execProcnode.c

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/*-------------------------------------------------------------------------
 *
 * execProcnode.c
 *   contains dispatch functions which call the appropriate "initialize",
 *   "get a tuple", and "cleanup" routines for the given node type.
 *   If the node has children, then it will presumably call ExecInitNode,
 *   ExecProcNode, or ExecEndNode on its subnodes and do the appropriate
 *   processing.
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/execProcnode.c
 *
 *-------------------------------------------------------------------------
 */
/*
 *   NOTES
 *      This used to be three files.  It is now all combined into
 *      one file so that it is easier to keep the dispatch routines
 *      in sync when new nodes are added.
 *
 *   EXAMPLE
 *      Suppose we want the age of the manager of the shoe department and
 *      the number of employees in that department.  So we have the query:
 *
 *              select DEPT.no_emps, EMP.age
 *              from DEPT, EMP
 *              where EMP.name = DEPT.mgr and
 *                    DEPT.name = "shoe"
 *
 *      Suppose the planner gives us the following plan:
 *
 *                      Nest Loop (DEPT.mgr = EMP.name)
 *                      /       \
 *                     /         \
 *                 Seq Scan     Seq Scan
 *                  DEPT          EMP
 *              (name = "shoe")
 *
 *      ExecutorStart() is called first.
 *      It calls InitPlan() which calls ExecInitNode() on
 *      the root of the plan -- the nest loop node.
 *
 *    * ExecInitNode() notices that it is looking at a nest loop and
 *      as the code below demonstrates, it calls ExecInitNestLoop().
 *      Eventually this calls ExecInitNode() on the right and left subplans
 *      and so forth until the entire plan is initialized.  The result
 *      of ExecInitNode() is a plan state tree built with the same structure
 *      as the underlying plan tree.
 *
 *    * Then when ExecutorRun() is called, it calls ExecutePlan() which calls
 *      ExecProcNode() repeatedly on the top node of the plan state tree.
 *      Each time this happens, ExecProcNode() will end up calling
 *      ExecNestLoop(), which calls ExecProcNode() on its subplans.
 *      Each of these subplans is a sequential scan so ExecSeqScan() is
 *      called.  The slots returned by ExecSeqScan() may contain
 *      tuples which contain the attributes ExecNestLoop() uses to
 *      form the tuples it returns.
 *
 *    * Eventually ExecSeqScan() stops returning tuples and the nest
 *      loop join ends.  Lastly, ExecutorEnd() calls ExecEndNode() which
 *      calls ExecEndNestLoop() which in turn calls ExecEndNode() on
 *      its subplans which result in ExecEndSeqScan().
 *
 *      This should show how the executor works by having
 *      ExecInitNode(), ExecProcNode() and ExecEndNode() dispatch
 *      their work to the appropriate node support routines which may
 *      in turn call these routines themselves on their subplans.
 */
#include "postgres.h"

#include "executor/executor.h"
#include "executor/nodeAgg.h"
#include "executor/nodeAppend.h"
#include "executor/nodeBitmapAnd.h"
#include "executor/nodeBitmapHeapscan.h"
#include "executor/nodeBitmapIndexscan.h"
#include "executor/nodeBitmapOr.h"
#include "executor/nodeCtescan.h"
#include "executor/nodeCustom.h"
#include "executor/nodeForeignscan.h"
#include "executor/nodeFunctionscan.h"
#include "executor/nodeGather.h"
#include "executor/nodeGatherMerge.h"
#include "executor/nodeGroup.h"
#include "executor/nodeHash.h"
#include "executor/nodeHashjoin.h"
#include "executor/nodeIndexonlyscan.h"
#include "executor/nodeIndexscan.h"
#include "executor/nodeLimit.h"
#include "executor/nodeLockRows.h"
#include "executor/nodeMaterial.h"
#include "executor/nodeMergeAppend.h"
#include "executor/nodeMergejoin.h"
#include "executor/nodeModifyTable.h"
#include "executor/nodeNamedtuplestorescan.h"
#include "executor/nodeNestloop.h"
#include "executor/nodeProjectSet.h"
#include "executor/nodeRecursiveunion.h"
#include "executor/nodeResult.h"
#include "executor/nodeSamplescan.h"
#include "executor/nodeSeqscan.h"
#include "executor/nodeSetOp.h"
#include "executor/nodeSort.h"
#include "executor/nodeSubplan.h"
#include "executor/nodeSubqueryscan.h"
#include "executor/nodeTableFuncscan.h"
#include "executor/nodeTidscan.h"
#include "executor/nodeUnique.h"
#include "executor/nodeValuesscan.h"
#include "executor/nodeWindowAgg.h"
#include "executor/nodeWorktablescan.h"
#include "nodes/nodeFuncs.h"
#include "miscadmin.h"


static TupleTableSlot *ExecProcNodeFirst(PlanState *node);
static TupleTableSlot *ExecProcNodeInstr(PlanState *node);


/* ------------------------------------------------------------------------
 *      ExecInitNode
 *
 *      Recursively initializes all the nodes in the plan tree rooted
 *      at 'node'.
 *
 *      Inputs:
 *        'node' is the current node of the plan produced by the query planner
 *        'estate' is the shared execution state for the plan tree
 *        'eflags' is a bitwise OR of flag bits described in executor.h
 *
 *      Returns a PlanState node corresponding to the given Plan node.
 * ------------------------------------------------------------------------
 */
PlanState *
ExecInitNode(Plan *node, EState *estate, int eflags)
{
    PlanState  *result;
    List       *subps;
    ListCell   *l;

    /*
     * do nothing when we get to the end of a leaf on tree.
     */
    if (node == NULL)
        return NULL;

    /*
     * Make sure there's enough stack available. Need to check here, in
     * addition to ExecProcNode() (via ExecProcNodeFirst()), to ensure the
     * stack isn't overrun while initializing the node tree.
     */
    check_stack_depth();

    switch (nodeTag(node))
    {
            /*
             * control nodes
             */
        case T_Result:
            result = (PlanState *) ExecInitResult((Result *) node,
                                                  estate, eflags);
            break;

        case T_ProjectSet:
            result = (PlanState *) ExecInitProjectSet((ProjectSet *) node,
                                                      estate, eflags);
            break;

        case T_ModifyTable:
            result = (PlanState *) ExecInitModifyTable((ModifyTable *) node,
                                                       estate, eflags);
            break;

        case T_Append:
            result = (PlanState *) ExecInitAppend((Append *) node,
                                                  estate, eflags);
            break;

        case T_MergeAppend:
            result = (PlanState *) ExecInitMergeAppend((MergeAppend *) node,
                                                       estate, eflags);
            break;

        case T_RecursiveUnion:
            result = (PlanState *) ExecInitRecursiveUnion((RecursiveUnion *) node,
                                                          estate, eflags);
            break;

        case T_BitmapAnd:
            result = (PlanState *) ExecInitBitmapAnd((BitmapAnd *) node,
                                                     estate, eflags);
            break;

        case T_BitmapOr:
            result = (PlanState *) ExecInitBitmapOr((BitmapOr *) node,
                                                    estate, eflags);
            break;

            /*
             * scan nodes
             */
        case T_SeqScan:
            result = (PlanState *) ExecInitSeqScan((SeqScan *) node,
                                                   estate, eflags);
            break;

        case T_SampleScan:
            result = (PlanState *) ExecInitSampleScan((SampleScan *) node,
                                                      estate, eflags);
            break;

        case T_IndexScan:
            result = (PlanState *) ExecInitIndexScan((IndexScan *) node,
                                                     estate, eflags);
            break;

        case T_IndexOnlyScan:
            result = (PlanState *) ExecInitIndexOnlyScan((IndexOnlyScan *) node,
                                                         estate, eflags);
            break;

        case T_BitmapIndexScan:
            result = (PlanState *) ExecInitBitmapIndexScan((BitmapIndexScan *) node,
                                                           estate, eflags);
            break;

        case T_BitmapHeapScan:
            result = (PlanState *) ExecInitBitmapHeapScan((BitmapHeapScan *) node,
                                                          estate, eflags);
            break;

        case T_TidScan:
            result = (PlanState *) ExecInitTidScan((TidScan *) node,
                                                   estate, eflags);
            break;

        case T_SubqueryScan:
            result = (PlanState *) ExecInitSubqueryScan((SubqueryScan *) node,
                                                        estate, eflags);
            break;

        case T_FunctionScan:
            result = (PlanState *) ExecInitFunctionScan((FunctionScan *) node,
                                                        estate, eflags);
            break;

        case T_TableFuncScan:
            result = (PlanState *) ExecInitTableFuncScan((TableFuncScan *) node,
                                                         estate, eflags);
            break;

        case T_ValuesScan:
            result = (PlanState *) ExecInitValuesScan((ValuesScan *) node,
                                                      estate, eflags);
            break;

        case T_CteScan:
            result = (PlanState *) ExecInitCteScan((CteScan *) node,
                                                   estate, eflags);
            break;

        case T_NamedTuplestoreScan:
            result = (PlanState *) ExecInitNamedTuplestoreScan((NamedTuplestoreScan *) node,
                                                               estate, eflags);
            break;

        case T_WorkTableScan:
            result = (PlanState *) ExecInitWorkTableScan((WorkTableScan *) node,
                                                         estate, eflags);
            break;

        case T_ForeignScan:
            result = (PlanState *) ExecInitForeignScan((ForeignScan *) node,
                                                       estate, eflags);
            break;

        case T_CustomScan:
            result = (PlanState *) ExecInitCustomScan((CustomScan *) node,
                                                      estate, eflags);
            break;

            /*
             * join nodes
             */
        case T_NestLoop:
            result = (PlanState *) ExecInitNestLoop((NestLoop *) node,
                                                    estate, eflags);
            break;

        case T_MergeJoin:
            result = (PlanState *) ExecInitMergeJoin((MergeJoin *) node,
                                                     estate, eflags);
            break;

        case T_HashJoin:
            result = (PlanState *) ExecInitHashJoin((HashJoin *) node,
                                                    estate, eflags);
            break;

            /*
             * materialization nodes
             */
        case T_Material:
            result = (PlanState *) ExecInitMaterial((Material *) node,
                                                    estate, eflags);
            break;

        case T_Sort:
            result = (PlanState *) ExecInitSort((Sort *) node,
                                                estate, eflags);
            break;

        case T_Group:
            result = (PlanState *) ExecInitGroup((Group *) node,
                                                 estate, eflags);
            break;

        case T_Agg:
            result = (PlanState *) ExecInitAgg((Agg *) node,
                                               estate, eflags);
            break;

        case T_WindowAgg:
            result = (PlanState *) ExecInitWindowAgg((WindowAgg *) node,
                                                     estate, eflags);
            break;

        case T_Unique:
            result = (PlanState *) ExecInitUnique((Unique *) node,
                                                  estate, eflags);
            break;

        case T_Gather:
            result = (PlanState *) ExecInitGather((Gather *) node,
                                                  estate, eflags);
            break;

        case T_GatherMerge:
            result = (PlanState *) ExecInitGatherMerge((GatherMerge *) node,
                                                       estate, eflags);
            break;

        case T_Hash:
            result = (PlanState *) ExecInitHash((Hash *) node,
                                                estate, eflags);
            break;

        case T_SetOp:
            result = (PlanState *) ExecInitSetOp((SetOp *) node,
                                                 estate, eflags);
            break;

        case T_LockRows:
            result = (PlanState *) ExecInitLockRows((LockRows *) node,
                                                    estate, eflags);
            break;

        case T_Limit:
            result = (PlanState *) ExecInitLimit((Limit *) node,
                                                 estate, eflags);
            break;

        default:
            elog(ERROR, "unrecognized node type: %d", (int) nodeTag(node));
            result = NULL;      /* keep compiler quiet */
            break;
    }

    /*
     * Add a wrapper around the ExecProcNode callback that checks stack depth
     * during the first execution.
     */
    result->ExecProcNodeReal = result->ExecProcNode;
    result->ExecProcNode = ExecProcNodeFirst;

    /*
     * Initialize any initPlans present in this node.  The planner put them in
     * a separate list for us.
     */
    subps = NIL;
    foreach(l, node->initPlan)
    {
        SubPlan    *subplan = (SubPlan *) lfirst(l);
        SubPlanState *sstate;

        Assert(IsA(subplan, SubPlan));
        sstate = ExecInitSubPlan(subplan, result);
        subps = lappend(subps, sstate);
    }
    result->initPlan = subps;

    /* Set up instrumentation for this node if requested */
    if (estate->es_instrument)
        result->instrument = InstrAlloc(1, estate->es_instrument);

    return result;
}


/*
 * ExecProcNode wrapper that performs some one-time checks, before calling
 * the relevant node method (possibly via an instrumentation wrapper).
 */
static TupleTableSlot *
ExecProcNodeFirst(PlanState *node)
{
    /*
     * Perform stack depth check during the first execution of the node.  We
     * only do so the first time round because it turns out to not be cheap on
     * some common architectures (eg. x86).  This relies on the assumption
     * that ExecProcNode calls for a given plan node will always be made at
     * roughly the same stack depth.
     */
    check_stack_depth();

    /*
     * If instrumentation is required, change the wrapper to one that just
     * does instrumentation.  Otherwise we can dispense with all wrappers and
     * have ExecProcNode() directly call the relevant function from now on.
     */
    if (node->instrument)
        node->ExecProcNode = ExecProcNodeInstr;
    else
        node->ExecProcNode = node->ExecProcNodeReal;

    return node->ExecProcNode(node);
}


/*
 * ExecProcNode wrapper that performs instrumentation calls.  By keeping
 * this a separate function, we avoid overhead in the normal case where
 * no instrumentation is wanted.
 */
static TupleTableSlot *
ExecProcNodeInstr(PlanState *node)
{
    TupleTableSlot *result;

    InstrStartNode(node->instrument);

    result = node->ExecProcNodeReal(node);

    InstrStopNode(node->instrument, TupIsNull(result) ? 0.0 : 1.0);

    return result;
}


/* ----------------------------------------------------------------
 *      MultiExecProcNode
 *
 *      Execute a node that doesn't return individual tuples
 *      (it might return a hashtable, bitmap, etc).  Caller should
 *      check it got back the expected kind of Node.
 *
 * This has essentially the same responsibilities as ExecProcNode,
 * but it does not do InstrStartNode/InstrStopNode (mainly because
 * it can't tell how many returned tuples to count).  Each per-node
 * function must provide its own instrumentation support.
 * ----------------------------------------------------------------
 */
Node *
MultiExecProcNode(PlanState *node)
{
    Node       *result;

    check_stack_depth();

    CHECK_FOR_INTERRUPTS();

    if (node->chgParam != NULL) /* something changed */
        ExecReScan(node);       /* let ReScan handle this */

    switch (nodeTag(node))
    {
            /*
             * Only node types that actually support multiexec will be listed
             */

        case T_HashState:
            result = MultiExecHash((HashState *) node);
            break;

        case T_BitmapIndexScanState:
            result = MultiExecBitmapIndexScan((BitmapIndexScanState *) node);
            break;

        case T_BitmapAndState:
            result = MultiExecBitmapAnd((BitmapAndState *) node);
            break;

        case T_BitmapOrState:
            result = MultiExecBitmapOr((BitmapOrState *) node);
            break;

        default:
            elog(ERROR, "unrecognized node type: %d", (int) nodeTag(node));
            result = NULL;
            break;
    }

    return result;
}


/* ----------------------------------------------------------------
 *      ExecEndNode
 *
 *      Recursively cleans up all the nodes in the plan rooted
 *      at 'node'.
 *
 *      After this operation, the query plan will not be able to be
 *      processed any further.  This should be called only after
 *      the query plan has been fully executed.
 * ----------------------------------------------------------------
 */
void
ExecEndNode(PlanState *node)
{
    /*
     * do nothing when we get to the end of a leaf on tree.
     */
    if (node == NULL)
        return;

    /*
     * Make sure there's enough stack available. Need to check here, in
     * addition to ExecProcNode() (via ExecProcNodeFirst()), because it's not
     * guaranteed that ExecProcNode() is reached for all nodes.
     */
    check_stack_depth();

    if (node->chgParam != NULL)
    {
        bms_free(node->chgParam);
        node->chgParam = NULL;
    }

    switch (nodeTag(node))
    {
            /*
             * control nodes
             */
        case T_ResultState:
            ExecEndResult((ResultState *) node);
            break;

        case T_ProjectSetState:
            ExecEndProjectSet((ProjectSetState *) node);
            break;

        case T_ModifyTableState:
            ExecEndModifyTable((ModifyTableState *) node);
            break;

        case T_AppendState:
            ExecEndAppend((AppendState *) node);
            break;

        case T_MergeAppendState:
            ExecEndMergeAppend((MergeAppendState *) node);
            break;

        case T_RecursiveUnionState:
            ExecEndRecursiveUnion((RecursiveUnionState *) node);
            break;

        case T_BitmapAndState:
            ExecEndBitmapAnd((BitmapAndState *) node);
            break;

        case T_BitmapOrState:
            ExecEndBitmapOr((BitmapOrState *) node);
            break;

            /*
             * scan nodes
             */
        case T_SeqScanState:
            ExecEndSeqScan((SeqScanState *) node);
            break;

        case T_SampleScanState:
            ExecEndSampleScan((SampleScanState *) node);
            break;

        case T_GatherState:
            ExecEndGather((GatherState *) node);
            break;

        case T_GatherMergeState:
            ExecEndGatherMerge((GatherMergeState *) node);
            break;

        case T_IndexScanState:
            ExecEndIndexScan((IndexScanState *) node);
            break;

        case T_IndexOnlyScanState:
            ExecEndIndexOnlyScan((IndexOnlyScanState *) node);
            break;

        case T_BitmapIndexScanState:
            ExecEndBitmapIndexScan((BitmapIndexScanState *) node);
            break;

        case T_BitmapHeapScanState:
            ExecEndBitmapHeapScan((BitmapHeapScanState *) node);
            break;

        case T_TidScanState:
            ExecEndTidScan((TidScanState *) node);
            break;

        case T_SubqueryScanState:
            ExecEndSubqueryScan((SubqueryScanState *) node);
            break;

        case T_FunctionScanState:
            ExecEndFunctionScan((FunctionScanState *) node);
            break;

        case T_TableFuncScanState:
            ExecEndTableFuncScan((TableFuncScanState *) node);
            break;

        case T_ValuesScanState:
            ExecEndValuesScan((ValuesScanState *) node);
            break;

        case T_CteScanState:
            ExecEndCteScan((CteScanState *) node);
            break;

        case T_NamedTuplestoreScanState:
            ExecEndNamedTuplestoreScan((NamedTuplestoreScanState *) node);
            break;

        case T_WorkTableScanState:
            ExecEndWorkTableScan((WorkTableScanState *) node);
            break;

        case T_ForeignScanState:
            ExecEndForeignScan((ForeignScanState *) node);
            break;

        case T_CustomScanState:
            ExecEndCustomScan((CustomScanState *) node);
            break;

            /*
             * join nodes
             */
        case T_NestLoopState:
            ExecEndNestLoop((NestLoopState *) node);
            break;

        case T_MergeJoinState:
            ExecEndMergeJoin((MergeJoinState *) node);
            break;

        case T_HashJoinState:
            ExecEndHashJoin((HashJoinState *) node);
            break;

            /*
             * materialization nodes
             */
        case T_MaterialState:
            ExecEndMaterial((MaterialState *) node);
            break;

        case T_SortState:
            ExecEndSort((SortState *) node);
            break;

        case T_GroupState:
            ExecEndGroup((GroupState *) node);
            break;

        case T_AggState:
            ExecEndAgg((AggState *) node);
            break;

        case T_WindowAggState:
            ExecEndWindowAgg((WindowAggState *) node);
            break;

        case T_UniqueState:
            ExecEndUnique((UniqueState *) node);
            break;

        case T_HashState:
            ExecEndHash((HashState *) node);
            break;

        case T_SetOpState:
            ExecEndSetOp((SetOpState *) node);
            break;

        case T_LockRowsState:
            ExecEndLockRows((LockRowsState *) node);
            break;

        case T_LimitState:
            ExecEndLimit((LimitState *) node);
            break;

        default:
            elog(ERROR, "unrecognized node type: %d", (int) nodeTag(node));
            break;
    }
}

/*
 * ExecShutdownNode
 *
 * Give execution nodes a chance to stop asynchronous resource consumption
 * and release any resources still held.  Currently, this is only used for
 * parallel query, but we might want to extend it to other cases also (e.g.
 * FDW).  We might also want to call it sooner, as soon as it's evident that
 * no more rows will be needed (e.g. when a Limit is filled) rather than only
 * at the end of ExecutorRun.
 */
bool
ExecShutdownNode(PlanState *node)
{
    if (node == NULL)
        return false;

    check_stack_depth();

    planstate_tree_walker(node, ExecShutdownNode, NULL);

    switch (nodeTag(node))
    {
        case T_GatherState:
            ExecShutdownGather((GatherState *) node);
            break;
        case T_ForeignScanState:
            ExecShutdownForeignScan((ForeignScanState *) node);
            break;
        case T_CustomScanState:
            ExecShutdownCustomScan((CustomScanState *) node);
            break;
        case T_GatherMergeState:
            ExecShutdownGatherMerge((GatherMergeState *) node);
            break;
        default:
            break;
    }

    return false;
}

/*
 * ExecSetTupleBound
 *
 * Set a tuple bound for a planstate node.  This lets child plan nodes
 * optimize based on the knowledge that the maximum number of tuples that
 * their parent will demand is limited.  The tuple bound for a node may
 * only be changed between scans (i.e., after node initialization or just
 * before an ExecReScan call).
 *
 * Any negative tuples_needed value means "no limit", which should be the
 * default assumption when this is not called at all for a particular node.
 *
 * Note: if this is called repeatedly on a plan tree, the exact same set
 * of nodes must be updated with the new limit each time; be careful that
 * only unchanging conditions are tested here.
 */
void
ExecSetTupleBound(int64 tuples_needed, PlanState *child_node)
{
    /*
     * Since this function recurses, in principle we should check stack depth
     * here.  In practice, it's probably pointless since the earlier node
     * initialization tree traversal would surely have consumed more stack.
     */

    if (IsA(child_node, SortState))
    {
        /*
         * If it is a Sort node, notify it that it can use bounded sort.
         *
         * Note: it is the responsibility of nodeSort.c to react properly to
         * changes of these parameters.  If we ever redesign this, it'd be a
         * good idea to integrate this signaling with the parameter-change
         * mechanism.
         */
        SortState  *sortState = (SortState *) child_node;

        if (tuples_needed < 0)
        {
            /* make sure flag gets reset if needed upon rescan */
            sortState->bounded = false;
        }
        else
        {
            sortState->bounded = true;
            sortState->bound = tuples_needed;
        }
    }
    else if (IsA(child_node, MergeAppendState))
    {
        /*
         * If it is a MergeAppend, we can apply the bound to any nodes that
         * are children of the MergeAppend, since the MergeAppend surely need
         * read no more than that many tuples from any one input.
         */
        MergeAppendState *maState = (MergeAppendState *) child_node;
        int         i;

        for (i = 0; i < maState->ms_nplans; i++)
            ExecSetTupleBound(tuples_needed, maState->mergeplans[i]);
    }
    else if (IsA(child_node, ResultState))
    {
        /*
         * Similarly, for a projecting Result, we can apply the bound to its
         * child node.
         *
         * If Result supported qual checking, we'd have to punt on seeing a
         * qual.  Note that having a resconstantqual is not a showstopper: if
         * that condition succeeds it affects nothing, while if it fails, no
         * rows will be demanded from the Result child anyway.
         */
        if (outerPlanState(child_node))
            ExecSetTupleBound(tuples_needed, outerPlanState(child_node));
    }
    else if (IsA(child_node, SubqueryScanState))
    {
        /*
         * We can also descend through SubqueryScan, but only if it has no
         * qual (otherwise it might discard rows).
         */
        SubqueryScanState *subqueryState = (SubqueryScanState *) child_node;

        if (subqueryState->ss.ps.qual == NULL)
            ExecSetTupleBound(tuples_needed, subqueryState->subplan);
    }
    else if (IsA(child_node, GatherState))
    {
        /*
         * A Gather node can propagate the bound to its workers.  As with
         * MergeAppend, no one worker could possibly need to return more
         * tuples than the Gather itself needs to.
         *
         * Note: As with Sort, the Gather node is responsible for reacting
         * properly to changes to this parameter.
         */
        GatherState *gstate = (GatherState *) child_node;

        gstate->tuples_needed = tuples_needed;

        /* Also pass down the bound to our own copy of the child plan */
        ExecSetTupleBound(tuples_needed, outerPlanState(child_node));
    }
    else if (IsA(child_node, GatherMergeState))
    {
        /* Same comments as for Gather */
        GatherMergeState *gstate = (GatherMergeState *) child_node;

        gstate->tuples_needed = tuples_needed;

        ExecSetTupleBound(tuples_needed, outerPlanState(child_node));
    }

    /*
     * In principle we could descend through any plan node type that is
     * certain not to discard or combine input rows; but on seeing a node that
     * can do that, we can't propagate the bound any further.  For the moment
     * it's unclear that any other cases are worth checking here.
     */
}