nodeAppend.c

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/*-------------------------------------------------------------------------
 *
 * nodeAppend.c
 *    routines to handle append nodes.
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeAppend.c
 *
 *-------------------------------------------------------------------------
 */
/* INTERFACE ROUTINES
 *      ExecInitAppend  - initialize the append node
 *      ExecAppend      - retrieve the next tuple from the node
 *      ExecEndAppend   - shut down the append node
 *      ExecReScanAppend - rescan the append node
 *
 *   NOTES
 *      Each append node contains a list of one or more subplans which
 *      must be iteratively processed (forwards or backwards).
 *      Tuples are retrieved by executing the 'whichplan'th subplan
 *      until the subplan stops returning tuples, at which point that
 *      plan is shut down and the next started up.
 *
 *      Append nodes don't make use of their left and right
 *      subtrees, rather they maintain a list of subplans so
 *      a typical append node looks like this in the plan tree:
 *
 *                 ...
 *                 /
 *              Append -------+------+------+--- nil
 *              /   \         |      |      |
 *            nil   nil      ...    ...    ...
 *                               subplans
 *
 *      Append nodes are currently used for unions, and to support
 *      inheritance queries, where several relations need to be scanned.
 *      For example, in our standard person/student/employee/student-emp
 *      example, where student and employee inherit from person
 *      and student-emp inherits from student and employee, the
 *      query:
 *
 *              select name from person
 *
 *      generates the plan:
 *
 *                |
 *              Append -------+-------+--------+--------+
 *              /   \         |       |        |        |
 *            nil   nil      Scan    Scan     Scan     Scan
 *                            |       |        |        |
 *                          person employee student student-emp
 */

#include "postgres.h"

#include "executor/execAsync.h"
#include "executor/execdebug.h"
#include "executor/execPartition.h"
#include "executor/nodeAppend.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/latch.h"

/* Shared state for parallel-aware Append. */
struct ParallelAppendState
{
    LWLock      pa_lock;        /* mutual exclusion to choose next subplan */
    int         pa_next_plan;   /* next plan to choose by any worker */

    /*
     * pa_finished[i] should be true if no more workers should select subplan
     * i.  for a non-partial plan, this should be set to true as soon as a
     * worker selects the plan; for a partial plan, it remains false until
     * some worker executes the plan to completion.
     */
    bool        pa_finished[FLEXIBLE_ARRAY_MEMBER];
};

#define INVALID_SUBPLAN_INDEX       -1
#define EVENT_BUFFER_SIZE           16

static TupleTableSlot *ExecAppend(PlanState *pstate);
static bool choose_next_subplan_locally(AppendState *node);
static bool choose_next_subplan_for_leader(AppendState *node);
static bool choose_next_subplan_for_worker(AppendState *node);
static void mark_invalid_subplans_as_finished(AppendState *node);
static void ExecAppendAsyncBegin(AppendState *node);
static bool ExecAppendAsyncGetNext(AppendState *node, TupleTableSlot **result);
static bool ExecAppendAsyncRequest(AppendState *node, TupleTableSlot **result);
static void ExecAppendAsyncEventWait(AppendState *node);
static void classify_matching_subplans(AppendState *node);

/* ----------------------------------------------------------------
 *      ExecInitAppend
 *
 *      Begin all of the subscans of the append node.
 *
 *     (This is potentially wasteful, since the entire result of the
 *      append node may not be scanned, but this way all of the
 *      structures get allocated in the executor's top level memory
 *      block instead of that of the call to ExecAppend.)
 * ----------------------------------------------------------------
 */
AppendState *
ExecInitAppend(Append *node, EState *estate, int eflags)
{
    AppendState *appendstate = makeNode(AppendState);
    PlanState **appendplanstates;
    Bitmapset  *validsubplans;
    Bitmapset  *asyncplans;
    int         nplans;
    int         nasyncplans;
    int         firstvalid;
    int         i,
                j;

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

    /*
     * create new AppendState for our append node
     */
    appendstate->ps.plan = (Plan *) node;
    appendstate->ps.state = estate;
    appendstate->ps.ExecProcNode = ExecAppend;

    /* Let choose_next_subplan_* function handle setting the first subplan */
    appendstate->as_whichplan = INVALID_SUBPLAN_INDEX;
    appendstate->as_syncdone = false;
    appendstate->as_begun = false;

    /* If run-time partition pruning is enabled, then set that up now */
    if (node->part_prune_info != NULL)
    {
        PartitionPruneState *prunestate;

        /* We may need an expression context to evaluate partition exprs */
        ExecAssignExprContext(estate, &appendstate->ps);

        /* Create the working data structure for pruning. */
        prunestate = ExecCreatePartitionPruneState(&appendstate->ps,
                                                   node->part_prune_info);
        appendstate->as_prune_state = prunestate;

        /* Perform an initial partition prune, if required. */
        if (prunestate->do_initial_prune)
        {
            /* Determine which subplans survive initial pruning */
            validsubplans = ExecFindInitialMatchingSubPlans(prunestate,
                                                            list_length(node->appendplans));

            nplans = bms_num_members(validsubplans);
        }
        else
        {
            /* We'll need to initialize all subplans */
            nplans = list_length(node->appendplans);
            Assert(nplans > 0);
            validsubplans = bms_add_range(NULL, 0, nplans - 1);
        }

        /*
         * When no run-time pruning is required and there's at least one
         * subplan, we can fill as_valid_subplans immediately, preventing
         * later calls to ExecFindMatchingSubPlans.
         */
        if (!prunestate->do_exec_prune && nplans > 0)
            appendstate->as_valid_subplans = bms_add_range(NULL, 0, nplans - 1);
    }
    else
    {
        nplans = list_length(node->appendplans);

        /*
         * When run-time partition pruning is not enabled we can just mark all
         * subplans as valid; they must also all be initialized.
         */
        Assert(nplans > 0);
        appendstate->as_valid_subplans = validsubplans =
            bms_add_range(NULL, 0, nplans - 1);
        appendstate->as_prune_state = NULL;
    }

    /*
     * Initialize result tuple type and slot.
     */
    ExecInitResultTupleSlotTL(&appendstate->ps, &TTSOpsVirtual);

    /* node returns slots from each of its subnodes, therefore not fixed */
    appendstate->ps.resultopsset = true;
    appendstate->ps.resultopsfixed = false;

    appendplanstates = (PlanState **) palloc(nplans *
                                             sizeof(PlanState *));

    /*
     * call ExecInitNode on each of the valid plans to be executed and save
     * the results into the appendplanstates array.
     *
     * While at it, find out the first valid partial plan.
     */
    j = 0;
    asyncplans = NULL;
    nasyncplans = 0;
    firstvalid = nplans;
    i = -1;
    while ((i = bms_next_member(validsubplans, i)) >= 0)
    {
        Plan       *initNode = (Plan *) list_nth(node->appendplans, i);

        /*
         * Record async subplans.  When executing EvalPlanQual, we treat them
         * as sync ones; don't do this when initializing an EvalPlanQual plan
         * tree.
         */
        if (initNode->async_capable && estate->es_epq_active == NULL)
        {
            asyncplans = bms_add_member(asyncplans, j);
            nasyncplans++;
        }

        /*
         * Record the lowest appendplans index which is a valid partial plan.
         */
        if (i >= node->first_partial_plan && j < firstvalid)
            firstvalid = j;

        appendplanstates[j++] = ExecInitNode(initNode, estate, eflags);
    }

    appendstate->as_first_partial_plan = firstvalid;
    appendstate->appendplans = appendplanstates;
    appendstate->as_nplans = nplans;

    /* Initialize async state */
    appendstate->as_asyncplans = asyncplans;
    appendstate->as_nasyncplans = nasyncplans;
    appendstate->as_asyncrequests = NULL;
    appendstate->as_asyncresults = (TupleTableSlot **)
        palloc0(nasyncplans * sizeof(TupleTableSlot *));
    appendstate->as_needrequest = NULL;
    appendstate->as_eventset = NULL;

    if (nasyncplans > 0)
    {
        appendstate->as_asyncrequests = (AsyncRequest **)
            palloc0(nplans * sizeof(AsyncRequest *));

        i = -1;
        while ((i = bms_next_member(asyncplans, i)) >= 0)
        {
            AsyncRequest *areq;

            areq = palloc(sizeof(AsyncRequest));
            areq->requestor = (PlanState *) appendstate;
            areq->requestee = appendplanstates[i];
            areq->request_index = i;
            areq->callback_pending = false;
            areq->request_complete = false;
            areq->result = NULL;

            appendstate->as_asyncrequests[i] = areq;
        }
    }

    /*
     * Miscellaneous initialization
     */

    appendstate->ps.ps_ProjInfo = NULL;

    /* For parallel query, this will be overridden later. */
    appendstate->choose_next_subplan = choose_next_subplan_locally;

    return appendstate;
}

/* ----------------------------------------------------------------
 *     ExecAppend
 *
 *      Handles iteration over multiple subplans.
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecAppend(PlanState *pstate)
{
    AppendState *node = castNode(AppendState, pstate);
    TupleTableSlot *result;

    /*
     * If this is the first call after Init or ReScan, we need to do the
     * initialization work.
     */
    if (!node->as_begun)
    {
        Assert(node->as_whichplan == INVALID_SUBPLAN_INDEX);
        Assert(!node->as_syncdone);

        /* Nothing to do if there are no subplans */
        if (node->as_nplans == 0)
            return ExecClearTuple(node->ps.ps_ResultTupleSlot);

        /* If there are any async subplans, begin executing them. */
        if (node->as_nasyncplans > 0)
            ExecAppendAsyncBegin(node);

        /*
         * If no sync subplan has been chosen, we must choose one before
         * proceeding.
         */
        if (!node->choose_next_subplan(node) && node->as_nasyncremain == 0)
            return ExecClearTuple(node->ps.ps_ResultTupleSlot);

        Assert(node->as_syncdone ||
               (node->as_whichplan >= 0 &&
                node->as_whichplan < node->as_nplans));

        /* And we're initialized. */
        node->as_begun = true;
    }

    for (;;)
    {
        PlanState  *subnode;

        CHECK_FOR_INTERRUPTS();

        /*
         * try to get a tuple from an async subplan if any
         */
        if (node->as_syncdone || !bms_is_empty(node->as_needrequest))
        {
            if (ExecAppendAsyncGetNext(node, &result))
                return result;
            Assert(!node->as_syncdone);
            Assert(bms_is_empty(node->as_needrequest));
        }

        /*
         * figure out which sync subplan we are currently processing
         */
        Assert(node->as_whichplan >= 0 && node->as_whichplan < node->as_nplans);
        subnode = node->appendplans[node->as_whichplan];

        /*
         * get a tuple from the subplan
         */
        result = ExecProcNode(subnode);

        if (!TupIsNull(result))
        {
            /*
             * If the subplan gave us something then return it as-is. We do
             * NOT make use of the result slot that was set up in
             * ExecInitAppend; there's no need for it.
             */
            return result;
        }

        /*
         * wait or poll async events if any. We do this before checking for
         * the end of iteration, because it might drain the remaining async
         * subplans.
         */
        if (node->as_nasyncremain > 0)
            ExecAppendAsyncEventWait(node);

        /* choose new sync subplan; if no sync/async subplans, we're done */
        if (!node->choose_next_subplan(node) && node->as_nasyncremain == 0)
            return ExecClearTuple(node->ps.ps_ResultTupleSlot);
    }
}

/* ----------------------------------------------------------------
 *      ExecEndAppend
 *
 *      Shuts down the subscans of the append node.
 *
 *      Returns nothing of interest.
 * ----------------------------------------------------------------
 */
void
ExecEndAppend(AppendState *node)
{
    PlanState **appendplans;
    int         nplans;
    int         i;

    /*
     * get information from the node
     */
    appendplans = node->appendplans;
    nplans = node->as_nplans;

    /*
     * shut down each of the subscans
     */
    for (i = 0; i < nplans; i++)
        ExecEndNode(appendplans[i]);
}

void
ExecReScanAppend(AppendState *node)
{
    int         nasyncplans = node->as_nasyncplans;
    int         i;

    /*
     * If any PARAM_EXEC Params used in pruning expressions have changed, then
     * we'd better unset the valid subplans so that they are reselected for
     * the new parameter values.
     */
    if (node->as_prune_state &&
        bms_overlap(node->ps.chgParam,
                    node->as_prune_state->execparamids))
    {
        bms_free(node->as_valid_subplans);
        node->as_valid_subplans = NULL;
        if (nasyncplans > 0)
        {
            bms_free(node->as_valid_asyncplans);
            node->as_valid_asyncplans = NULL;
        }
    }

    for (i = 0; i < node->as_nplans; i++)
    {
        PlanState  *subnode = node->appendplans[i];

        /*
         * ExecReScan doesn't know about my subplans, so I have to do
         * changed-parameter signaling myself.
         */
        if (node->ps.chgParam != NULL)
            UpdateChangedParamSet(subnode, node->ps.chgParam);

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

    /* Reset async state */
    if (nasyncplans > 0)
    {
        i = -1;
        while ((i = bms_next_member(node->as_asyncplans, i)) >= 0)
        {
            AsyncRequest *areq = node->as_asyncrequests[i];

            areq->callback_pending = false;
            areq->request_complete = false;
            areq->result = NULL;
        }

        bms_free(node->as_needrequest);
        node->as_needrequest = NULL;
    }

    /* Let choose_next_subplan_* function handle setting the first subplan */
    node->as_whichplan = INVALID_SUBPLAN_INDEX;
    node->as_syncdone = false;
    node->as_begun = false;
}

/* ----------------------------------------------------------------
 *                      Parallel Append Support
 * ----------------------------------------------------------------
 */

/* ----------------------------------------------------------------
 *      ExecAppendEstimate
 *
 *      Compute the amount of space we'll need in the parallel
 *      query DSM, and inform pcxt->estimator about our needs.
 * ----------------------------------------------------------------
 */
void
ExecAppendEstimate(AppendState *node,
                   ParallelContext *pcxt)
{
    node->pstate_len =
        add_size(offsetof(ParallelAppendState, pa_finished),
                 sizeof(bool) * node->as_nplans);

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


/* ----------------------------------------------------------------
 *      ExecAppendInitializeDSM
 *
 *      Set up shared state for Parallel Append.
 * ----------------------------------------------------------------
 */
void
ExecAppendInitializeDSM(AppendState *node,
                        ParallelContext *pcxt)
{
    ParallelAppendState *pstate;

    pstate = shm_toc_allocate(pcxt->toc, node->pstate_len);
    memset(pstate, 0, node->pstate_len);
    LWLockInitialize(&pstate->pa_lock, LWTRANCHE_PARALLEL_APPEND);
    shm_toc_insert(pcxt->toc, node->ps.plan->plan_node_id, pstate);

    node->as_pstate = pstate;
    node->choose_next_subplan = choose_next_subplan_for_leader;
}

/* ----------------------------------------------------------------
 *      ExecAppendReInitializeDSM
 *
 *      Reset shared state before beginning a fresh scan.
 * ----------------------------------------------------------------
 */
void
ExecAppendReInitializeDSM(AppendState *node, ParallelContext *pcxt)
{
    ParallelAppendState *pstate = node->as_pstate;

    pstate->pa_next_plan = 0;
    memset(pstate->pa_finished, 0, sizeof(bool) * node->as_nplans);
}

/* ----------------------------------------------------------------
 *      ExecAppendInitializeWorker
 *
 *      Copy relevant information from TOC into planstate, and initialize
 *      whatever is required to choose and execute the optimal subplan.
 * ----------------------------------------------------------------
 */
void
ExecAppendInitializeWorker(AppendState *node, ParallelWorkerContext *pwcxt)
{
    node->as_pstate = shm_toc_lookup(pwcxt->toc, node->ps.plan->plan_node_id, false);
    node->choose_next_subplan = choose_next_subplan_for_worker;
}

/* ----------------------------------------------------------------
 *      choose_next_subplan_locally
 *
 *      Choose next sync subplan for a non-parallel-aware Append,
 *      returning false if there are no more.
 * ----------------------------------------------------------------
 */
static bool
choose_next_subplan_locally(AppendState *node)
{
    int         whichplan = node->as_whichplan;
    int         nextplan;

    /* We should never be called when there are no subplans */
    Assert(node->as_nplans > 0);

    /* Nothing to do if syncdone */
    if (node->as_syncdone)
        return false;

    /*
     * If first call then have the bms member function choose the first valid
     * sync subplan by initializing whichplan to -1.  If there happen to be
     * no valid sync subplans then the bms member function will handle that
     * by returning a negative number which will allow us to exit returning a
     * false value.
     */
    if (whichplan == INVALID_SUBPLAN_INDEX)
    {
        if (node->as_nasyncplans > 0)
        {
            /* We'd have filled as_valid_subplans already */
            Assert(node->as_valid_subplans);
        }
        else if (node->as_valid_subplans == NULL)
            node->as_valid_subplans =
                ExecFindMatchingSubPlans(node->as_prune_state);

        whichplan = -1;
    }

    /* Ensure whichplan is within the expected range */
    Assert(whichplan >= -1 && whichplan <= node->as_nplans);

    if (ScanDirectionIsForward(node->ps.state->es_direction))
        nextplan = bms_next_member(node->as_valid_subplans, whichplan);
    else
        nextplan = bms_prev_member(node->as_valid_subplans, whichplan);

    if (nextplan < 0)
    {
        /* Set as_syncdone if in async mode */
        if (node->as_nasyncplans > 0)
            node->as_syncdone = true;
        return false;
    }

    node->as_whichplan = nextplan;

    return true;
}

/* ----------------------------------------------------------------
 *      choose_next_subplan_for_leader
 *
 *      Try to pick a plan which doesn't commit us to doing much
 *      work locally, so that as much work as possible is done in
 *      the workers.  Cheapest subplans are at the end.
 * ----------------------------------------------------------------
 */
static bool
choose_next_subplan_for_leader(AppendState *node)
{
    ParallelAppendState *pstate = node->as_pstate;

    /* Backward scan is not supported by parallel-aware plans */
    Assert(ScanDirectionIsForward(node->ps.state->es_direction));

    /* We should never be called when there are no subplans */
    Assert(node->as_nplans > 0);

    LWLockAcquire(&pstate->pa_lock, LW_EXCLUSIVE);

    if (node->as_whichplan != INVALID_SUBPLAN_INDEX)
    {
        /* Mark just-completed subplan as finished. */
        node->as_pstate->pa_finished[node->as_whichplan] = true;
    }
    else
    {
        /* Start with last subplan. */
        node->as_whichplan = node->as_nplans - 1;

        /*
         * If we've yet to determine the valid subplans then do so now.  If
         * run-time pruning is disabled then the valid subplans will always be
         * set to all subplans.
         */
        if (node->as_valid_subplans == NULL)
        {
            node->as_valid_subplans =
                ExecFindMatchingSubPlans(node->as_prune_state);

            /*
             * Mark each invalid plan as finished to allow the loop below to
             * select the first valid subplan.
             */
            mark_invalid_subplans_as_finished(node);
        }
    }

    /* Loop until we find a subplan to execute. */
    while (pstate->pa_finished[node->as_whichplan])
    {
        if (node->as_whichplan == 0)
        {
            pstate->pa_next_plan = INVALID_SUBPLAN_INDEX;
            node->as_whichplan = INVALID_SUBPLAN_INDEX;
            LWLockRelease(&pstate->pa_lock);
            return false;
        }

        /*
         * We needn't pay attention to as_valid_subplans here as all invalid
         * plans have been marked as finished.
         */
        node->as_whichplan--;
    }

    /* If non-partial, immediately mark as finished. */
    if (node->as_whichplan < node->as_first_partial_plan)
        node->as_pstate->pa_finished[node->as_whichplan] = true;

    LWLockRelease(&pstate->pa_lock);

    return true;
}

/* ----------------------------------------------------------------
 *      choose_next_subplan_for_worker
 *
 *      Choose next subplan for a parallel-aware Append, returning
 *      false if there are no more.
 *
 *      We start from the first plan and advance through the list;
 *      when we get back to the end, we loop back to the first
 *      partial plan.  This assigns the non-partial plans first in
 *      order of descending cost and then spreads out the workers
 *      as evenly as possible across the remaining partial plans.
 * ----------------------------------------------------------------
 */
static bool
choose_next_subplan_for_worker(AppendState *node)
{
    ParallelAppendState *pstate = node->as_pstate;

    /* Backward scan is not supported by parallel-aware plans */
    Assert(ScanDirectionIsForward(node->ps.state->es_direction));

    /* We should never be called when there are no subplans */
    Assert(node->as_nplans > 0);

    LWLockAcquire(&pstate->pa_lock, LW_EXCLUSIVE);

    /* Mark just-completed subplan as finished. */
    if (node->as_whichplan != INVALID_SUBPLAN_INDEX)
        node->as_pstate->pa_finished[node->as_whichplan] = true;

    /*
     * If we've yet to determine the valid subplans then do so now.  If
     * run-time pruning is disabled then the valid subplans will always be set
     * to all subplans.
     */
    else if (node->as_valid_subplans == NULL)
    {
        node->as_valid_subplans =
            ExecFindMatchingSubPlans(node->as_prune_state);
        mark_invalid_subplans_as_finished(node);
    }

    /* If all the plans are already done, we have nothing to do */
    if (pstate->pa_next_plan == INVALID_SUBPLAN_INDEX)
    {
        LWLockRelease(&pstate->pa_lock);
        return false;
    }

    /* Save the plan from which we are starting the search. */
    node->as_whichplan = pstate->pa_next_plan;

    /* Loop until we find a valid subplan to execute. */
    while (pstate->pa_finished[pstate->pa_next_plan])
    {
        int         nextplan;

        nextplan = bms_next_member(node->as_valid_subplans,
                                   pstate->pa_next_plan);
        if (nextplan >= 0)
        {
            /* Advance to the next valid plan. */
            pstate->pa_next_plan = nextplan;
        }
        else if (node->as_whichplan > node->as_first_partial_plan)
        {
            /*
             * Try looping back to the first valid partial plan, if there is
             * one.  If there isn't, arrange to bail out below.
             */
            nextplan = bms_next_member(node->as_valid_subplans,
                                       node->as_first_partial_plan - 1);
            pstate->pa_next_plan =
                nextplan < 0 ? node->as_whichplan : nextplan;
        }
        else
        {
            /*
             * At last plan, and either there are no partial plans or we've
             * tried them all.  Arrange to bail out.
             */
            pstate->pa_next_plan = node->as_whichplan;
        }

        if (pstate->pa_next_plan == node->as_whichplan)
        {
            /* We've tried everything! */
            pstate->pa_next_plan = INVALID_SUBPLAN_INDEX;
            LWLockRelease(&pstate->pa_lock);
            return false;
        }
    }

    /* Pick the plan we found, and advance pa_next_plan one more time. */
    node->as_whichplan = pstate->pa_next_plan;
    pstate->pa_next_plan = bms_next_member(node->as_valid_subplans,
                                           pstate->pa_next_plan);

    /*
     * If there are no more valid plans then try setting the next plan to the
     * first valid partial plan.
     */
    if (pstate->pa_next_plan < 0)
    {
        int         nextplan = bms_next_member(node->as_valid_subplans,
                                               node->as_first_partial_plan - 1);

        if (nextplan >= 0)
            pstate->pa_next_plan = nextplan;
        else
        {
            /*
             * There are no valid partial plans, and we already chose the last
             * non-partial plan; so flag that there's nothing more for our
             * fellow workers to do.
             */
            pstate->pa_next_plan = INVALID_SUBPLAN_INDEX;
        }
    }

    /* If non-partial, immediately mark as finished. */
    if (node->as_whichplan < node->as_first_partial_plan)
        node->as_pstate->pa_finished[node->as_whichplan] = true;

    LWLockRelease(&pstate->pa_lock);

    return true;
}

/*
 * mark_invalid_subplans_as_finished
 *      Marks the ParallelAppendState's pa_finished as true for each invalid
 *      subplan.
 *
 * This function should only be called for parallel Append with run-time
 * pruning enabled.
 */
static void
mark_invalid_subplans_as_finished(AppendState *node)
{
    int         i;

    /* Only valid to call this while in parallel Append mode */
    Assert(node->as_pstate);

    /* Shouldn't have been called when run-time pruning is not enabled */
    Assert(node->as_prune_state);

    /* Nothing to do if all plans are valid */
    if (bms_num_members(node->as_valid_subplans) == node->as_nplans)
        return;

    /* Mark all non-valid plans as finished */
    for (i = 0; i < node->as_nplans; i++)
    {
        if (!bms_is_member(i, node->as_valid_subplans))
            node->as_pstate->pa_finished[i] = true;
    }
}

/* ----------------------------------------------------------------
 *                      Asynchronous Append Support
 * ----------------------------------------------------------------
 */

/* ----------------------------------------------------------------
 *      ExecAppendAsyncBegin
 *
 *      Begin executing designed async-capable subplans.
 * ----------------------------------------------------------------
 */
static void
ExecAppendAsyncBegin(AppendState *node)
{
    int         i;

    /* Backward scan is not supported by async-aware Appends. */
    Assert(ScanDirectionIsForward(node->ps.state->es_direction));

    /* We should never be called when there are no async subplans. */
    Assert(node->as_nasyncplans > 0);

    /* If we've yet to determine the valid subplans then do so now. */
    if (node->as_valid_subplans == NULL)
        node->as_valid_subplans =
            ExecFindMatchingSubPlans(node->as_prune_state);

    classify_matching_subplans(node);

    /* Nothing to do if there are no valid async subplans. */
    if (node->as_nasyncremain == 0)
        return;

    /* Make a request for each of the valid async subplans. */
    i = -1;
    while ((i = bms_next_member(node->as_valid_asyncplans, i)) >= 0)
    {
        AsyncRequest *areq = node->as_asyncrequests[i];

        Assert(areq->request_index == i);
        Assert(!areq->callback_pending);

        /* Do the actual work. */
        ExecAsyncRequest(areq);
    }
}

/* ----------------------------------------------------------------
 *      ExecAppendAsyncGetNext
 *
 *      Get the next tuple from any of the asynchronous subplans.
 * ----------------------------------------------------------------
 */
static bool
ExecAppendAsyncGetNext(AppendState *node, TupleTableSlot **result)
{
    *result = NULL;

    /* We should never be called when there are no valid async subplans. */
    Assert(node->as_nasyncremain > 0);

    /* Request a tuple asynchronously. */
    if (ExecAppendAsyncRequest(node, result))
        return true;

    while (node->as_nasyncremain > 0)
    {
        CHECK_FOR_INTERRUPTS();

        /* Wait or poll async events. */
        ExecAppendAsyncEventWait(node);

        /* Request a tuple asynchronously. */
        if (ExecAppendAsyncRequest(node, result))
            return true;

        /* Break from loop if there's any sync subplan that isn't complete. */
        if (!node->as_syncdone)
            break;
    }

    /*
     * If all sync subplans are complete, we're totally done scanning the
     * given node.  Otherwise, we're done with the asynchronous stuff but
     * must continue scanning the sync subplans.
     */
    if (node->as_syncdone)
    {
        Assert(node->as_nasyncremain == 0);
        *result = ExecClearTuple(node->ps.ps_ResultTupleSlot);
        return true;
    }

    return false;
}

/* ----------------------------------------------------------------
 *      ExecAppendAsyncRequest
 *
 *      Request a tuple asynchronously.
 * ----------------------------------------------------------------
 */
static bool
ExecAppendAsyncRequest(AppendState *node, TupleTableSlot **result)
{
    Bitmapset  *needrequest;
    int         i;

    /* Nothing to do if there are no async subplans needing a new request. */
    if (bms_is_empty(node->as_needrequest))
        return false;

    /*
     * If there are any asynchronously-generated results that have not yet
     * been returned, we have nothing to do; just return one of them.
     */
    if (node->as_nasyncresults > 0)
    {
        --node->as_nasyncresults;
        *result = node->as_asyncresults[node->as_nasyncresults];
        return true;
    }

    /* Make a new request for each of the async subplans that need it. */
    needrequest = node->as_needrequest;
    node->as_needrequest = NULL;
    i = -1;
    while ((i = bms_next_member(needrequest, i)) >= 0)
    {
        AsyncRequest *areq = node->as_asyncrequests[i];

        /* Do the actual work. */
        ExecAsyncRequest(areq);
    }
    bms_free(needrequest);

    /* Return one of the asynchronously-generated results if any. */
    if (node->as_nasyncresults > 0)
    {
        --node->as_nasyncresults;
        *result = node->as_asyncresults[node->as_nasyncresults];
        return true;
    }

    return false;
}

/* ----------------------------------------------------------------
 *      ExecAppendAsyncEventWait
 *
 *      Wait or poll for file descriptor events and fire callbacks.
 * ----------------------------------------------------------------
 */
static void
ExecAppendAsyncEventWait(AppendState *node)
{
    long        timeout = node->as_syncdone ? -1 : 0;
    WaitEvent   occurred_event[EVENT_BUFFER_SIZE];
    int         noccurred;
    int         nevents;
    int         i;

    /* We should never be called when there are no valid async subplans. */
    Assert(node->as_nasyncremain > 0);

    node->as_eventset = CreateWaitEventSet(CurrentMemoryContext,
                                           node->as_nasyncplans + 1);
    AddWaitEventToSet(node->as_eventset, WL_EXIT_ON_PM_DEATH, PGINVALID_SOCKET,
                      NULL, NULL);

    /* Give each waiting subplan a chance to add an event. */
    i = -1;
    while ((i = bms_next_member(node->as_asyncplans, i)) >= 0)
    {
        AsyncRequest *areq = node->as_asyncrequests[i];

        if (areq->callback_pending)
            ExecAsyncConfigureWait(areq);
    }

    /* Wait for at least one event to occur. */
    nevents = Min(node->as_nasyncplans + 1, EVENT_BUFFER_SIZE);
    noccurred = WaitEventSetWait(node->as_eventset, timeout, occurred_event,
                                 nevents, WAIT_EVENT_APPEND_READY);
    FreeWaitEventSet(node->as_eventset);
    node->as_eventset = NULL;
    if (noccurred == 0)
        return;

    /* Deliver notifications. */
    for (i = 0; i < noccurred; i++)
    {
        WaitEvent  *w = &occurred_event[i];

        /*
         * Each waiting subplan should have registered its wait event with
         * user_data pointing back to its AsyncRequest.
         */
        if ((w->events & WL_SOCKET_READABLE) != 0)
        {
            AsyncRequest *areq = (AsyncRequest *) w->user_data;

            /*
             * Mark it as no longer needing a callback.  We must do this
             * before dispatching the callback in case the callback resets
             * the flag.
             */
            Assert(areq->callback_pending);
            areq->callback_pending = false;

            /* Do the actual work. */
            ExecAsyncNotify(areq);
        }
    }
}

/* ----------------------------------------------------------------
 *      ExecAsyncAppendResponse
 *
 *      Receive a response from an asynchronous request we made.
 * ----------------------------------------------------------------
 */
void
ExecAsyncAppendResponse(AsyncRequest *areq)
{
    AppendState *node = (AppendState *) areq->requestor;
    TupleTableSlot *slot = areq->result;

    /* The result should be a TupleTableSlot or NULL. */
    Assert(slot == NULL || IsA(slot, TupleTableSlot));

    /* Nothing to do if the request is pending. */
    if (!areq->request_complete)
    {
        /* The request would have been pending for a callback */
        Assert(areq->callback_pending);
        return;
    }

    /* If the result is NULL or an empty slot, there's nothing more to do. */
    if (TupIsNull(slot))
    {
        /* The ending subplan wouldn't have been pending for a callback. */
        Assert(!areq->callback_pending);
        --node->as_nasyncremain;
        return;
    }

    /* Save result so we can return it. */
    Assert(node->as_nasyncresults < node->as_nasyncplans);
    node->as_asyncresults[node->as_nasyncresults++] = slot;

    /*
     * Mark the subplan that returned a result as ready for a new request.  We
     * don't launch another one here immediately because it might complete.
     */
    node->as_needrequest = bms_add_member(node->as_needrequest,
                                          areq->request_index);
}

/* ----------------------------------------------------------------
 *      classify_matching_subplans
 *
 *      Classify the node's as_valid_subplans into sync ones and
 *      async ones, adjust it to contain sync ones only, and save
 *      async ones in the node's as_valid_asyncplans.
 * ----------------------------------------------------------------
 */
static void
classify_matching_subplans(AppendState *node)
{
    Bitmapset  *valid_asyncplans;

    Assert(node->as_valid_asyncplans == NULL);

    /* Nothing to do if there are no valid subplans. */
    if (bms_is_empty(node->as_valid_subplans))
    {
        node->as_syncdone = true;
        node->as_nasyncremain = 0;
        return;
    }

    /* Nothing to do if there are no valid async subplans. */
    if (!bms_overlap(node->as_valid_subplans, node->as_asyncplans))
    {
        node->as_nasyncremain = 0;
        return;
    }

    /* Get valid async subplans. */
    valid_asyncplans = bms_copy(node->as_asyncplans);
    valid_asyncplans = bms_int_members(valid_asyncplans,
                                       node->as_valid_subplans);

    /* Adjust the valid subplans to contain sync subplans only. */
    node->as_valid_subplans = bms_del_members(node->as_valid_subplans,
                                              valid_asyncplans);
    node->as_syncdone = bms_is_empty(node->as_valid_subplans);

    /* Save valid async subplans. */
    node->as_valid_asyncplans = valid_asyncplans;
    node->as_nasyncremain = bms_num_members(valid_asyncplans);
}