execParallel.c

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/*-------------------------------------------------------------------------
 *
 * execParallel.c
 *    Support routines for parallel execution.
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * This file contains routines that are intended to support setting up,
 * using, and tearing down a ParallelContext from within the PostgreSQL
 * executor.  The ParallelContext machinery will handle starting the
 * workers and ensuring that their state generally matches that of the
 * leader; see src/backend/access/transam/README.parallel for details.
 * However, we must save and restore relevant executor state, such as
 * any ParamListInfo associated with the query, buffer usage info, and
 * the actual plan to be passed down to the worker.
 *
 * IDENTIFICATION
 *    src/backend/executor/execParallel.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "executor/execParallel.h"
#include "executor/executor.h"
#include "executor/nodeBitmapHeapscan.h"
#include "executor/nodeCustom.h"
#include "executor/nodeForeignscan.h"
#include "executor/nodeSeqscan.h"
#include "executor/nodeIndexscan.h"
#include "executor/nodeIndexonlyscan.h"
#include "executor/tqueue.h"
#include "nodes/nodeFuncs.h"
#include "optimizer/planmain.h"
#include "optimizer/planner.h"
#include "storage/spin.h"
#include "tcop/tcopprot.h"
#include "utils/dsa.h"
#include "utils/memutils.h"
#include "utils/snapmgr.h"
#include "pgstat.h"

/*
 * Magic numbers for parallel executor communication.  We use constants
 * greater than any 32-bit integer here so that values < 2^32 can be used
 * by individual parallel nodes to store their own state.
 */
#define PARALLEL_KEY_PLANNEDSTMT        UINT64CONST(0xE000000000000001)
#define PARALLEL_KEY_PARAMS             UINT64CONST(0xE000000000000002)
#define PARALLEL_KEY_BUFFER_USAGE       UINT64CONST(0xE000000000000003)
#define PARALLEL_KEY_TUPLE_QUEUE        UINT64CONST(0xE000000000000004)
#define PARALLEL_KEY_INSTRUMENTATION    UINT64CONST(0xE000000000000005)
#define PARALLEL_KEY_DSA                UINT64CONST(0xE000000000000006)
#define PARALLEL_KEY_QUERY_TEXT     UINT64CONST(0xE000000000000007)

#define PARALLEL_TUPLE_QUEUE_SIZE       65536

/*
 * DSM structure for accumulating per-PlanState instrumentation.
 *
 * instrument_options: Same meaning here as in instrument.c.
 *
 * instrument_offset: Offset, relative to the start of this structure,
 * of the first Instrumentation object.  This will depend on the length of
 * the plan_node_id array.
 *
 * num_workers: Number of workers.
 *
 * num_plan_nodes: Number of plan nodes.
 *
 * plan_node_id: Array of plan nodes for which we are gathering instrumentation
 * from parallel workers.  The length of this array is given by num_plan_nodes.
 */
struct SharedExecutorInstrumentation
{
    int         instrument_options;
    int         instrument_offset;
    int         num_workers;
    int         num_plan_nodes;
    int         plan_node_id[FLEXIBLE_ARRAY_MEMBER];
    /* array of num_plan_nodes * num_workers Instrumentation objects follows */
};
#define GetInstrumentationArray(sei) \
    (AssertVariableIsOfTypeMacro(sei, SharedExecutorInstrumentation *), \
     (Instrumentation *) (((char *) sei) + sei->instrument_offset))

/* Context object for ExecParallelEstimate. */
typedef struct ExecParallelEstimateContext
{
    ParallelContext *pcxt;
    int         nnodes;
} ExecParallelEstimateContext;

/* Context object for ExecParallelInitializeDSM. */
typedef struct ExecParallelInitializeDSMContext
{
    ParallelContext *pcxt;
    SharedExecutorInstrumentation *instrumentation;
    int         nnodes;
} ExecParallelInitializeDSMContext;

/* Helper functions that run in the parallel leader. */
static char *ExecSerializePlan(Plan *plan, EState *estate);
static bool ExecParallelEstimate(PlanState *node,
                     ExecParallelEstimateContext *e);
static bool ExecParallelInitializeDSM(PlanState *node,
                          ExecParallelInitializeDSMContext *d);
static shm_mq_handle **ExecParallelSetupTupleQueues(ParallelContext *pcxt,
                             bool reinitialize);
static bool ExecParallelRetrieveInstrumentation(PlanState *planstate,
                                    SharedExecutorInstrumentation *instrumentation);

/* Helper function that runs in the parallel worker. */
static DestReceiver *ExecParallelGetReceiver(dsm_segment *seg, shm_toc *toc);

/*
 * Create a serialized representation of the plan to be sent to each worker.
 */
static char *
ExecSerializePlan(Plan *plan, EState *estate)
{
    PlannedStmt *pstmt;
    ListCell   *lc;

    /* We can't scribble on the original plan, so make a copy. */
    plan = copyObject(plan);

    /*
     * The worker will start its own copy of the executor, and that copy will
     * insert a junk filter if the toplevel node has any resjunk entries. We
     * don't want that to happen, because while resjunk columns shouldn't be
     * sent back to the user, here the tuples are coming back to another
     * backend which may very well need them.  So mutate the target list
     * accordingly.  This is sort of a hack; there might be better ways to do
     * this...
     */
    foreach(lc, plan->targetlist)
    {
        TargetEntry *tle = lfirst_node(TargetEntry, lc);

        tle->resjunk = false;
    }

    /*
     * Create a dummy PlannedStmt.  Most of the fields don't need to be valid
     * for our purposes, but the worker will need at least a minimal
     * PlannedStmt to start the executor.
     */
    pstmt = makeNode(PlannedStmt);
    pstmt->commandType = CMD_SELECT;
    pstmt->queryId = 0;
    pstmt->hasReturning = false;
    pstmt->hasModifyingCTE = false;
    pstmt->canSetTag = true;
    pstmt->transientPlan = false;
    pstmt->dependsOnRole = false;
    pstmt->parallelModeNeeded = false;
    pstmt->planTree = plan;
    pstmt->rtable = estate->es_range_table;
    pstmt->resultRelations = NIL;
    pstmt->nonleafResultRelations = NIL;

    /*
     * Transfer only parallel-safe subplans, leaving a NULL "hole" in the list
     * for unsafe ones (so that the list indexes of the safe ones are
     * preserved).  This positively ensures that the worker won't try to run,
     * or even do ExecInitNode on, an unsafe subplan.  That's important to
     * protect, eg, non-parallel-aware FDWs from getting into trouble.
     */
    pstmt->subplans = NIL;
    foreach(lc, estate->es_plannedstmt->subplans)
    {
        Plan       *subplan = (Plan *) lfirst(lc);

        if (subplan && !subplan->parallel_safe)
            subplan = NULL;
        pstmt->subplans = lappend(pstmt->subplans, subplan);
    }

    pstmt->rewindPlanIDs = NULL;
    pstmt->rowMarks = NIL;
    pstmt->relationOids = NIL;
    pstmt->invalItems = NIL;    /* workers can't replan anyway... */
    pstmt->nParamExec = estate->es_plannedstmt->nParamExec;
    pstmt->utilityStmt = NULL;
    pstmt->stmt_location = -1;
    pstmt->stmt_len = -1;

    /* Return serialized copy of our dummy PlannedStmt. */
    return nodeToString(pstmt);
}

/*
 * Ordinary plan nodes won't do anything here, but parallel-aware plan nodes
 * may need some state which is shared across all parallel workers.  Before
 * we size the DSM, give them a chance to call shm_toc_estimate_chunk or
 * shm_toc_estimate_keys on &pcxt->estimator.
 *
 * While we're at it, count the number of PlanState nodes in the tree, so
 * we know how many SharedPlanStateInstrumentation structures we need.
 */
static bool
ExecParallelEstimate(PlanState *planstate, ExecParallelEstimateContext *e)
{
    if (planstate == NULL)
        return false;

    /* Count this node. */
    e->nnodes++;

    /* Call estimators for parallel-aware nodes. */
    if (planstate->plan->parallel_aware)
    {
        switch (nodeTag(planstate))
        {
            case T_SeqScanState:
                ExecSeqScanEstimate((SeqScanState *) planstate,
                                    e->pcxt);
                break;
            case T_IndexScanState:
                ExecIndexScanEstimate((IndexScanState *) planstate,
                                      e->pcxt);
                break;
            case T_IndexOnlyScanState:
                ExecIndexOnlyScanEstimate((IndexOnlyScanState *) planstate,
                                          e->pcxt);
                break;
            case T_ForeignScanState:
                ExecForeignScanEstimate((ForeignScanState *) planstate,
                                        e->pcxt);
                break;
            case T_CustomScanState:
                ExecCustomScanEstimate((CustomScanState *) planstate,
                                       e->pcxt);
                break;
            case T_BitmapHeapScanState:
                ExecBitmapHeapEstimate((BitmapHeapScanState *) planstate,
                                       e->pcxt);
                break;
            default:
                break;
        }
    }

    return planstate_tree_walker(planstate, ExecParallelEstimate, e);
}

/*
 * Initialize the dynamic shared memory segment that will be used to control
 * parallel execution.
 */
static bool
ExecParallelInitializeDSM(PlanState *planstate,
                          ExecParallelInitializeDSMContext *d)
{
    if (planstate == NULL)
        return false;

    /* If instrumentation is enabled, initialize slot for this node. */
    if (d->instrumentation != NULL)
        d->instrumentation->plan_node_id[d->nnodes] =
            planstate->plan->plan_node_id;

    /* Count this node. */
    d->nnodes++;

    /*
     * Call initializers for parallel-aware plan nodes.
     *
     * Ordinary plan nodes won't do anything here, but parallel-aware plan
     * nodes may need to initialize shared state in the DSM before parallel
     * workers are available.  They can allocate the space they previously
     * estimated using shm_toc_allocate, and add the keys they previously
     * estimated using shm_toc_insert, in each case targeting pcxt->toc.
     */
    if (planstate->plan->parallel_aware)
    {
        switch (nodeTag(planstate))
        {
            case T_SeqScanState:
                ExecSeqScanInitializeDSM((SeqScanState *) planstate,
                                         d->pcxt);
                break;
            case T_IndexScanState:
                ExecIndexScanInitializeDSM((IndexScanState *) planstate,
                                           d->pcxt);
                break;
            case T_IndexOnlyScanState:
                ExecIndexOnlyScanInitializeDSM((IndexOnlyScanState *) planstate,
                                               d->pcxt);
                break;
            case T_ForeignScanState:
                ExecForeignScanInitializeDSM((ForeignScanState *) planstate,
                                             d->pcxt);
                break;
            case T_CustomScanState:
                ExecCustomScanInitializeDSM((CustomScanState *) planstate,
                                            d->pcxt);
                break;
            case T_BitmapHeapScanState:
                ExecBitmapHeapInitializeDSM((BitmapHeapScanState *) planstate,
                                            d->pcxt);
                break;

            default:
                break;
        }
    }

    return planstate_tree_walker(planstate, ExecParallelInitializeDSM, d);
}

/*
 * It sets up the response queues for backend workers to return tuples
 * to the main backend and start the workers.
 */
static shm_mq_handle **
ExecParallelSetupTupleQueues(ParallelContext *pcxt, bool reinitialize)
{
    shm_mq_handle **responseq;
    char       *tqueuespace;
    int         i;

    /* Skip this if no workers. */
    if (pcxt->nworkers == 0)
        return NULL;

    /* Allocate memory for shared memory queue handles. */
    responseq = (shm_mq_handle **)
        palloc(pcxt->nworkers * sizeof(shm_mq_handle *));

    /*
     * If not reinitializing, allocate space from the DSM for the queues;
     * otherwise, find the already allocated space.
     */
    if (!reinitialize)
        tqueuespace =
            shm_toc_allocate(pcxt->toc,
                             mul_size(PARALLEL_TUPLE_QUEUE_SIZE,
                                      pcxt->nworkers));
    else
        tqueuespace = shm_toc_lookup(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE, false);

    /* Create the queues, and become the receiver for each. */
    for (i = 0; i < pcxt->nworkers; ++i)
    {
        shm_mq     *mq;

        mq = shm_mq_create(tqueuespace +
                           ((Size) i) * PARALLEL_TUPLE_QUEUE_SIZE,
                           (Size) PARALLEL_TUPLE_QUEUE_SIZE);

        shm_mq_set_receiver(mq, MyProc);
        responseq[i] = shm_mq_attach(mq, pcxt->seg, NULL);
    }

    /* Add array of queues to shm_toc, so others can find it. */
    if (!reinitialize)
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_TUPLE_QUEUE, tqueuespace);

    /* Return array of handles. */
    return responseq;
}

/*
 * Re-initialize the parallel executor info such that it can be reused by
 * workers.
 */
void
ExecParallelReinitialize(ParallelExecutorInfo *pei)
{
    ReinitializeParallelDSM(pei->pcxt);
    pei->tqueue = ExecParallelSetupTupleQueues(pei->pcxt, true);
    pei->finished = false;
}

/*
 * Sets up the required infrastructure for backend workers to perform
 * execution and return results to the main backend.
 */
ParallelExecutorInfo *
ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
{
    ParallelExecutorInfo *pei;
    ParallelContext *pcxt;
    ExecParallelEstimateContext e;
    ExecParallelInitializeDSMContext d;
    char       *pstmt_data;
    char       *pstmt_space;
    char       *param_space;
    BufferUsage *bufusage_space;
    SharedExecutorInstrumentation *instrumentation = NULL;
    int         pstmt_len;
    int         param_len;
    int         instrumentation_len = 0;
    int         instrument_offset = 0;
    Size        dsa_minsize = dsa_minimum_size();
    char       *query_string;
    int         query_len;

    /* Allocate object for return value. */
    pei = palloc0(sizeof(ParallelExecutorInfo));
    pei->finished = false;
    pei->planstate = planstate;

    /* Fix up and serialize plan to be sent to workers. */
    pstmt_data = ExecSerializePlan(planstate->plan, estate);

    /* Create a parallel context. */
    pcxt = CreateParallelContext("postgres", "ParallelQueryMain", nworkers);
    pei->pcxt = pcxt;

    /*
     * Before telling the parallel context to create a dynamic shared memory
     * segment, we need to figure out how big it should be.  Estimate space
     * for the various things we need to store.
     */

    /* Estimate space for query text. */
    query_len = strlen(estate->es_sourceText);
    shm_toc_estimate_chunk(&pcxt->estimator, query_len);
    shm_toc_estimate_keys(&pcxt->estimator, 1);

    /* Estimate space for serialized PlannedStmt. */
    pstmt_len = strlen(pstmt_data) + 1;
    shm_toc_estimate_chunk(&pcxt->estimator, pstmt_len);
    shm_toc_estimate_keys(&pcxt->estimator, 1);

    /* Estimate space for serialized ParamListInfo. */
    param_len = EstimateParamListSpace(estate->es_param_list_info);
    shm_toc_estimate_chunk(&pcxt->estimator, param_len);
    shm_toc_estimate_keys(&pcxt->estimator, 1);

    /*
     * Estimate space for BufferUsage.
     *
     * If EXPLAIN is not in use and there are no extensions loaded that care,
     * we could skip this.  But we have no way of knowing whether anyone's
     * looking at pgBufferUsage, so do it unconditionally.
     */
    shm_toc_estimate_chunk(&pcxt->estimator,
                           mul_size(sizeof(BufferUsage), pcxt->nworkers));
    shm_toc_estimate_keys(&pcxt->estimator, 1);

    /* Estimate space for tuple queues. */
    shm_toc_estimate_chunk(&pcxt->estimator,
                           mul_size(PARALLEL_TUPLE_QUEUE_SIZE, pcxt->nworkers));
    shm_toc_estimate_keys(&pcxt->estimator, 1);

    /*
     * Give parallel-aware nodes a chance to add to the estimates, and get a
     * count of how many PlanState nodes there are.
     */
    e.pcxt = pcxt;
    e.nnodes = 0;
    ExecParallelEstimate(planstate, &e);

    /* Estimate space for instrumentation, if required. */
    if (estate->es_instrument)
    {
        instrumentation_len =
            offsetof(SharedExecutorInstrumentation, plan_node_id) +
            sizeof(int) * e.nnodes;
        instrumentation_len = MAXALIGN(instrumentation_len);
        instrument_offset = instrumentation_len;
        instrumentation_len +=
            mul_size(sizeof(Instrumentation),
                     mul_size(e.nnodes, nworkers));
        shm_toc_estimate_chunk(&pcxt->estimator, instrumentation_len);
        shm_toc_estimate_keys(&pcxt->estimator, 1);
    }

    /* Estimate space for DSA area. */
    shm_toc_estimate_chunk(&pcxt->estimator, dsa_minsize);
    shm_toc_estimate_keys(&pcxt->estimator, 1);

    /* Everyone's had a chance to ask for space, so now create the DSM. */
    InitializeParallelDSM(pcxt);

    /*
     * OK, now we have a dynamic shared memory segment, and it should be big
     * enough to store all of the data we estimated we would want to put into
     * it, plus whatever general stuff (not specifically executor-related) the
     * ParallelContext itself needs to store there.  None of the space we
     * asked for has been allocated or initialized yet, though, so do that.
     */

    /* Store query string */
    query_string = shm_toc_allocate(pcxt->toc, query_len);
    memcpy(query_string, estate->es_sourceText, query_len);
    shm_toc_insert(pcxt->toc, PARALLEL_KEY_QUERY_TEXT, query_string);

    /* Store serialized PlannedStmt. */
    pstmt_space = shm_toc_allocate(pcxt->toc, pstmt_len);
    memcpy(pstmt_space, pstmt_data, pstmt_len);
    shm_toc_insert(pcxt->toc, PARALLEL_KEY_PLANNEDSTMT, pstmt_space);

    /* Store serialized ParamListInfo. */
    param_space = shm_toc_allocate(pcxt->toc, param_len);
    shm_toc_insert(pcxt->toc, PARALLEL_KEY_PARAMS, param_space);
    SerializeParamList(estate->es_param_list_info, &param_space);

    /* Allocate space for each worker's BufferUsage; no need to initialize. */
    bufusage_space = shm_toc_allocate(pcxt->toc,
                                      mul_size(sizeof(BufferUsage), pcxt->nworkers));
    shm_toc_insert(pcxt->toc, PARALLEL_KEY_BUFFER_USAGE, bufusage_space);
    pei->buffer_usage = bufusage_space;

    /* Set up tuple queues. */
    pei->tqueue = ExecParallelSetupTupleQueues(pcxt, false);

    /*
     * If instrumentation options were supplied, allocate space for the data.
     * It only gets partially initialized here; the rest happens during
     * ExecParallelInitializeDSM.
     */
    if (estate->es_instrument)
    {
        Instrumentation *instrument;
        int         i;

        instrumentation = shm_toc_allocate(pcxt->toc, instrumentation_len);
        instrumentation->instrument_options = estate->es_instrument;
        instrumentation->instrument_offset = instrument_offset;
        instrumentation->num_workers = nworkers;
        instrumentation->num_plan_nodes = e.nnodes;
        instrument = GetInstrumentationArray(instrumentation);
        for (i = 0; i < nworkers * e.nnodes; ++i)
            InstrInit(&instrument[i], estate->es_instrument);
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_INSTRUMENTATION,
                       instrumentation);
        pei->instrumentation = instrumentation;
    }

    /*
     * Create a DSA area that can be used by the leader and all workers.
     * (However, if we failed to create a DSM and are using private memory
     * instead, then skip this.)
     */
    if (pcxt->seg != NULL)
    {
        char       *area_space;

        area_space = shm_toc_allocate(pcxt->toc, dsa_minsize);
        shm_toc_insert(pcxt->toc, PARALLEL_KEY_DSA, area_space);
        pei->area = dsa_create_in_place(area_space, dsa_minsize,
                                        LWTRANCHE_PARALLEL_QUERY_DSA,
                                        pcxt->seg);
    }

    /*
     * Make the area available to executor nodes running in the leader.  See
     * also ParallelQueryMain which makes it available to workers.
     */
    estate->es_query_dsa = pei->area;

    /*
     * Give parallel-aware nodes a chance to initialize their shared data.
     * This also initializes the elements of instrumentation->ps_instrument,
     * if it exists.
     */
    d.pcxt = pcxt;
    d.instrumentation = instrumentation;
    d.nnodes = 0;
    ExecParallelInitializeDSM(planstate, &d);

    /*
     * Make sure that the world hasn't shifted under our feat.  This could
     * probably just be an Assert(), but let's be conservative for now.
     */
    if (e.nnodes != d.nnodes)
        elog(ERROR, "inconsistent count of PlanState nodes");

    /* OK, we're ready to rock and roll. */
    return pei;
}

/*
 * Copy instrumentation information about this node and its descendants from
 * dynamic shared memory.
 */
static bool
ExecParallelRetrieveInstrumentation(PlanState *planstate,
                                    SharedExecutorInstrumentation *instrumentation)
{
    Instrumentation *instrument;
    int         i;
    int         n;
    int         ibytes;
    int         plan_node_id = planstate->plan->plan_node_id;
    MemoryContext oldcontext;

    /* Find the instrumentation for this node. */
    for (i = 0; i < instrumentation->num_plan_nodes; ++i)
        if (instrumentation->plan_node_id[i] == plan_node_id)
            break;
    if (i >= instrumentation->num_plan_nodes)
        elog(ERROR, "plan node %d not found", plan_node_id);

    /* Accumulate the statistics from all workers. */
    instrument = GetInstrumentationArray(instrumentation);
    instrument += i * instrumentation->num_workers;
    for (n = 0; n < instrumentation->num_workers; ++n)
        InstrAggNode(planstate->instrument, &instrument[n]);

    /*
     * Also store the per-worker detail.
     *
     * Worker instrumentation should be allocated in the same context as the
     * regular instrumentation information, which is the per-query context.
     * Switch into per-query memory context.
     */
    oldcontext = MemoryContextSwitchTo(planstate->state->es_query_cxt);
    ibytes = mul_size(instrumentation->num_workers, sizeof(Instrumentation));
    planstate->worker_instrument =
        palloc(ibytes + offsetof(WorkerInstrumentation, instrument));
    MemoryContextSwitchTo(oldcontext);

    planstate->worker_instrument->num_workers = instrumentation->num_workers;
    memcpy(&planstate->worker_instrument->instrument, instrument, ibytes);

    return planstate_tree_walker(planstate, ExecParallelRetrieveInstrumentation,
                                 instrumentation);
}

/*
 * Finish parallel execution.  We wait for parallel workers to finish, and
 * accumulate their buffer usage and instrumentation.
 */
void
ExecParallelFinish(ParallelExecutorInfo *pei)
{
    int         i;

    if (pei->finished)
        return;

    /* First, wait for the workers to finish. */
    WaitForParallelWorkersToFinish(pei->pcxt);

    /* Next, accumulate buffer usage. */
    for (i = 0; i < pei->pcxt->nworkers_launched; ++i)
        InstrAccumParallelQuery(&pei->buffer_usage[i]);

    /* Finally, accumulate instrumentation, if any. */
    if (pei->instrumentation)
        ExecParallelRetrieveInstrumentation(pei->planstate,
                                            pei->instrumentation);

    pei->finished = true;
}

/*
 * Clean up whatever ParallelExecutorInfo resources still exist after
 * ExecParallelFinish.  We separate these routines because someone might
 * want to examine the contents of the DSM after ExecParallelFinish and
 * before calling this routine.
 */
void
ExecParallelCleanup(ParallelExecutorInfo *pei)
{
    if (pei->area != NULL)
    {
        dsa_detach(pei->area);
        pei->area = NULL;
    }
    if (pei->pcxt != NULL)
    {
        DestroyParallelContext(pei->pcxt);
        pei->pcxt = NULL;
    }
    pfree(pei);
}

/*
 * Create a DestReceiver to write tuples we produce to the shm_mq designated
 * for that purpose.
 */
static DestReceiver *
ExecParallelGetReceiver(dsm_segment *seg, shm_toc *toc)
{
    char       *mqspace;
    shm_mq     *mq;

    mqspace = shm_toc_lookup(toc, PARALLEL_KEY_TUPLE_QUEUE, false);
    mqspace += ParallelWorkerNumber * PARALLEL_TUPLE_QUEUE_SIZE;
    mq = (shm_mq *) mqspace;
    shm_mq_set_sender(mq, MyProc);
    return CreateTupleQueueDestReceiver(shm_mq_attach(mq, seg, NULL));
}

/*
 * Create a QueryDesc for the PlannedStmt we are to execute, and return it.
 */
static QueryDesc *
ExecParallelGetQueryDesc(shm_toc *toc, DestReceiver *receiver,
                         int instrument_options)
{
    char       *pstmtspace;
    char       *paramspace;
    PlannedStmt *pstmt;
    ParamListInfo paramLI;
    char       *queryString;

    /* Get the query string from shared memory */
    queryString = shm_toc_lookup(toc, PARALLEL_KEY_QUERY_TEXT, false);

    /* Reconstruct leader-supplied PlannedStmt. */
    pstmtspace = shm_toc_lookup(toc, PARALLEL_KEY_PLANNEDSTMT, false);
    pstmt = (PlannedStmt *) stringToNode(pstmtspace);

    /* Reconstruct ParamListInfo. */
    paramspace = shm_toc_lookup(toc, PARALLEL_KEY_PARAMS, false);
    paramLI = RestoreParamList(&paramspace);

    /*
     * Create a QueryDesc for the query.
     *
     * It's not obvious how to obtain the query string from here; and even if
     * we could copying it would take more cycles than not copying it. But
     * it's a bit unsatisfying to just use a dummy string here, so consider
     * revising this someday.
     */
    return CreateQueryDesc(pstmt,
                           queryString,
                           GetActiveSnapshot(), InvalidSnapshot,
                           receiver, paramLI, NULL, instrument_options);
}

/*
 * Copy instrumentation information from this node and its descendants into
 * dynamic shared memory, so that the parallel leader can retrieve it.
 */
static bool
ExecParallelReportInstrumentation(PlanState *planstate,
                                  SharedExecutorInstrumentation *instrumentation)
{
    int         i;
    int         plan_node_id = planstate->plan->plan_node_id;
    Instrumentation *instrument;

    InstrEndLoop(planstate->instrument);

    /*
     * If we shuffled the plan_node_id values in ps_instrument into sorted
     * order, we could use binary search here.  This might matter someday if
     * we're pushing down sufficiently large plan trees.  For now, do it the
     * slow, dumb way.
     */
    for (i = 0; i < instrumentation->num_plan_nodes; ++i)
        if (instrumentation->plan_node_id[i] == plan_node_id)
            break;
    if (i >= instrumentation->num_plan_nodes)
        elog(ERROR, "plan node %d not found", plan_node_id);

    /*
     * Add our statistics to the per-node, per-worker totals.  It's possible
     * that this could happen more than once if we relaunched workers.
     */
    instrument = GetInstrumentationArray(instrumentation);
    instrument += i * instrumentation->num_workers;
    Assert(IsParallelWorker());
    Assert(ParallelWorkerNumber < instrumentation->num_workers);
    InstrAggNode(&instrument[ParallelWorkerNumber], planstate->instrument);

    return planstate_tree_walker(planstate, ExecParallelReportInstrumentation,
                                 instrumentation);
}

/*
 * Initialize the PlanState and its descendants with the information
 * retrieved from shared memory.  This has to be done once the PlanState
 * is allocated and initialized by executor; that is, after ExecutorStart().
 */
static bool
ExecParallelInitializeWorker(PlanState *planstate, shm_toc *toc)
{
    if (planstate == NULL)
        return false;

    /* Call initializers for parallel-aware plan nodes. */
    if (planstate->plan->parallel_aware)
    {
        switch (nodeTag(planstate))
        {
            case T_SeqScanState:
                ExecSeqScanInitializeWorker((SeqScanState *) planstate, toc);
                break;
            case T_IndexScanState:
                ExecIndexScanInitializeWorker((IndexScanState *) planstate, toc);
                break;
            case T_IndexOnlyScanState:
                ExecIndexOnlyScanInitializeWorker((IndexOnlyScanState *) planstate, toc);
                break;
            case T_ForeignScanState:
                ExecForeignScanInitializeWorker((ForeignScanState *) planstate,
                                                toc);
                break;
            case T_CustomScanState:
                ExecCustomScanInitializeWorker((CustomScanState *) planstate,
                                               toc);
                break;
            case T_BitmapHeapScanState:
                ExecBitmapHeapInitializeWorker(
                                               (BitmapHeapScanState *) planstate, toc);
                break;
            default:
                break;
        }
    }

    return planstate_tree_walker(planstate, ExecParallelInitializeWorker, toc);
}

/*
 * Main entrypoint for parallel query worker processes.
 *
 * We reach this function from ParallelWorkerMain, so the setup necessary to
 * create a sensible parallel environment has already been done;
 * ParallelWorkerMain worries about stuff like the transaction state, combo
 * CID mappings, and GUC values, so we don't need to deal with any of that
 * here.
 *
 * Our job is to deal with concerns specific to the executor.  The parallel
 * group leader will have stored a serialized PlannedStmt, and it's our job
 * to execute that plan and write the resulting tuples to the appropriate
 * tuple queue.  Various bits of supporting information that we need in order
 * to do this are also stored in the dsm_segment and can be accessed through
 * the shm_toc.
 */
void
ParallelQueryMain(dsm_segment *seg, shm_toc *toc)
{
    BufferUsage *buffer_usage;
    DestReceiver *receiver;
    QueryDesc  *queryDesc;
    SharedExecutorInstrumentation *instrumentation;
    int         instrument_options = 0;
    void       *area_space;
    dsa_area   *area;

    /* Set up DestReceiver, SharedExecutorInstrumentation, and QueryDesc. */
    receiver = ExecParallelGetReceiver(seg, toc);
    instrumentation = shm_toc_lookup(toc, PARALLEL_KEY_INSTRUMENTATION, true);
    if (instrumentation != NULL)
        instrument_options = instrumentation->instrument_options;
    queryDesc = ExecParallelGetQueryDesc(toc, receiver, instrument_options);

    /* Setting debug_query_string for individual workers */
    debug_query_string = queryDesc->sourceText;

    /* Report workers' query for monitoring purposes */
    pgstat_report_activity(STATE_RUNNING, debug_query_string);

    /* Prepare to track buffer usage during query execution. */
    InstrStartParallelQuery();

    /* Attach to the dynamic shared memory area. */
    area_space = shm_toc_lookup(toc, PARALLEL_KEY_DSA, false);
    area = dsa_attach_in_place(area_space, seg);

    /* Start up the executor */
    ExecutorStart(queryDesc, 0);

    /* Special executor initialization steps for parallel workers */
    queryDesc->planstate->state->es_query_dsa = area;
    ExecParallelInitializeWorker(queryDesc->planstate, toc);

    /* Run the plan */
    ExecutorRun(queryDesc, ForwardScanDirection, 0L, true);

    /* Shut down the executor */
    ExecutorFinish(queryDesc);

    /* Report buffer usage during parallel execution. */
    buffer_usage = shm_toc_lookup(toc, PARALLEL_KEY_BUFFER_USAGE, false);
    InstrEndParallelQuery(&buffer_usage[ParallelWorkerNumber]);

    /* Report instrumentation data if any instrumentation options are set. */
    if (instrumentation != NULL)
        ExecParallelReportInstrumentation(queryDesc->planstate,
                                          instrumentation);

    /* Must do this after capturing instrumentation. */
    ExecutorEnd(queryDesc);

    /* Cleanup. */
    dsa_detach(area);
    FreeQueryDesc(queryDesc);
    (*receiver->rDestroy) (receiver);
}