pquery.c

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/*-------------------------------------------------------------------------
 *
 * pquery.c
 *    POSTGRES process query command code
 *
 * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/tcop/pquery.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include <limits.h>
 
#include "access/xact.h"
#include "commands/prepare.h"
#include "executor/execdesc.h"
#include "executor/executor.h"
#include "executor/tstoreReceiver.h"
#include "miscadmin.h"
#include "pg_trace.h"
#include "tcop/pquery.h"
#include "tcop/utility.h"
#include "utils/memutils.h"
#include "utils/snapmgr.h"
 
 
/*
 * ActivePortal is the currently executing Portal (the most closely nested,
 * if there are several).
 */
Portal      ActivePortal = NULL;
 
 
static void ProcessQuery(PlannedStmt *plan,
                         CachedPlan *cplan,
                         CachedPlanSource *plansource,
                         int query_index,
                         const char *sourceText,
                         ParamListInfo params,
                         QueryEnvironment *queryEnv,
                         DestReceiver *dest,
                         QueryCompletion *qc);
static void FillPortalStore(Portal portal, bool isTopLevel);
static uint64 RunFromStore(Portal portal, ScanDirection direction, uint64 count,
                           DestReceiver *dest);
static uint64 PortalRunSelect(Portal portal, bool forward, long count,
                              DestReceiver *dest);
static void PortalRunUtility(Portal portal, PlannedStmt *pstmt,
                             bool isTopLevel, bool setHoldSnapshot,
                             DestReceiver *dest, QueryCompletion *qc);
static void PortalRunMulti(Portal portal,
                           bool isTopLevel, bool setHoldSnapshot,
                           DestReceiver *dest, DestReceiver *altdest,
                           QueryCompletion *qc);
static uint64 DoPortalRunFetch(Portal portal,
                               FetchDirection fdirection,
                               long count,
                               DestReceiver *dest);
static void DoPortalRewind(Portal portal);
 
 
/*
 * CreateQueryDesc
 */
QueryDesc *
CreateQueryDesc(PlannedStmt *plannedstmt,
                CachedPlan *cplan,
                const char *sourceText,
                Snapshot snapshot,
                Snapshot crosscheck_snapshot,
                DestReceiver *dest,
                ParamListInfo params,
                QueryEnvironment *queryEnv,
                int instrument_options)
{
    QueryDesc  *qd = (QueryDesc *) palloc(sizeof(QueryDesc));
 
    qd->operation = plannedstmt->commandType;   /* operation */
    qd->plannedstmt = plannedstmt;  /* plan */
    qd->cplan = cplan;          /* CachedPlan supplying the plannedstmt */
    qd->sourceText = sourceText;    /* query text */
    qd->snapshot = RegisterSnapshot(snapshot);  /* snapshot */
    /* RI check snapshot */
    qd->crosscheck_snapshot = RegisterSnapshot(crosscheck_snapshot);
    qd->dest = dest;            /* output dest */
    qd->params = params;        /* parameter values passed into query */
    qd->queryEnv = queryEnv;
    qd->instrument_options = instrument_options;    /* instrumentation wanted? */
 
    /* null these fields until set by ExecutorStart */
    qd->tupDesc = NULL;
    qd->estate = NULL;
    qd->planstate = NULL;
    qd->totaltime = NULL;
 
    /* not yet executed */
    qd->already_executed = false;
 
    return qd;
}
 
/*
 * FreeQueryDesc
 */
void
FreeQueryDesc(QueryDesc *qdesc)
{
    /* Can't be a live query */
    Assert(qdesc->estate == NULL);
 
    /* forget our snapshots */
    UnregisterSnapshot(qdesc->snapshot);
    UnregisterSnapshot(qdesc->crosscheck_snapshot);
 
    /* Only the QueryDesc itself need be freed */
    pfree(qdesc);
}
 
 
/*
 * ProcessQuery
 *      Execute a single plannable query within a PORTAL_MULTI_QUERY,
 *      PORTAL_ONE_RETURNING, or PORTAL_ONE_MOD_WITH portal
 *
 *  plan: the plan tree for the query
 *  cplan: CachedPlan supplying the plan
 *  plansource: CachedPlanSource supplying the cplan
 *  query_index: index of the query in plansource->query_list
 *  sourceText: the source text of the query
 *  params: any parameters needed
 *  dest: where to send results
 *  qc: where to store the command completion status data.
 *
 * qc may be NULL if caller doesn't want a status string.
 *
 * Must be called in a memory context that will be reset or deleted on
 * error; otherwise the executor's memory usage will be leaked.
 */
static void
ProcessQuery(PlannedStmt *plan,
             CachedPlan *cplan,
             CachedPlanSource *plansource,
             int query_index,
             const char *sourceText,
             ParamListInfo params,
             QueryEnvironment *queryEnv,
             DestReceiver *dest,
             QueryCompletion *qc)
{
    QueryDesc  *queryDesc;
 
    /*
     * Create the QueryDesc object
     */
    queryDesc = CreateQueryDesc(plan, cplan, sourceText,
                                GetActiveSnapshot(), InvalidSnapshot,
                                dest, params, queryEnv, 0);
 
    /*
     * Prepare the plan for execution
     */
    if (queryDesc->cplan)
    {
        ExecutorStartCachedPlan(queryDesc, 0, plansource, query_index);
        Assert(queryDesc->planstate);
    }
    else
    {
        if (!ExecutorStart(queryDesc, 0))
            elog(ERROR, "ExecutorStart() failed unexpectedly");
    }
 
    /*
     * Run the plan to completion.
     */
    ExecutorRun(queryDesc, ForwardScanDirection, 0);
 
    /*
     * Build command completion status data, if caller wants one.
     */
    if (qc)
    {
        switch (queryDesc->operation)
        {
            case CMD_SELECT:
                SetQueryCompletion(qc, CMDTAG_SELECT, queryDesc->estate->es_processed);
                break;
            case CMD_INSERT:
                SetQueryCompletion(qc, CMDTAG_INSERT, queryDesc->estate->es_processed);
                break;
            case CMD_UPDATE:
                SetQueryCompletion(qc, CMDTAG_UPDATE, queryDesc->estate->es_processed);
                break;
            case CMD_DELETE:
                SetQueryCompletion(qc, CMDTAG_DELETE, queryDesc->estate->es_processed);
                break;
            case CMD_MERGE:
                SetQueryCompletion(qc, CMDTAG_MERGE, queryDesc->estate->es_processed);
                break;
            default:
                SetQueryCompletion(qc, CMDTAG_UNKNOWN, queryDesc->estate->es_processed);
                break;
        }
    }
 
    /*
     * Now, we close down all the scans and free allocated resources.
     */
    ExecutorFinish(queryDesc);
    ExecutorEnd(queryDesc);
 
    FreeQueryDesc(queryDesc);
}
 
/*
 * ChoosePortalStrategy
 *      Select portal execution strategy given the intended statement list.
 *
 * The list elements can be Querys or PlannedStmts.
 * That's more general than portals need, but plancache.c uses this too.
 *
 * See the comments in portal.h.
 */
PortalStrategy
ChoosePortalStrategy(List *stmts)
{
    int         nSetTag;
    ListCell   *lc;
 
    /*
     * PORTAL_ONE_SELECT and PORTAL_UTIL_SELECT need only consider the
     * single-statement case, since there are no rewrite rules that can add
     * auxiliary queries to a SELECT or a utility command. PORTAL_ONE_MOD_WITH
     * likewise allows only one top-level statement.
     */
    if (list_length(stmts) == 1)
    {
        Node       *stmt = (Node *) linitial(stmts);
 
        if (IsA(stmt, Query))
        {
            Query      *query = (Query *) stmt;
 
            if (query->canSetTag)
            {
                if (query->commandType == CMD_SELECT)
                {
                    if (query->hasModifyingCTE)
                        return PORTAL_ONE_MOD_WITH;
                    else
                        return PORTAL_ONE_SELECT;
                }
                if (query->commandType == CMD_UTILITY)
                {
                    if (UtilityReturnsTuples(query->utilityStmt))
                        return PORTAL_UTIL_SELECT;
                    /* it can't be ONE_RETURNING, so give up */
                    return PORTAL_MULTI_QUERY;
                }
            }
        }
        else if (IsA(stmt, PlannedStmt))
        {
            PlannedStmt *pstmt = (PlannedStmt *) stmt;
 
            if (pstmt->canSetTag)
            {
                if (pstmt->commandType == CMD_SELECT)
                {
                    if (pstmt->hasModifyingCTE)
                        return PORTAL_ONE_MOD_WITH;
                    else
                        return PORTAL_ONE_SELECT;
                }
                if (pstmt->commandType == CMD_UTILITY)
                {
                    if (UtilityReturnsTuples(pstmt->utilityStmt))
                        return PORTAL_UTIL_SELECT;
                    /* it can't be ONE_RETURNING, so give up */
                    return PORTAL_MULTI_QUERY;
                }
            }
        }
        else
            elog(ERROR, "unrecognized node type: %d", (int) nodeTag(stmt));
    }
 
    /*
     * PORTAL_ONE_RETURNING has to allow auxiliary queries added by rewrite.
     * Choose PORTAL_ONE_RETURNING if there is exactly one canSetTag query and
     * it has a RETURNING list.
     */
    nSetTag = 0;
    foreach(lc, stmts)
    {
        Node       *stmt = (Node *) lfirst(lc);
 
        if (IsA(stmt, Query))
        {
            Query      *query = (Query *) stmt;
 
            if (query->canSetTag)
            {
                if (++nSetTag > 1)
                    return PORTAL_MULTI_QUERY;  /* no need to look further */
                if (query->commandType == CMD_UTILITY ||
                    query->returningList == NIL)
                    return PORTAL_MULTI_QUERY;  /* no need to look further */
            }
        }
        else if (IsA(stmt, PlannedStmt))
        {
            PlannedStmt *pstmt = (PlannedStmt *) stmt;
 
            if (pstmt->canSetTag)
            {
                if (++nSetTag > 1)
                    return PORTAL_MULTI_QUERY;  /* no need to look further */
                if (pstmt->commandType == CMD_UTILITY ||
                    !pstmt->hasReturning)
                    return PORTAL_MULTI_QUERY;  /* no need to look further */
            }
        }
        else
            elog(ERROR, "unrecognized node type: %d", (int) nodeTag(stmt));
    }
    if (nSetTag == 1)
        return PORTAL_ONE_RETURNING;
 
    /* Else, it's the general case... */
    return PORTAL_MULTI_QUERY;
}
 
/*
 * FetchPortalTargetList
 *      Given a portal that returns tuples, extract the query targetlist.
 *      Returns NIL if the portal doesn't have a determinable targetlist.
 *
 * Note: do not modify the result.
 */
List *
FetchPortalTargetList(Portal portal)
{
    /* no point in looking if we determined it doesn't return tuples */
    if (portal->strategy == PORTAL_MULTI_QUERY)
        return NIL;
    /* get the primary statement and find out what it returns */
    return FetchStatementTargetList((Node *) PortalGetPrimaryStmt(portal));
}
 
/*
 * FetchStatementTargetList
 *      Given a statement that returns tuples, extract the query targetlist.
 *      Returns NIL if the statement doesn't have a determinable targetlist.
 *
 * This can be applied to a Query or a PlannedStmt.
 * That's more general than portals need, but plancache.c uses this too.
 *
 * Note: do not modify the result.
 *
 * XXX be careful to keep this in sync with UtilityReturnsTuples.
 */
List *
FetchStatementTargetList(Node *stmt)
{
    if (stmt == NULL)
        return NIL;
    if (IsA(stmt, Query))
    {
        Query      *query = (Query *) stmt;
 
        if (query->commandType == CMD_UTILITY)
        {
            /* transfer attention to utility statement */
            stmt = query->utilityStmt;
        }
        else
        {
            if (query->commandType == CMD_SELECT)
                return query->targetList;
            if (query->returningList)
                return query->returningList;
            return NIL;
        }
    }
    if (IsA(stmt, PlannedStmt))
    {
        PlannedStmt *pstmt = (PlannedStmt *) stmt;
 
        if (pstmt->commandType == CMD_UTILITY)
        {
            /* transfer attention to utility statement */
            stmt = pstmt->utilityStmt;
        }
        else
        {
            if (pstmt->commandType == CMD_SELECT)
                return pstmt->planTree->targetlist;
            if (pstmt->hasReturning)
                return pstmt->planTree->targetlist;
            return NIL;
        }
    }
    if (IsA(stmt, FetchStmt))
    {
        FetchStmt  *fstmt = (FetchStmt *) stmt;
        Portal      subportal;
 
        Assert(!fstmt->ismove);
        subportal = GetPortalByName(fstmt->portalname);
        Assert(PortalIsValid(subportal));
        return FetchPortalTargetList(subportal);
    }
    if (IsA(stmt, ExecuteStmt))
    {
        ExecuteStmt *estmt = (ExecuteStmt *) stmt;
        PreparedStatement *entry;
 
        entry = FetchPreparedStatement(estmt->name, true);
        return FetchPreparedStatementTargetList(entry);
    }
    return NIL;
}
 
/*
 * PortalStart
 *      Prepare a portal for execution.
 *
 * Caller must already have created the portal, done PortalDefineQuery(),
 * and adjusted portal options if needed.
 *
 * If parameters are needed by the query, they must be passed in "params"
 * (caller is responsible for giving them appropriate lifetime).
 *
 * The caller can also provide an initial set of "eflags" to be passed to
 * ExecutorStart (but note these can be modified internally, and they are
 * currently only honored for PORTAL_ONE_SELECT portals).  Most callers
 * should simply pass zero.
 *
 * The caller can optionally pass a snapshot to be used; pass InvalidSnapshot
 * for the normal behavior of setting a new snapshot.  This parameter is
 * presently ignored for non-PORTAL_ONE_SELECT portals (it's only intended
 * to be used for cursors).
 *
 * On return, portal is ready to accept PortalRun() calls, and the result
 * tupdesc (if any) is known.
 */
void
PortalStart(Portal portal, ParamListInfo params,
            int eflags, Snapshot snapshot)
{
    Portal      saveActivePortal;
    ResourceOwner saveResourceOwner;
    MemoryContext savePortalContext;
    MemoryContext oldContext;
    QueryDesc  *queryDesc;
    int         myeflags;
 
    Assert(PortalIsValid(portal));
    Assert(portal->status == PORTAL_DEFINED);
 
    /*
     * Set up global portal context pointers.
     */
    saveActivePortal = ActivePortal;
    saveResourceOwner = CurrentResourceOwner;
    savePortalContext = PortalContext;
    PG_TRY();
    {
        ActivePortal = portal;
        if (portal->resowner)
            CurrentResourceOwner = portal->resowner;
        PortalContext = portal->portalContext;
 
        oldContext = MemoryContextSwitchTo(PortalContext);
 
        /* Must remember portal param list, if any */
        portal->portalParams = params;
 
        /*
         * Determine the portal execution strategy
         */
        portal->strategy = ChoosePortalStrategy(portal->stmts);
 
        /*
         * Fire her up according to the strategy
         */
        switch (portal->strategy)
        {
            case PORTAL_ONE_SELECT:
 
                /* Must set snapshot before starting executor. */
                if (snapshot)
                    PushActiveSnapshot(snapshot);
                else
                    PushActiveSnapshot(GetTransactionSnapshot());
 
                /*
                 * We could remember the snapshot in portal->portalSnapshot,
                 * but presently there seems no need to, as this code path
                 * cannot be used for non-atomic execution.  Hence there can't
                 * be any commit/abort that might destroy the snapshot.  Since
                 * we don't do that, there's also no need to force a
                 * non-default nesting level for the snapshot.
                 */
 
                /*
                 * Create QueryDesc in portal's context; for the moment, set
                 * the destination to DestNone.
                 */
                queryDesc = CreateQueryDesc(linitial_node(PlannedStmt, portal->stmts),
                                            portal->cplan,
                                            portal->sourceText,
                                            GetActiveSnapshot(),
                                            InvalidSnapshot,
                                            None_Receiver,
                                            params,
                                            portal->queryEnv,
                                            0);
 
                /*
                 * If it's a scrollable cursor, executor needs to support
                 * REWIND and backwards scan, as well as whatever the caller
                 * might've asked for.
                 */
                if (portal->cursorOptions & CURSOR_OPT_SCROLL)
                    myeflags = eflags | EXEC_FLAG_REWIND | EXEC_FLAG_BACKWARD;
                else
                    myeflags = eflags;
 
                /*
                 * Prepare the plan for execution.
                 */
                if (portal->cplan)
                {
                    ExecutorStartCachedPlan(queryDesc, myeflags,
                                            portal->plansource, 0);
                    Assert(queryDesc->planstate);
                }
                else
                {
                    if (!ExecutorStart(queryDesc, myeflags))
                        elog(ERROR, "ExecutorStart() failed unexpectedly");
                }
 
                /*
                 * This tells PortalCleanup to shut down the executor
                 */
                portal->queryDesc = queryDesc;
 
                /*
                 * Remember tuple descriptor (computed by ExecutorStart)
                 */
                portal->tupDesc = queryDesc->tupDesc;
 
                /*
                 * Reset cursor position data to "start of query"
                 */
                portal->atStart = true;
                portal->atEnd = false;  /* allow fetches */
                portal->portalPos = 0;
 
                PopActiveSnapshot();
                break;
 
            case PORTAL_ONE_RETURNING:
            case PORTAL_ONE_MOD_WITH:
 
                /*
                 * We don't start the executor until we are told to run the
                 * portal.  We do need to set up the result tupdesc.
                 */
                {
                    PlannedStmt *pstmt;
 
                    pstmt = PortalGetPrimaryStmt(portal);
                    portal->tupDesc =
                        ExecCleanTypeFromTL(pstmt->planTree->targetlist);
                }
 
                /*
                 * Reset cursor position data to "start of query"
                 */
                portal->atStart = true;
                portal->atEnd = false;  /* allow fetches */
                portal->portalPos = 0;
                break;
 
            case PORTAL_UTIL_SELECT:
 
                /*
                 * We don't set snapshot here, because PortalRunUtility will
                 * take care of it if needed.
                 */
                {
                    PlannedStmt *pstmt = PortalGetPrimaryStmt(portal);
 
                    Assert(pstmt->commandType == CMD_UTILITY);
                    portal->tupDesc = UtilityTupleDescriptor(pstmt->utilityStmt);
                }
 
                /*
                 * Reset cursor position data to "start of query"
                 */
                portal->atStart = true;
                portal->atEnd = false;  /* allow fetches */
                portal->portalPos = 0;
                break;
 
            case PORTAL_MULTI_QUERY:
                /* Need do nothing now */
                portal->tupDesc = NULL;
                break;
        }
    }
    PG_CATCH();
    {
        /* Uncaught error while executing portal: mark it dead */
        MarkPortalFailed(portal);
 
        /* Restore global vars and propagate error */
        ActivePortal = saveActivePortal;
        CurrentResourceOwner = saveResourceOwner;
        PortalContext = savePortalContext;
 
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    MemoryContextSwitchTo(oldContext);
 
    ActivePortal = saveActivePortal;
    CurrentResourceOwner = saveResourceOwner;
    PortalContext = savePortalContext;
 
    portal->status = PORTAL_READY;
}
 
/*
 * PortalSetResultFormat
 *      Select the format codes for a portal's output.
 *
 * This must be run after PortalStart for a portal that will be read by
 * a DestRemote or DestRemoteExecute destination.  It is not presently needed
 * for other destination types.
 *
 * formats[] is the client format request, as per Bind message conventions.
 */
void
PortalSetResultFormat(Portal portal, int nFormats, int16 *formats)
{
    int         natts;
    int         i;
 
    /* Do nothing if portal won't return tuples */
    if (portal->tupDesc == NULL)
        return;
    natts = portal->tupDesc->natts;
    portal->formats = (int16 *)
        MemoryContextAlloc(portal->portalContext,
                           natts * sizeof(int16));
    if (nFormats > 1)
    {
        /* format specified for each column */
        if (nFormats != natts)
            ereport(ERROR,
                    (errcode(ERRCODE_PROTOCOL_VIOLATION),
                     errmsg("bind message has %d result formats but query has %d columns",
                            nFormats, natts)));
        memcpy(portal->formats, formats, natts * sizeof(int16));
    }
    else if (nFormats > 0)
    {
        /* single format specified, use for all columns */
        int16       format1 = formats[0];
 
        for (i = 0; i < natts; i++)
            portal->formats[i] = format1;
    }
    else
    {
        /* use default format for all columns */
        for (i = 0; i < natts; i++)
            portal->formats[i] = 0;
    }
}
 
/*
 * PortalRun
 *      Run a portal's query or queries.
 *
 * count <= 0 is interpreted as a no-op: the destination gets started up
 * and shut down, but nothing else happens.  Also, count == FETCH_ALL is
 * interpreted as "all rows".  Note that count is ignored in multi-query
 * situations, where we always run the portal to completion.
 *
 * isTopLevel: true if query is being executed at backend "top level"
 * (that is, directly from a client command message)
 *
 * dest: where to send output of primary (canSetTag) query
 *
 * altdest: where to send output of non-primary queries
 *
 * qc: where to store command completion status data.
 *      May be NULL if caller doesn't want status data.
 *
 * Returns true if the portal's execution is complete, false if it was
 * suspended due to exhaustion of the count parameter.
 */
bool
PortalRun(Portal portal, long count, bool isTopLevel,
          DestReceiver *dest, DestReceiver *altdest,
          QueryCompletion *qc)
{
    bool        result;
    uint64      nprocessed;
    ResourceOwner saveTopTransactionResourceOwner;
    MemoryContext saveTopTransactionContext;
    Portal      saveActivePortal;
    ResourceOwner saveResourceOwner;
    MemoryContext savePortalContext;
    MemoryContext saveMemoryContext;
 
    Assert(PortalIsValid(portal));
 
    TRACE_POSTGRESQL_QUERY_EXECUTE_START();
 
    /* Initialize empty completion data */
    if (qc)
        InitializeQueryCompletion(qc);
 
    if (log_executor_stats && portal->strategy != PORTAL_MULTI_QUERY)
    {
        elog(DEBUG3, "PortalRun");
        /* PORTAL_MULTI_QUERY logs its own stats per query */
        ResetUsage();
    }
 
    /*
     * Check for improper portal use, and mark portal active.
     */
    MarkPortalActive(portal);
 
    /*
     * Set up global portal context pointers.
     *
     * We have to play a special game here to support utility commands like
     * VACUUM and CLUSTER, which internally start and commit transactions.
     * When we are called to execute such a command, CurrentResourceOwner will
     * be pointing to the TopTransactionResourceOwner --- which will be
     * destroyed and replaced in the course of the internal commit and
     * restart.  So we need to be prepared to restore it as pointing to the
     * exit-time TopTransactionResourceOwner.  (Ain't that ugly?  This idea of
     * internally starting whole new transactions is not good.)
     * CurrentMemoryContext has a similar problem, but the other pointers we
     * save here will be NULL or pointing to longer-lived objects.
     */
    saveTopTransactionResourceOwner = TopTransactionResourceOwner;
    saveTopTransactionContext = TopTransactionContext;
    saveActivePortal = ActivePortal;
    saveResourceOwner = CurrentResourceOwner;
    savePortalContext = PortalContext;
    saveMemoryContext = CurrentMemoryContext;
    PG_TRY();
    {
        ActivePortal = portal;
        if (portal->resowner)
            CurrentResourceOwner = portal->resowner;
        PortalContext = portal->portalContext;
 
        MemoryContextSwitchTo(PortalContext);
 
        switch (portal->strategy)
        {
            case PORTAL_ONE_SELECT:
            case PORTAL_ONE_RETURNING:
            case PORTAL_ONE_MOD_WITH:
            case PORTAL_UTIL_SELECT:
 
                /*
                 * If we have not yet run the command, do so, storing its
                 * results in the portal's tuplestore.  But we don't do that
                 * for the PORTAL_ONE_SELECT case.
                 */
                if (portal->strategy != PORTAL_ONE_SELECT && !portal->holdStore)
                    FillPortalStore(portal, isTopLevel);
 
                /*
                 * Now fetch desired portion of results.
                 */
                nprocessed = PortalRunSelect(portal, true, count, dest);
 
                /*
                 * If the portal result contains a command tag and the caller
                 * gave us a pointer to store it, copy it and update the
                 * rowcount.
                 */
                if (qc && portal->qc.commandTag != CMDTAG_UNKNOWN)
                {
                    CopyQueryCompletion(qc, &portal->qc);
                    qc->nprocessed = nprocessed;
                }
 
                /* Mark portal not active */
                portal->status = PORTAL_READY;
 
                /*
                 * Since it's a forward fetch, say DONE iff atEnd is now true.
                 */
                result = portal->atEnd;
                break;
 
            case PORTAL_MULTI_QUERY:
                PortalRunMulti(portal, isTopLevel, false,
                               dest, altdest, qc);
 
                /* Prevent portal's commands from being re-executed */
                MarkPortalDone(portal);
 
                /* Always complete at end of RunMulti */
                result = true;
                break;
 
            default:
                elog(ERROR, "unrecognized portal strategy: %d",
                     (int) portal->strategy);
                result = false; /* keep compiler quiet */
                break;
        }
    }
    PG_CATCH();
    {
        /* Uncaught error while executing portal: mark it dead */
        MarkPortalFailed(portal);
 
        /* Restore global vars and propagate error */
        if (saveMemoryContext == saveTopTransactionContext)
            MemoryContextSwitchTo(TopTransactionContext);
        else
            MemoryContextSwitchTo(saveMemoryContext);
        ActivePortal = saveActivePortal;
        if (saveResourceOwner == saveTopTransactionResourceOwner)
            CurrentResourceOwner = TopTransactionResourceOwner;
        else
            CurrentResourceOwner = saveResourceOwner;
        PortalContext = savePortalContext;
 
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    if (saveMemoryContext == saveTopTransactionContext)
        MemoryContextSwitchTo(TopTransactionContext);
    else
        MemoryContextSwitchTo(saveMemoryContext);
    ActivePortal = saveActivePortal;
    if (saveResourceOwner == saveTopTransactionResourceOwner)
        CurrentResourceOwner = TopTransactionResourceOwner;
    else
        CurrentResourceOwner = saveResourceOwner;
    PortalContext = savePortalContext;
 
    if (log_executor_stats && portal->strategy != PORTAL_MULTI_QUERY)
        ShowUsage("EXECUTOR STATISTICS");
 
    TRACE_POSTGRESQL_QUERY_EXECUTE_DONE();
 
    return result;
}
 
/*
 * PortalRunSelect
 *      Execute a portal's query in PORTAL_ONE_SELECT mode, and also
 *      when fetching from a completed holdStore in PORTAL_ONE_RETURNING,
 *      PORTAL_ONE_MOD_WITH, and PORTAL_UTIL_SELECT cases.
 *
 * This handles simple N-rows-forward-or-backward cases.  For more complex
 * nonsequential access to a portal, see PortalRunFetch.
 *
 * count <= 0 is interpreted as a no-op: the destination gets started up
 * and shut down, but nothing else happens.  Also, count == FETCH_ALL is
 * interpreted as "all rows".  (cf FetchStmt.howMany)
 *
 * Caller must already have validated the Portal and done appropriate
 * setup (cf. PortalRun).
 *
 * Returns number of rows processed (suitable for use in result tag)
 */
static uint64
PortalRunSelect(Portal portal,
                bool forward,
                long count,
                DestReceiver *dest)
{
    QueryDesc  *queryDesc;
    ScanDirection direction;
    uint64      nprocessed;
 
    /*
     * NB: queryDesc will be NULL if we are fetching from a held cursor or a
     * completed utility query; can't use it in that path.
     */
    queryDesc = portal->queryDesc;
 
    /* Caller messed up if we have neither a ready query nor held data. */
    Assert(queryDesc || portal->holdStore);
 
    /*
     * Force the queryDesc destination to the right thing.  This supports
     * MOVE, for example, which will pass in dest = DestNone.  This is okay to
     * change as long as we do it on every fetch.  (The Executor must not
     * assume that dest never changes.)
     */
    if (queryDesc)
        queryDesc->dest = dest;
 
    /*
     * Determine which direction to go in, and check to see if we're already
     * at the end of the available tuples in that direction.  If so, set the
     * direction to NoMovement to avoid trying to fetch any tuples.  (This
     * check exists because not all plan node types are robust about being
     * called again if they've already returned NULL once.)  Then call the
     * executor (we must not skip this, because the destination needs to see a
     * setup and shutdown even if no tuples are available).  Finally, update
     * the portal position state depending on the number of tuples that were
     * retrieved.
     */
    if (forward)
    {
        if (portal->atEnd || count <= 0)
        {
            direction = NoMovementScanDirection;
            count = 0;          /* don't pass negative count to executor */
        }
        else
            direction = ForwardScanDirection;
 
        /* In the executor, zero count processes all rows */
        if (count == FETCH_ALL)
            count = 0;
 
        if (portal->holdStore)
            nprocessed = RunFromStore(portal, direction, (uint64) count, dest);
        else
        {
            PushActiveSnapshot(queryDesc->snapshot);
            ExecutorRun(queryDesc, direction, (uint64) count);
            nprocessed = queryDesc->estate->es_processed;
            PopActiveSnapshot();
        }
 
        if (!ScanDirectionIsNoMovement(direction))
        {
            if (nprocessed > 0)
                portal->atStart = false;    /* OK to go backward now */
            if (count == 0 || nprocessed < (uint64) count)
                portal->atEnd = true;   /* we retrieved 'em all */
            portal->portalPos += nprocessed;
        }
    }
    else
    {
        if (portal->cursorOptions & CURSOR_OPT_NO_SCROLL)
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("cursor can only scan forward"),
                     errhint("Declare it with SCROLL option to enable backward scan.")));
 
        if (portal->atStart || count <= 0)
        {
            direction = NoMovementScanDirection;
            count = 0;          /* don't pass negative count to executor */
        }
        else
            direction = BackwardScanDirection;
 
        /* In the executor, zero count processes all rows */
        if (count == FETCH_ALL)
            count = 0;
 
        if (portal->holdStore)
            nprocessed = RunFromStore(portal, direction, (uint64) count, dest);
        else
        {
            PushActiveSnapshot(queryDesc->snapshot);
            ExecutorRun(queryDesc, direction, (uint64) count);
            nprocessed = queryDesc->estate->es_processed;
            PopActiveSnapshot();
        }
 
        if (!ScanDirectionIsNoMovement(direction))
        {
            if (nprocessed > 0 && portal->atEnd)
            {
                portal->atEnd = false;  /* OK to go forward now */
                portal->portalPos++;    /* adjust for endpoint case */
            }
            if (count == 0 || nprocessed < (uint64) count)
            {
                portal->atStart = true; /* we retrieved 'em all */
                portal->portalPos = 0;
            }
            else
            {
                portal->portalPos -= nprocessed;
            }
        }
    }
 
    return nprocessed;
}
 
/*
 * FillPortalStore
 *      Run the query and load result tuples into the portal's tuple store.
 *
 * This is used for PORTAL_ONE_RETURNING, PORTAL_ONE_MOD_WITH, and
 * PORTAL_UTIL_SELECT cases only.
 */
static void
FillPortalStore(Portal portal, bool isTopLevel)
{
    DestReceiver *treceiver;
    QueryCompletion qc;
 
    InitializeQueryCompletion(&qc);
    PortalCreateHoldStore(portal);
    treceiver = CreateDestReceiver(DestTuplestore);
    SetTuplestoreDestReceiverParams(treceiver,
                                    portal->holdStore,
                                    portal->holdContext,
                                    false,
                                    NULL,
                                    NULL);
 
    switch (portal->strategy)
    {
        case PORTAL_ONE_RETURNING:
        case PORTAL_ONE_MOD_WITH:
 
            /*
             * Run the portal to completion just as for the default
             * PORTAL_MULTI_QUERY case, but send the primary query's output to
             * the tuplestore.  Auxiliary query outputs are discarded. Set the
             * portal's holdSnapshot to the snapshot used (or a copy of it).
             */
            PortalRunMulti(portal, isTopLevel, true,
                           treceiver, None_Receiver, &qc);
            break;
 
        case PORTAL_UTIL_SELECT:
            PortalRunUtility(portal, linitial_node(PlannedStmt, portal->stmts),
                             isTopLevel, true, treceiver, &qc);
            break;
 
        default:
            elog(ERROR, "unsupported portal strategy: %d",
                 (int) portal->strategy);
            break;
    }
 
    /* Override portal completion data with actual command results */
    if (qc.commandTag != CMDTAG_UNKNOWN)
        CopyQueryCompletion(&portal->qc, &qc);
 
    treceiver->rDestroy(treceiver);
}
 
/*
 * RunFromStore
 *      Fetch tuples from the portal's tuple store.
 *
 * Calling conventions are similar to ExecutorRun, except that we
 * do not depend on having a queryDesc or estate.  Therefore we return the
 * number of tuples processed as the result, not in estate->es_processed.
 *
 * One difference from ExecutorRun is that the destination receiver functions
 * are run in the caller's memory context (since we have no estate).  Watch
 * out for memory leaks.
 */
static uint64
RunFromStore(Portal portal, ScanDirection direction, uint64 count,
             DestReceiver *dest)
{
    uint64      current_tuple_count = 0;
    TupleTableSlot *slot;
 
    slot = MakeSingleTupleTableSlot(portal->tupDesc, &TTSOpsMinimalTuple);
 
    dest->rStartup(dest, CMD_SELECT, portal->tupDesc);
 
    if (ScanDirectionIsNoMovement(direction))
    {
        /* do nothing except start/stop the destination */
    }
    else
    {
        bool        forward = ScanDirectionIsForward(direction);
 
        for (;;)
        {
            MemoryContext oldcontext;
            bool        ok;
 
            oldcontext = MemoryContextSwitchTo(portal->holdContext);
 
            ok = tuplestore_gettupleslot(portal->holdStore, forward, false,
                                         slot);
 
            MemoryContextSwitchTo(oldcontext);
 
            if (!ok)
                break;
 
            /*
             * If we are not able to send the tuple, we assume the destination
             * has closed and no more tuples can be sent. If that's the case,
             * end the loop.
             */
            if (!dest->receiveSlot(slot, dest))
                break;
 
            ExecClearTuple(slot);
 
            /*
             * check our tuple count.. if we've processed the proper number
             * then quit, else loop again and process more tuples. Zero count
             * means no limit.
             */
            current_tuple_count++;
            if (count && count == current_tuple_count)
                break;
        }
    }
 
    dest->rShutdown(dest);
 
    ExecDropSingleTupleTableSlot(slot);
 
    return current_tuple_count;
}
 
/*
 * PortalRunUtility
 *      Execute a utility statement inside a portal.
 */
static void
PortalRunUtility(Portal portal, PlannedStmt *pstmt,
                 bool isTopLevel, bool setHoldSnapshot,
                 DestReceiver *dest, QueryCompletion *qc)
{
    /*
     * Set snapshot if utility stmt needs one.
     */
    if (PlannedStmtRequiresSnapshot(pstmt))
    {
        Snapshot    snapshot = GetTransactionSnapshot();
 
        /* If told to, register the snapshot we're using and save in portal */
        if (setHoldSnapshot)
        {
            snapshot = RegisterSnapshot(snapshot);
            portal->holdSnapshot = snapshot;
        }
 
        /*
         * In any case, make the snapshot active and remember it in portal.
         * Because the portal now references the snapshot, we must tell
         * snapmgr.c that the snapshot belongs to the portal's transaction
         * level, else we risk portalSnapshot becoming a dangling pointer.
         */
        PushActiveSnapshotWithLevel(snapshot, portal->createLevel);
        /* PushActiveSnapshotWithLevel might have copied the snapshot */
        portal->portalSnapshot = GetActiveSnapshot();
    }
    else
        portal->portalSnapshot = NULL;
 
    ProcessUtility(pstmt,
                   portal->sourceText,
                   (portal->cplan != NULL), /* protect tree if in plancache */
                   isTopLevel ? PROCESS_UTILITY_TOPLEVEL : PROCESS_UTILITY_QUERY,
                   portal->portalParams,
                   portal->queryEnv,
                   dest,
                   qc);
 
    /* Some utility statements may change context on us */
    MemoryContextSwitchTo(portal->portalContext);
 
    /*
     * Some utility commands (e.g., VACUUM) pop the ActiveSnapshot stack from
     * under us, so don't complain if it's now empty.  Otherwise, our snapshot
     * should be the top one; pop it.  Note that this could be a different
     * snapshot from the one we made above; see EnsurePortalSnapshotExists.
     */
    if (portal->portalSnapshot != NULL && ActiveSnapshotSet())
    {
        Assert(portal->portalSnapshot == GetActiveSnapshot());
        PopActiveSnapshot();
    }
    portal->portalSnapshot = NULL;
}
 
/*
 * PortalRunMulti
 *      Execute a portal's queries in the general case (multi queries
 *      or non-SELECT-like queries)
 */
static void
PortalRunMulti(Portal portal,
               bool isTopLevel, bool setHoldSnapshot,
               DestReceiver *dest, DestReceiver *altdest,
               QueryCompletion *qc)
{
    bool        active_snapshot_set = false;
    ListCell   *stmtlist_item;
    int         query_index = 0;
 
    /*
     * If the destination is DestRemoteExecute, change to DestNone.  The
     * reason is that the client won't be expecting any tuples, and indeed has
     * no way to know what they are, since there is no provision for Describe
     * to send a RowDescription message when this portal execution strategy is
     * in effect.  This presently will only affect SELECT commands added to
     * non-SELECT queries by rewrite rules: such commands will be executed,
     * but the results will be discarded unless you use "simple Query"
     * protocol.
     */
    if (dest->mydest == DestRemoteExecute)
        dest = None_Receiver;
    if (altdest->mydest == DestRemoteExecute)
        altdest = None_Receiver;
 
    /*
     * Loop to handle the individual queries generated from a single parsetree
     * by analysis and rewrite.
     */
    foreach(stmtlist_item, portal->stmts)
    {
        PlannedStmt *pstmt = lfirst_node(PlannedStmt, stmtlist_item);
 
        /*
         * If we got a cancel signal in prior command, quit
         */
        CHECK_FOR_INTERRUPTS();
 
        if (pstmt->utilityStmt == NULL)
        {
            /*
             * process a plannable query.
             */
            TRACE_POSTGRESQL_QUERY_EXECUTE_START();
 
            if (log_executor_stats)
                ResetUsage();
 
            /*
             * Must always have a snapshot for plannable queries.  First time
             * through, take a new snapshot; for subsequent queries in the
             * same portal, just update the snapshot's copy of the command
             * counter.
             */
            if (!active_snapshot_set)
            {
                Snapshot    snapshot = GetTransactionSnapshot();
 
                /* If told to, register the snapshot and save in portal */
                if (setHoldSnapshot)
                {
                    snapshot = RegisterSnapshot(snapshot);
                    portal->holdSnapshot = snapshot;
                }
 
                /*
                 * We can't have the holdSnapshot also be the active one,
                 * because UpdateActiveSnapshotCommandId would complain.  So
                 * force an extra snapshot copy.  Plain PushActiveSnapshot
                 * would have copied the transaction snapshot anyway, so this
                 * only adds a copy step when setHoldSnapshot is true.  (It's
                 * okay for the command ID of the active snapshot to diverge
                 * from what holdSnapshot has.)
                 */
                PushCopiedSnapshot(snapshot);
 
                /*
                 * As for PORTAL_ONE_SELECT portals, it does not seem
                 * necessary to maintain portal->portalSnapshot here.
                 */
 
                active_snapshot_set = true;
            }
            else
                UpdateActiveSnapshotCommandId();
 
            if (pstmt->canSetTag)
            {
                /* statement can set tag string */
                ProcessQuery(pstmt,
                             portal->cplan,
                             portal->plansource,
                             query_index,
                             portal->sourceText,
                             portal->portalParams,
                             portal->queryEnv,
                             dest, qc);
            }
            else
            {
                /* stmt added by rewrite cannot set tag */
                ProcessQuery(pstmt,
                             portal->cplan,
                             portal->plansource,
                             query_index,
                             portal->sourceText,
                             portal->portalParams,
                             portal->queryEnv,
                             altdest, NULL);
            }
 
            if (log_executor_stats)
                ShowUsage("EXECUTOR STATISTICS");
 
            TRACE_POSTGRESQL_QUERY_EXECUTE_DONE();
        }
        else
        {
            /*
             * process utility functions (create, destroy, etc..)
             *
             * We must not set a snapshot here for utility commands (if one is
             * needed, PortalRunUtility will do it).  If a utility command is
             * alone in a portal then everything's fine.  The only case where
             * a utility command can be part of a longer list is that rules
             * are allowed to include NotifyStmt.  NotifyStmt doesn't care
             * whether it has a snapshot or not, so we just leave the current
             * snapshot alone if we have one.
             */
            if (pstmt->canSetTag)
            {
                Assert(!active_snapshot_set);
                /* statement can set tag string */
                PortalRunUtility(portal, pstmt, isTopLevel, false,
                                 dest, qc);
            }
            else
            {
                Assert(IsA(pstmt->utilityStmt, NotifyStmt));
                /* stmt added by rewrite cannot set tag */
                PortalRunUtility(portal, pstmt, isTopLevel, false,
                                 altdest, NULL);
            }
        }
 
        /*
         * Clear subsidiary contexts to recover temporary memory.
         */
        Assert(portal->portalContext == CurrentMemoryContext);
 
        MemoryContextDeleteChildren(portal->portalContext);
 
        /*
         * Avoid crashing if portal->stmts has been reset.  This can only
         * occur if a CALL or DO utility statement executed an internal
         * COMMIT/ROLLBACK (cf PortalReleaseCachedPlan).  The CALL or DO must
         * have been the only statement in the portal, so there's nothing left
         * for us to do; but we don't want to dereference a now-dangling list
         * pointer.
         */
        if (portal->stmts == NIL)
            break;
 
        /*
         * Increment command counter between queries, but not after the last
         * one.
         */
        if (lnext(portal->stmts, stmtlist_item) != NULL)
            CommandCounterIncrement();
 
        query_index++;
    }
 
    /* Pop the snapshot if we pushed one. */
    if (active_snapshot_set)
        PopActiveSnapshot();
 
    /*
     * If a query completion data was supplied, use it.  Otherwise use the
     * portal's query completion data.
     *
     * Exception: Clients expect INSERT/UPDATE/DELETE tags to have counts, so
     * fake them with zeros.  This can happen with DO INSTEAD rules if there
     * is no replacement query of the same type as the original.  We print "0
     * 0" here because technically there is no query of the matching tag type,
     * and printing a non-zero count for a different query type seems wrong,
     * e.g.  an INSERT that does an UPDATE instead should not print "0 1" if
     * one row was updated.  See QueryRewrite(), step 3, for details.
     */
    if (qc && qc->commandTag == CMDTAG_UNKNOWN)
    {
        if (portal->qc.commandTag != CMDTAG_UNKNOWN)
            CopyQueryCompletion(qc, &portal->qc);
        /* If the caller supplied a qc, we should have set it by now. */
        Assert(qc->commandTag != CMDTAG_UNKNOWN);
    }
}
 
/*
 * PortalRunFetch
 *      Variant form of PortalRun that supports SQL FETCH directions.
 *
 * Note: we presently assume that no callers of this want isTopLevel = true.
 *
 * count <= 0 is interpreted as a no-op: the destination gets started up
 * and shut down, but nothing else happens.  Also, count == FETCH_ALL is
 * interpreted as "all rows".  (cf FetchStmt.howMany)
 *
 * Returns number of rows processed (suitable for use in result tag)
 */
uint64
PortalRunFetch(Portal portal,
               FetchDirection fdirection,
               long count,
               DestReceiver *dest)
{
    uint64      result;
    Portal      saveActivePortal;
    ResourceOwner saveResourceOwner;
    MemoryContext savePortalContext;
    MemoryContext oldContext;
 
    Assert(PortalIsValid(portal));
 
    /*
     * Check for improper portal use, and mark portal active.
     */
    MarkPortalActive(portal);
 
    /*
     * Set up global portal context pointers.
     */
    saveActivePortal = ActivePortal;
    saveResourceOwner = CurrentResourceOwner;
    savePortalContext = PortalContext;
    PG_TRY();
    {
        ActivePortal = portal;
        if (portal->resowner)
            CurrentResourceOwner = portal->resowner;
        PortalContext = portal->portalContext;
 
        oldContext = MemoryContextSwitchTo(PortalContext);
 
        switch (portal->strategy)
        {
            case PORTAL_ONE_SELECT:
                result = DoPortalRunFetch(portal, fdirection, count, dest);
                break;
 
            case PORTAL_ONE_RETURNING:
            case PORTAL_ONE_MOD_WITH:
            case PORTAL_UTIL_SELECT:
 
                /*
                 * If we have not yet run the command, do so, storing its
                 * results in the portal's tuplestore.
                 */
                if (!portal->holdStore)
                    FillPortalStore(portal, false /* isTopLevel */ );
 
                /*
                 * Now fetch desired portion of results.
                 */
                result = DoPortalRunFetch(portal, fdirection, count, dest);
                break;
 
            default:
                elog(ERROR, "unsupported portal strategy");
                result = 0;     /* keep compiler quiet */
                break;
        }
    }
    PG_CATCH();
    {
        /* Uncaught error while executing portal: mark it dead */
        MarkPortalFailed(portal);
 
        /* Restore global vars and propagate error */
        ActivePortal = saveActivePortal;
        CurrentResourceOwner = saveResourceOwner;
        PortalContext = savePortalContext;
 
        PG_RE_THROW();
    }
    PG_END_TRY();
 
    MemoryContextSwitchTo(oldContext);
 
    /* Mark portal not active */
    portal->status = PORTAL_READY;
 
    ActivePortal = saveActivePortal;
    CurrentResourceOwner = saveResourceOwner;
    PortalContext = savePortalContext;
 
    return result;
}
 
/*
 * DoPortalRunFetch
 *      Guts of PortalRunFetch --- the portal context is already set up
 *
 * Here, count < 0 typically reverses the direction.  Also, count == FETCH_ALL
 * is interpreted as "all rows".  (cf FetchStmt.howMany)
 *
 * Returns number of rows processed (suitable for use in result tag)
 */
static uint64
DoPortalRunFetch(Portal portal,
                 FetchDirection fdirection,
                 long count,
                 DestReceiver *dest)
{
    bool        forward;
 
    Assert(portal->strategy == PORTAL_ONE_SELECT ||
           portal->strategy == PORTAL_ONE_RETURNING ||
           portal->strategy == PORTAL_ONE_MOD_WITH ||
           portal->strategy == PORTAL_UTIL_SELECT);
 
    /*
     * Note: we disallow backwards fetch (including re-fetch of current row)
     * for NO SCROLL cursors, but we interpret that very loosely: you can use
     * any of the FetchDirection options, so long as the end result is to move
     * forwards by at least one row.  Currently it's sufficient to check for
     * NO SCROLL in DoPortalRewind() and in the forward == false path in
     * PortalRunSelect(); but someday we might prefer to account for that
     * restriction explicitly here.
     */
    switch (fdirection)
    {
        case FETCH_FORWARD:
            if (count < 0)
            {
                fdirection = FETCH_BACKWARD;
                count = -count;
            }
            /* fall out of switch to share code with FETCH_BACKWARD */
            break;
        case FETCH_BACKWARD:
            if (count < 0)
            {
                fdirection = FETCH_FORWARD;
                count = -count;
            }
            /* fall out of switch to share code with FETCH_FORWARD */
            break;
        case FETCH_ABSOLUTE:
            if (count > 0)
            {
                /*
                 * Definition: Rewind to start, advance count-1 rows, return
                 * next row (if any).
                 *
                 * In practice, if the goal is less than halfway back to the
                 * start, it's better to scan from where we are.
                 *
                 * Also, if current portalPos is outside the range of "long",
                 * do it the hard way to avoid possible overflow of the count
                 * argument to PortalRunSelect.  We must exclude exactly
                 * LONG_MAX, as well, lest the count look like FETCH_ALL.
                 *
                 * In any case, we arrange to fetch the target row going
                 * forwards.
                 */
                if ((uint64) (count - 1) <= portal->portalPos / 2 ||
                    portal->portalPos >= (uint64) LONG_MAX)
                {
                    DoPortalRewind(portal);
                    if (count > 1)
                        PortalRunSelect(portal, true, count - 1,
                                        None_Receiver);
                }
                else
                {
                    long        pos = (long) portal->portalPos;
 
                    if (portal->atEnd)
                        pos++;  /* need one extra fetch if off end */
                    if (count <= pos)
                        PortalRunSelect(portal, false, pos - count + 1,
                                        None_Receiver);
                    else if (count > pos + 1)
                        PortalRunSelect(portal, true, count - pos - 1,
                                        None_Receiver);
                }
                return PortalRunSelect(portal, true, 1L, dest);
            }
            else if (count < 0)
            {
                /*
                 * Definition: Advance to end, back up abs(count)-1 rows,
                 * return prior row (if any).  We could optimize this if we
                 * knew in advance where the end was, but typically we won't.
                 * (Is it worth considering case where count > half of size of
                 * query?  We could rewind once we know the size ...)
                 */
                PortalRunSelect(portal, true, FETCH_ALL, None_Receiver);
                if (count < -1)
                    PortalRunSelect(portal, false, -count - 1, None_Receiver);
                return PortalRunSelect(portal, false, 1L, dest);
            }
            else
            {
                /* count == 0 */
                /* Rewind to start, return zero rows */
                DoPortalRewind(portal);
                return PortalRunSelect(portal, true, 0L, dest);
            }
            break;
        case FETCH_RELATIVE:
            if (count > 0)
            {
                /*
                 * Definition: advance count-1 rows, return next row (if any).
                 */
                if (count > 1)
                    PortalRunSelect(portal, true, count - 1, None_Receiver);
                return PortalRunSelect(portal, true, 1L, dest);
            }
            else if (count < 0)
            {
                /*
                 * Definition: back up abs(count)-1 rows, return prior row (if
                 * any).
                 */
                if (count < -1)
                    PortalRunSelect(portal, false, -count - 1, None_Receiver);
                return PortalRunSelect(portal, false, 1L, dest);
            }
            else
            {
                /* count == 0 */
                /* Same as FETCH FORWARD 0, so fall out of switch */
                fdirection = FETCH_FORWARD;
            }
            break;
        default:
            elog(ERROR, "bogus direction");
            break;
    }
 
    /*
     * Get here with fdirection == FETCH_FORWARD or FETCH_BACKWARD, and count
     * >= 0.
     */
    forward = (fdirection == FETCH_FORWARD);
 
    /*
     * Zero count means to re-fetch the current row, if any (per SQL)
     */
    if (count == 0)
    {
        bool        on_row;
 
        /* Are we sitting on a row? */
        on_row = (!portal->atStart && !portal->atEnd);
 
        if (dest->mydest == DestNone)
        {
            /* MOVE 0 returns 0/1 based on if FETCH 0 would return a row */
            return on_row ? 1 : 0;
        }
        else
        {
            /*
             * If we are sitting on a row, back up one so we can re-fetch it.
             * If we are not sitting on a row, we still have to start up and
             * shut down the executor so that the destination is initialized
             * and shut down correctly; so keep going.  To PortalRunSelect,
             * count == 0 means we will retrieve no row.
             */
            if (on_row)
            {
                PortalRunSelect(portal, false, 1L, None_Receiver);
                /* Set up to fetch one row forward */
                count = 1;
                forward = true;
            }
        }
    }
 
    /*
     * Optimize MOVE BACKWARD ALL into a Rewind.
     */
    if (!forward && count == FETCH_ALL && dest->mydest == DestNone)
    {
        uint64      result = portal->portalPos;
 
        if (result > 0 && !portal->atEnd)
            result--;
        DoPortalRewind(portal);
        return result;
    }
 
    return PortalRunSelect(portal, forward, count, dest);
}
 
/*
 * DoPortalRewind - rewind a Portal to starting point
 */
static void
DoPortalRewind(Portal portal)
{
    QueryDesc  *queryDesc;
 
    /*
     * No work is needed if we've not advanced nor attempted to advance the
     * cursor (and we don't want to throw a NO SCROLL error in this case).
     */
    if (portal->atStart && !portal->atEnd)
        return;
 
    /* Otherwise, cursor must allow scrolling */
    if (portal->cursorOptions & CURSOR_OPT_NO_SCROLL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("cursor can only scan forward"),
                 errhint("Declare it with SCROLL option to enable backward scan.")));
 
    /* Rewind holdStore, if we have one */
    if (portal->holdStore)
    {
        MemoryContext oldcontext;
 
        oldcontext = MemoryContextSwitchTo(portal->holdContext);
        tuplestore_rescan(portal->holdStore);
        MemoryContextSwitchTo(oldcontext);
    }
 
    /* Rewind executor, if active */
    queryDesc = portal->queryDesc;
    if (queryDesc)
    {
        PushActiveSnapshot(queryDesc->snapshot);
        ExecutorRewind(queryDesc);
        PopActiveSnapshot();
    }
 
    portal->atStart = true;
    portal->atEnd = false;
    portal->portalPos = 0;
}
 
/*
 * PlannedStmtRequiresSnapshot - what it says on the tin
 */
bool
PlannedStmtRequiresSnapshot(PlannedStmt *pstmt)
{
    Node       *utilityStmt = pstmt->utilityStmt;
 
    /* If it's not a utility statement, it definitely needs a snapshot */
    if (utilityStmt == NULL)
        return true;
 
    /*
     * Most utility statements need a snapshot, and the default presumption
     * about new ones should be that they do too.  Hence, enumerate those that
     * do not need one.
     *
     * Transaction control, LOCK, and SET must *not* set a snapshot, since
     * they need to be executable at the start of a transaction-snapshot-mode
     * transaction without freezing a snapshot.  By extension we allow SHOW
     * not to set a snapshot.  The other stmts listed are just efficiency
     * hacks.  Beware of listing anything that can modify the database --- if,
     * say, it has to update an index with expressions that invoke
     * user-defined functions, then it had better have a snapshot.
     */
    if (IsA(utilityStmt, TransactionStmt) ||
        IsA(utilityStmt, LockStmt) ||
        IsA(utilityStmt, VariableSetStmt) ||
        IsA(utilityStmt, VariableShowStmt) ||
        IsA(utilityStmt, ConstraintsSetStmt) ||
    /* efficiency hacks from here down */
        IsA(utilityStmt, FetchStmt) ||
        IsA(utilityStmt, ListenStmt) ||
        IsA(utilityStmt, NotifyStmt) ||
        IsA(utilityStmt, UnlistenStmt) ||
        IsA(utilityStmt, CheckPointStmt))
        return false;
 
    return true;
}
 
/*
 * EnsurePortalSnapshotExists - recreate Portal-level snapshot, if needed
 *
 * Generally, we will have an active snapshot whenever we are executing
 * inside a Portal, unless the Portal's query is one of the utility
 * statements exempted from that rule (see PlannedStmtRequiresSnapshot).
 * However, procedures and DO blocks can commit or abort the transaction,
 * and thereby destroy all snapshots.  This function can be called to
 * re-establish the Portal-level snapshot when none exists.
 */
void
EnsurePortalSnapshotExists(void)
{
    Portal      portal;
 
    /*
     * Nothing to do if a snapshot is set.  (We take it on faith that the
     * outermost active snapshot belongs to some Portal; or if there is no
     * Portal, it's somebody else's responsibility to manage things.)
     */
    if (ActiveSnapshotSet())
        return;
 
    /* Otherwise, we'd better have an active Portal */
    portal = ActivePortal;
    if (unlikely(portal == NULL))
        elog(ERROR, "cannot execute SQL without an outer snapshot or portal");
    Assert(portal->portalSnapshot == NULL);
 
    /*
     * Create a new snapshot, make it active, and remember it in portal.
     * Because the portal now references the snapshot, we must tell snapmgr.c
     * that the snapshot belongs to the portal's transaction level, else we
     * risk portalSnapshot becoming a dangling pointer.
     */
    PushActiveSnapshotWithLevel(GetTransactionSnapshot(), portal->createLevel);
    /* PushActiveSnapshotWithLevel might have copied the snapshot */
    portal->portalSnapshot = GetActiveSnapshot();
}