction,
                                     ((IndexOnlyScan *) node->ss.ps.plan)->indexorderdir);
    scandesc = node->ioss_ScanDesc;
    econtext = node->ss.ps.ps_ExprContext;
    slot = node->ss.ss_ScanTupleSlot;
 
    if (scandesc == NULL)
    {
        /*
         * We reach here if the index only scan is not parallel, or if we're
         * serially executing an index only scan that was planned to be
         * parallel.
         */
        scandesc = index_beginscan(node->ss.ss_currentRelation,
                                   node->ioss_RelationDesc,
                                   estate->es_snapshot,
                                   node->ioss_NumScanKeys,
                                   node->ioss_NumOrderByKeys);
 
        node->ioss_ScanDesc = scandesc;
 
 
        /* Set it up for index-only scan */
        node->ioss_ScanDesc->xs_want_itup = true;
        node->ioss_VMBuffer = InvalidBuffer;
 
        /*
         * If no run-time keys to calculate or they are ready, go ahead and
         * pass the scankeys to the index AM.
         */
        if (node->ioss_NumRuntimeKeys == 0 || node->ioss_RuntimeKeysReady)
            index_rescan(scandesc,
                         node->ioss_ScanKeys,
                         node->ioss_NumScanKeys,
                         node->ioss_OrderByKeys,
                         node->ioss_NumOrderByKeys);
    }
 
    /*
     * OK, now that we have what we need, fetch the next tuple.
     */
    while ((tid = index_getnext_tid(scandesc, direction)) != NULL)
    {
        bool        tuple_from_heap = false;
 
        CHECK_FOR_INTERRUPTS();
 
        /*
         * We can skip the heap fetch if the TID references a heap page on
         * which all tuples are known visible to everybody.  In any case,
         * we'll use the index tuple not the heap tuple as the data source.
         *
         * Note on Memory Ordering Effects: visibilitymap_get_status does not
         * lock the visibility map buffer, and therefore the result we read
         * here could be slightly stale.  However, it can't be stale enough to
         * matter.
         *
         * We need to detect clearing a VM bit due to an insert right away,
         * because the tuple is present in the index page but not visible. The
         * reading of the TID by this scan (using a shared lock on the index
         * buffer) is serialized with the insert of the TID into the index
         * (using an exclusive lock on the index buffer). Because the VM bit
         * is cleared before updating the index, and locking/unlocking of the
         * index page acts as a full memory barrier, we are sure to see the
         * cleared bit if we see a recently-inserted TID.
         *
         * Deletes do not update the index page (only VACUUM will clear out
         * the TID), so the clearing of the VM bit by a delete is not
         * serialized with this test below, and we may see a value that is
         * significantly stale. However, we don't care about the delete right
         * away, because the tuple is still visible until the deleting
         * transaction commits or the statement ends (if it's our
         * transaction). In either case, the lock on the VM buffer will have
         * been released (acting as a write barrier) after clearing the bit.
         * And for us to have a snapshot that includes the deleting
         * transaction (making the tuple invisible), we must have acquired
         * ProcArrayLock after that time, acting as a read barrier.
         *
         * It's worth going through this complexity to avoid needing to lock
         * the VM buffer, which could cause significant contention.
         */
        if (!VM_ALL_VISIBLE(scandesc->heapRelation,
                            ItemPointerGetBlockNumber(tid),
                            &node->ioss_VMBuffer))
        {
            /*
             * Rats, we have to visit the heap to check visibility.
             */
            InstrCountTuples2(node, 1);
            if (!index_fetch_heap(scandesc, node->ioss_TableSlot))
                continue;       /* no visible tuple, try next index entry */
 
            ExecClearTuple(node->ioss_TableSlot);
 
            /*
             * Only MVCC snapshots are supported here, so there should be no
             * need to keep following the HOT chain once a visible entry has
             * been found.  If we did want to allow that, we'd need to keep
             * more state to remember not to call index_getnext_tid next time.
             */
            if (scandesc->xs_heap_continue)
                elog(ERROR, "non-MVCC snapshots are not supported in index-only scans");
 
            /*
             * Note: at this point we are holding a pin on the heap page, as
             * recorded in scandesc->xs_cbuf.  We could release that pin now,
             * but it's not clear whether it's a win to do so.  The next index
             * entry might require a visit to the same heap page.
             */
 
            tuple_from_heap = true;
        }
 
        /*
         * Fill the scan tuple slot with data from the index.  This might be
         * provided in either HeapTuple or IndexTuple format.  Conceivably an
         * index AM might fill both fields, in which case we prefer the heap
         * format, since it's probably a bit cheaper to fill a slot from.
         */
        if (scandesc->xs_hitup)
        {
            /*
             * We don't take the trouble to verify that the provided tuple has
             * exactly the slot's format, but it seems worth doing a quick
             * check on the number of fields.
             */
            Assert(slot->tts_tupleDescriptor->natts ==
                   scandesc->xs_hitupdesc->natts);
            ExecForceStoreHeapTuple(scandesc->xs_hitup, slot, false);
        }
        else if (scandesc->xs_itup)
            StoreIndexTuple(slot, scandesc->xs_itup, scandesc->xs_itupdesc);
        else
            elog(ERROR, "no data returned for index-only scan");
 
        /*
         * If the index was lossy, we have to recheck the index quals.
         */
        if (scandesc->xs_recheck)
        {
            econtext->ecxt_scantuple = slot;
            if (!ExecQualAndReset(node->recheckqual, econtext))
            {
                /* Fails recheck, so drop it and loop back for another */
                InstrCountFiltered2(node, 1);
                continue;
            }
        }
 
        /*
         * We don't currently support rechecking ORDER BY distances.  (In
         * principle, if the index can support retrieval of the originally
         * indexed value, it should be able to produce an exact distance
         * calculation too.  So it's not clear that adding code here for
         * recheck/re-sort would be worth the trouble.  But we should at least
         * throw an error if someone tries it.)
         */
        if (scandesc->numberOfOrderBys > 0 && scandesc->xs_recheckorderby)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("lossy distance functions are not supported in index-only scans")));
 
        /*
         * If we didn't access the heap, then we'll need to take a predicate
         * lock explicitly, as if we had.  For now we do that at page level.
         */
        if (!tuple_from_heap)
            PredicateLockPage(scandesc->heapRelation,
                              ItemPointerGetBlockNumber(tid),
                              estate->es_snapshot);
 
        return slot;
    }
 
    /*
     * if we get here it means the index scan failed so we are at the end of
     * the scan..
     */
    return ExecClearTuple(slot);
}
 
/*
 * StoreIndexTuple
 *      Fill the slot with data from the index tuple.
 *
 * At some point this might be generally-useful functionality, but
 * right now we don't need it elsewhere.
 */
static void
StoreIndexTuple(TupleTableSlot *slot, IndexTuple itup, TupleDesc itupdesc)
{
    /*
     * Note: we must use the tupdesc supplied by the AM in index_deform_tuple,
     * not the slot's tupdesc, in case the latter has different datatypes
     * (this happens for btree name_ops in particular).  They'd better have
     * the same number of columns though, as well as being datatype-compatible
     * which is something we can't so easily check.
     */
    Assert(slot->tts_tupleDescriptor->natts == itupdesc->natts);
 
    ExecClearTuple(slot);
    index_deform_tuple(itup, itupdesc, slot->tts_values, slot->tts_isnull);
    ExecStoreVirtualTuple(slot);
}
 
/*
 * IndexOnlyRecheck -- access method routine to recheck a tuple in EvalPlanQual
 *
 * This can't really happen, since an index can't supply CTID which would
 * be necessary data for any potential EvalPlanQual target relation.  If it
 * did happen, the EPQ code would pass us the wrong data, namely a heap
 * tuple not an index tuple.  So throw an error.
 */
static bool
IndexOnlyRecheck(IndexOnlyScanState *node, TupleTableSlot *slot)
{
    elog(ERROR, "EvalPlanQual recheck is not supported in index-only scans");
    return false;               /* keep compiler quiet */
}
 
/* ----------------------------------------------------------------
 *      ExecIndexOnlyScan(node)
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecIndexOnlyScan(PlanState *pstate)
{
    IndexOnlyScanState *node = castNode(IndexOnlyScanState, pstate);
 
    /*
     * If we have runtime keys and they've not already been set up, do it now.
     */
    if (node->ioss_NumRuntimeKeys != 0 && !node->ioss_RuntimeKeysReady)
        ExecReScan((PlanState *) node);
 
    return ExecScan(&node->ss,
                    (ExecScanAccessMtd) IndexOnlyNext,
                    (ExecScanRecheckMtd) IndexOnlyRecheck);
}
 
/* ----------------------------------------------------------------
 *      ExecReScanIndexOnlyScan(node)
 *
 *      Recalculates the values of any scan keys whose value depends on
 *      information known at runtime, then rescans the indexed relation.
 *
 *      Updating the scan key was formerly done separately in
 *      ExecUpdateIndexScanKeys. Integrating it into ReScan makes
 *      rescans of indices and relations/general streams more uniform.
 * ----------------------------------------------------------------
 */
void
ExecReScanIndexOnlyScan(IndexOnlyScanState *node)
{
    /*
     * If we are doing runtime key calculations (ie, any of the index key
     * values weren't simple Consts), compute the new key values.  But first,
     * reset the context so we don't leak memory as each outer tuple is
     * scanned.  Note this assumes that we will recalculate *all* runtime keys
     * on each call.
     */
    if (node->ioss_NumRuntimeKeys != 0)
    {
        ExprContext *econtext = node->ioss_RuntimeContext;
 
        ResetExprContext(econtext);
        ExecIndexEvalRuntimeKeys(econtext,
                                 node->ioss_RuntimeKeys,
                                 node->ioss_NumRuntimeKeys);
    }
    node->ioss_RuntimeKeysReady = true;
 
    /* reset index scan */
    if (node->ioss_ScanDesc)
        index_rescan(node->ioss_ScanDesc,
                     node->ioss_ScanKeys, node->ioss_NumScanKeys,
                     node->ioss_OrderByKeys, node->ioss_NumOrderByKeys);
 
    ExecScanReScan(&node->ss);
}
 
 
/* ----------------------------------------------------------------
 *      ExecEndIndexOnlyScan
 * ----------------------------------------------------------------
 */
void
ExecEndIndexOnlyScan(IndexOnlyScanState *node)
{
    Relation    indexRelationDesc;
    IndexScanDesc indexScanDesc;
 
    /*
     * extract information from the node
     */
    indexRelationDesc = node->ioss_RelationDesc;
    indexScanDesc = node->ioss_ScanDesc;
 
    /* Release VM buffer pin, if any. */
    if (node->ioss_VMBuffer != InvalidBuffer)
    {
        ReleaseBuffer(node->ioss_VMBuffer);
        node->ioss_VMBuffer = InvalidBuffer;
    }
 
    /*
     * close the index relation (no-op if we didn't open it)
     */
    if (indexScanDesc)
        index_endscan(indexScanDesc);
    if (indexRelationDesc)
        index_close(indexRelationDesc, NoLock);
}
 
/* ----------------------------------------------------------------
 *      ExecIndexOnlyMarkPos
 *
 * Note: we assume that no caller attempts to set a mark before having read
 * at least one tuple.  Otherwise, ioss_ScanDesc might still be NULL.
 * ----------------------------------------------------------------
 */
void
ExecIndexOnlyMarkPos(IndexOnlyScanState *node)
{
    EState     *estate = node->ss.ps.state;
    EPQState   *epqstate = estate->es_epq_active;
 
    if (epqstate != NULL)
    {
        /*
         * We are inside an EvalPlanQual recheck.  If a test tuple exists for
         * this relation, then we shouldn't access the index at all.  We would
         * instead need to save, and later restore, the state of the
         * relsubs_done flag, so that re-fetching the test tuple is possible.
         * However, given the assumption that no caller sets a mark at the
         * start of the scan, we can only get here with relsubs_done[i]
         * already set, and so no state need be saved.
         */
        Index       scanrelid = ((Scan *) node->ss.ps.plan)->scanrelid;
 
        Assert(scanrelid > 0);
        if (epqstate->relsubs_slot[scanrelid - 1] != NULL ||
            epqstate->relsubs_rowmark[scanrelid - 1] != NULL)
        {
            /* Verify the claim above */
            if (!epqstate->relsubs_done[scanrelid - 1])
                elog(ERROR, "unexpected ExecIndexOnlyMarkPos call in EPQ recheck");
            return;
        }
    }
 
    index_markpos(node->ioss_ScanDesc);
}
 
/* ----------------------------------------------------------------
 *      ExecIndexOnlyRestrPos
 * ----------------------------------------------------------------
 */
void
ExecIndexOnlyRestrPos(IndexOnlyScanState *node)
{
    EState     *estate = node->ss.ps.state;
    EPQState   *epqstate = estate->es_epq_active;
 
    if (estate->es_epq_active != NULL)
    {
        /* See comments in ExecIndexMarkPos */
        Index       scanrelid = ((Scan *) node->ss.ps.plan)->scanrelid;
 
        Assert(scanrelid > 0);
        if (epqstate->relsubs_slot[scanrelid - 1] != NULL ||
            epqstate->relsubs_rowmark[scanrelid - 1] != NULL)
        {
            /* Verify the claim above */
            if (!epqstate->relsubs_done[scanrelid - 1])
                elog(ERROR, "unexpected ExecIndexOnlyRestrPos call in EPQ recheck");
            return;
        }
    }
 
    index_restrpos(node->ioss_ScanDesc);
}
 
/* ----------------------------------------------------------------
 *      ExecInitIndexOnlyScan
 *
 *      Initializes the index scan's state information, creates
 *      scan keys, and opens the base and index relations.
 *
 *      Note: index scans have 2 sets of state information because
 *            we have to keep track of the base relation and the
 *            index relation.
 * ----------------------------------------------------------------
 */
IndexOnlyScanState *
ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
{
    IndexOnlyScanState *indexstate;
    Relation    currentRelation;
    LOCKMODE    lockmode;
    TupleDesc   tupDesc;
 
    /*
     * create state structure
     */
    indexstate = makeNode(IndexOnlyScanState);
    indexstate->ss.ps.plan = (Plan *) node;
    indexstate->ss.ps.state = estate;
    indexstate->ss.ps.ExecProcNode = ExecIndexOnlyScan;
 
    /*
     * Miscellaneous initialization
     *
     * create expression context for node
     */
    ExecAssignExprContext(estate, &indexstate->ss.ps);
 
    /*
     * open the scan relation
     */
    currentRelation = ExecOpenScanRelation(estate, node->scan.scanrelid, eflags);
 
    indexstate->ss.ss_currentRelation = currentRelation;
    indexstate->ss.ss_currentScanDesc = NULL;   /* no heap scan here */
 
    /*
     * Build the scan tuple type using the indextlist generated by the
     * planner.  We use this, rather than the index's physical tuple
     * descriptor, because the latter contains storage column types not the
     * types of the original datums.  (It's the AM's responsibility to return
     * suitable data anyway.)
     */
    tupDesc = ExecTypeFromTL(node->indextlist);
    ExecInitScanTupleSlot(estate, &indexstate->ss, tupDesc,
                          &TTSOpsVirtual);
 
    /*
     * We need another slot, in a format that's suitable for the table AM, for
     * when we need to fetch a tuple from the table for rechecking visibility.
     */
    indexstate->ioss_TableSlot =
        ExecAllocTableSlot(&estate->es_tupleTable,
                           RelationGetDescr(currentRelation),
                           table_slot_callbacks(currentRelation));
 
    /*
     * Initialize result type and projection info.  The node's targetlist will
     * contain Vars with varno = INDEX_VAR, referencing the scan tuple.
     */
    ExecInitResultTypeTL(&indexstate->ss.ps);
    ExecAssignScanProjectionInfoWithVarno(&indexstate->ss, INDEX_VAR);
 
    /*
     * initialize child expressions
     *
     * Note: we don't initialize all of the indexorderby expression, only the
     * sub-parts corresponding to runtime keys (see below).
     */
    indexstate->ss.ps.qual =
        ExecInitQual(node->scan.plan.qual, (PlanState *) indexstate);
    indexstate->recheckqual =
        ExecInitQual(node->recheckqual, (PlanState *) indexstate);
 
    /*
     * If we are just doing EXPLAIN (ie, aren't going to run the plan), stop
     * here.  This allows an index-advisor plugin to EXPLAIN a plan containing
     * references to nonexistent indexes.
     */
    if (eflags & EXEC_FLAG_EXPLAIN_ONLY)
        return indexstate;
 
    /* Open the index relation. */
    lockmode = exec_rt_fetch(node->scan.scanrelid, estate)->rellockmode;
    indexstate->ioss_RelationDesc = index_open(node->indexid, lockmode);
 
    /*
     * Initialize index-specific scan state
     */
    indexstate->ioss_RuntimeKeysReady = false;
    indexstate->ioss_RuntimeKeys = NULL;
    indexstate->ioss_NumRuntimeKeys = 0;
 
    /*
     * build the index scan keys from the index qualification
     */
    ExecIndexBuildScanKeys((PlanState *) indexstate,
                           indexstate->ioss_RelationDesc,
                           node->indexqual,
                           false,
                           &indexstate->ioss_ScanKeys,
                           &indexstate->ioss_NumScanKeys,
                           &indexstate->ioss_RuntimeKeys,
                           &indexstate->ioss_NumRuntimeKeys,
                           NULL,    /* no ArrayKeys */
                           NULL);
 
    /*
     * any ORDER BY exprs have to be turned into scankeys in the same way
     */
    ExecIndexBuildScanKeys((PlanState *) indexstate,
                           indexstate->ioss_RelationDesc,
                           node->indexorderby,
                           true,
                           &indexstate->ioss_OrderByKeys,
                           &indexstate->ioss_NumOrderByKeys,
                           &indexstate->ioss_RuntimeKeys,
                           &indexstate->ioss_NumRuntimeKeys,
                           NULL,    /* no ArrayKeys */
                           NULL);
 
    /*
     * If we have runtime keys, we need an ExprContext to evaluate them. The
     * node's standard context won't do because we want to reset that context
     * for every tuple.  So, build another context just like the other one...
     * -tgl 7/11/00
     */
    if (indexstate->ioss_NumRuntimeKeys != 0)
    {
        ExprContext *stdecontext = indexstate->ss.ps.ps_ExprContext;
 
        ExecAssignExprContext(estate, &indexstate->ss.ps);
        indexstate->ioss_RuntimeContext = indexstate->ss.ps.ps_ExprContext;
        indexstate->ss.ps.ps_ExprContext = stdecontext;
    }
    else
    {
        indexstate->ioss_RuntimeContext = NULL;
    }
 
    /*
     * all done.
     */
    return indexstate;
}
 
/* ----------------------------------------------------------------
 *      Parallel Index-only Scan Support
 * ----------------------------------------------------------------
 */
 
/* ----------------------------------------------------------------
 *      ExecIndexOnlyScanEstimate
 *
 *      Compute the amount of space we'll need in the parallel
 *      query DSM, and inform pcxt->estimator about our needs.
 * ----------------------------------------------------------------
 */
void
ExecIndexOnlyScanEstimate(IndexOnlyScanState *node,
                          ParallelContext *pcxt)
{
    EState     *estate = node->ss.ps.state;
 
    node->ioss_PscanLen = index_parallelscan_estimate(node->ioss_RelationDesc,
                                                      node->ioss_NumScanKeys,
                                                      node->ioss_NumOrderByKeys,
                                                      estate->es_snapshot);
    shm_toc_estimate_chunk(&pcxt->estimator, node->ioss_PscanLen);
    shm_toc_estimate_keys(&pcxt->estimator, 1);
}
 
/* ----------------------------------------------------------------
 *      ExecIndexOnlyScanInitializeDSM
 *
 *      Set up a parallel index-only scan descriptor.
 * ----------------------------------------------------------------
 */
void
ExecIndexOnlyScanInitializeDSM(IndexOnlyScanState *node,
                               ParallelContext *pcxt)
{
    EState     *estate = node->ss.ps.state;
    ParallelIndexScanDesc piscan;
 
    piscan = shm_toc_allocate(pcxt->toc, node->ioss_PscanLen);
    index_parallelscan_initialize(node->ss.ss_currentRelation,
                                  node->ioss_RelationDesc,
                                  estate->es_snapshot,
                                  piscan);
    shm_toc_insert(pcxt->toc, node->ss.ps.plan->plan_node_id, piscan);
    node->ioss_ScanDesc =
        index_beginscan_parallel(node->ss.ss_currentRelation,
                                 node->ioss_RelationDesc,
                                 node->ioss_NumScanKeys,
                                 node->ioss_NumOrderByKeys,
                                 piscan);
    node->ioss_ScanDesc->xs_want_itup = true;
    node->ioss_VMBuffer = InvalidBuffer;
 
    /*
     * If no run-time keys to calculate or they are ready, go ahead and pass
     * the scankeys to the index AM.
     */
    if (node->ioss_NumRuntimeKeys == 0 || node->ioss_RuntimeKeysReady)
        index_rescan(node->ioss_ScanDesc,
                     node->ioss_ScanKeys, node->ioss_NumScanKeys,
                     node->ioss_OrderByKeys, node->ioss_NumOrderByKeys);
}
 
/* ----------------------------------------------------------------
 *      ExecIndexOnlyScanReInitializeDSM
 *
 *      Reset shared state before beginning a fresh scan.
 * ----------------------------------------------------------------
 */
void
ExecIndexOnlyScanReInitializeDSM(IndexOnlyScanState *node,
                                 ParallelContext *pcxt)
{
    index_parallelrescan(node->ioss_ScanDesc);
}
 
/* ----------------------------------------------------------------
 *      ExecIndexOnlyScanInitializeWorker
 *
 *      Copy relevant information from TOC into planstate.
 * ----------------------------------------------------------------
 */
void
ExecIndexOnlyScanInitializeWorker(IndexOnlyScanState *node,
                                  ParallelWorkerContext *pwcxt)
{
    ParallelIndexScanDesc piscan;
 
    piscan = shm_toc_lookup(pwcxt->toc, node->ss.ps.plan->plan_node_id, false);
    node->ioss_ScanDesc =
        index_beginscan_parallel(node->ss.ss_currentRelation,
                                 node->ioss_RelationDesc,
                                 node->ioss_NumScanKeys,
                                 node->ioss_NumOrderByKeys,
                                 piscan);
    node->ioss_ScanDesc->xs_want_itup = true;
 
    /*
     * If no run-time keys to calculate or they are ready, go ahead and pass
     * the scankeys to the index AM.
     */
    if (node->ioss_NumRuntimeKeys == 0 || node->ioss_RuntimeKeysReady)
        index_rescan(node->ioss_ScanDesc,
                     node->ioss_ScanKeys, node->ioss_NumScanKeys,
                     node->ioss_OrderByKeys, node->ioss_NumOrderByKeys);
}