nodeCtescan.c

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/*-------------------------------------------------------------------------
 *
 * nodeCtescan.c
 *    routines to handle CteScan nodes.
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeCtescan.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "executor/executor.h"
#include "executor/nodeCtescan.h"
#include "miscadmin.h"
 
static TupleTableSlot *CteScanNext(CteScanState *node);
 
/* ----------------------------------------------------------------
 *      CteScanNext
 *
 *      This is a workhorse for ExecCteScan
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
CteScanNext(CteScanState *node)
{
    EState     *estate;
    ScanDirection dir;
    bool        forward;
    Tuplestorestate *tuplestorestate;
    bool        eof_tuplestore;
    TupleTableSlot *slot;
 
    /*
     * get state info from node
     */
    estate = node->ss.ps.state;
    dir = estate->es_direction;
    forward = ScanDirectionIsForward(dir);
    tuplestorestate = node->leader->cte_table;
    tuplestore_select_read_pointer(tuplestorestate, node->readptr);
    slot = node->ss.ss_ScanTupleSlot;
 
    /*
     * If we are not at the end of the tuplestore, or are going backwards, try
     * to fetch a tuple from tuplestore.
     */
    eof_tuplestore = tuplestore_ateof(tuplestorestate);
 
    if (!forward && eof_tuplestore)
    {
        if (!node->leader->eof_cte)
        {
            /*
             * When reversing direction at tuplestore EOF, the first
             * gettupleslot call will fetch the last-added tuple; but we want
             * to return the one before that, if possible. So do an extra
             * fetch.
             */
            if (!tuplestore_advance(tuplestorestate, forward))
                return NULL;    /* the tuplestore must be empty */
        }
        eof_tuplestore = false;
    }
 
    /*
     * If we can fetch another tuple from the tuplestore, return it.
     *
     * Note: we have to use copy=true in the tuplestore_gettupleslot call,
     * because we are sharing the tuplestore with other nodes that might write
     * into the tuplestore before we get called again.
     */
    if (!eof_tuplestore)
    {
        if (tuplestore_gettupleslot(tuplestorestate, forward, true, slot))
            return slot;
        if (forward)
            eof_tuplestore = true;
    }
 
    /*
     * If necessary, try to fetch another row from the CTE query.
     *
     * Note: the eof_cte state variable exists to short-circuit further calls
     * of the CTE plan.  It's not optional, unfortunately, because some plan
     * node types are not robust about being called again when they've already
     * returned NULL.
     */
    if (eof_tuplestore && !node->leader->eof_cte)
    {
        TupleTableSlot *cteslot;
 
        /*
         * We can only get here with forward==true, so no need to worry about
         * which direction the subplan will go.
         */
        cteslot = ExecProcNode(node->cteplanstate);
        if (TupIsNull(cteslot))
        {
            node->leader->eof_cte = true;
            return NULL;
        }
 
        /*
         * There are corner cases where the subplan could change which
         * tuplestore read pointer is active, so be sure to reselect ours
         * before storing the tuple we got.
         */
        tuplestore_select_read_pointer(tuplestorestate, node->readptr);
 
        /*
         * Append a copy of the returned tuple to tuplestore.  NOTE: because
         * our read pointer is certainly in EOF state, its read position will
         * move forward over the added tuple.  This is what we want.  Also,
         * any other readers will *not* move past the new tuple, which is what
         * they want.
         */
        tuplestore_puttupleslot(tuplestorestate, cteslot);
 
        /*
         * We MUST copy the CTE query's output tuple into our own slot. This
         * is because other CteScan nodes might advance the CTE query before
         * we are called again, and our output tuple must stay stable over
         * that.
         */
        return ExecCopySlot(slot, cteslot);
    }
 
    /*
     * Nothing left ...
     */
    return ExecClearTuple(slot);
}
 
/*
 * CteScanRecheck -- access method routine to recheck a tuple in EvalPlanQual
 */
static bool
CteScanRecheck(CteScanState *node, TupleTableSlot *slot)
{
    /* nothing to check */
    return true;
}
 
/* ----------------------------------------------------------------
 *      ExecCteScan(node)
 *
 *      Scans the CTE sequentially and returns the next qualifying tuple.
 *      We call the ExecScan() routine and pass it the appropriate
 *      access method functions.
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecCteScan(PlanState *pstate)
{
    CteScanState *node = castNode(CteScanState, pstate);
 
    return ExecScan(&node->ss,
                    (ExecScanAccessMtd) CteScanNext,
                    (ExecScanRecheckMtd) CteScanRecheck);
}
 
 
/* ----------------------------------------------------------------
 *      ExecInitCteScan
 * ----------------------------------------------------------------
 */
CteScanState *
ExecInitCteScan(CteScan *node, EState *estate, int eflags)
{
    CteScanState *scanstate;
    ParamExecData *prmdata;
 
    /* check for unsupported flags */
    Assert(!(eflags & EXEC_FLAG_MARK));
 
    /*
     * For the moment we have to force the tuplestore to allow REWIND, because
     * we might be asked to rescan the CTE even though upper levels didn't
     * tell us to be prepared to do it efficiently.  Annoying, since this
     * prevents truncation of the tuplestore.  XXX FIXME
     *
     * Note: if we are in an EPQ recheck plan tree, it's likely that no access
     * to the tuplestore is needed at all, making this even more annoying.
     * It's not worth improving that as long as all the read pointers would
     * have REWIND anyway, but if we ever improve this logic then that aspect
     * should be considered too.
     */
    eflags |= EXEC_FLAG_REWIND;
 
    /*
     * CteScan should not have any children.
     */
    Assert(outerPlan(node) == NULL);
    Assert(innerPlan(node) == NULL);
 
    /*
     * create new CteScanState for node
     */
    scanstate = makeNode(CteScanState);
    scanstate->ss.ps.plan = (Plan *) node;
    scanstate->ss.ps.state = estate;
    scanstate->ss.ps.ExecProcNode = ExecCteScan;
    scanstate->eflags = eflags;
    scanstate->cte_table = NULL;
    scanstate->eof_cte = false;
 
    /*
     * Find the already-initialized plan for the CTE query.
     */
    scanstate->cteplanstate = (PlanState *) list_nth(estate->es_subplanstates,
                                                     node->ctePlanId - 1);
 
    /*
     * The Param slot associated with the CTE query is used to hold a pointer
     * to the CteState of the first CteScan node that initializes for this
     * CTE.  This node will be the one that holds the shared state for all the
     * CTEs, particularly the shared tuplestore.
     */
    prmdata = &(estate->es_param_exec_vals[node->cteParam]);
    Assert(prmdata->execPlan == NULL);
    Assert(!prmdata->isnull);
    scanstate->leader = castNode(CteScanState, DatumGetPointer(prmdata->value));
    if (scanstate->leader == NULL)
    {
        /* I am the leader */
        prmdata->value = PointerGetDatum(scanstate);
        scanstate->leader = scanstate;
        scanstate->cte_table = tuplestore_begin_heap(true, false, work_mem);
        tuplestore_set_eflags(scanstate->cte_table, scanstate->eflags);
        scanstate->readptr = 0;
    }
    else
    {
        /* Not the leader */
        /* Create my own read pointer, and ensure it is at start */
        scanstate->readptr =
            tuplestore_alloc_read_pointer(scanstate->leader->cte_table,
                                          scanstate->eflags);
        tuplestore_select_read_pointer(scanstate->leader->cte_table,
                                       scanstate->readptr);
        tuplestore_rescan(scanstate->leader->cte_table);
    }
 
    /*
     * Miscellaneous initialization
     *
     * create expression context for node
     */
    ExecAssignExprContext(estate, &scanstate->ss.ps);
 
    /*
     * The scan tuple type (ie, the rowtype we expect to find in the work
     * table) is the same as the result rowtype of the CTE query.
     */
    ExecInitScanTupleSlot(estate, &scanstate->ss,
                          ExecGetResultType(scanstate->cteplanstate),
                          &TTSOpsMinimalTuple);
 
    /*
     * Initialize result type and projection.
     */
    ExecInitResultTypeTL(&scanstate->ss.ps);
    ExecAssignScanProjectionInfo(&scanstate->ss);
 
    /*
     * initialize child expressions
     */
    scanstate->ss.ps.qual =
        ExecInitQual(node->scan.plan.qual, (PlanState *) scanstate);
 
    return scanstate;
}
 
/* ----------------------------------------------------------------
 *      ExecEndCteScan
 *
 *      frees any storage allocated through C routines.
 * ----------------------------------------------------------------
 */
void
ExecEndCteScan(CteScanState *node)
{
    /*
     * If I am the leader, free the tuplestore.
     */
    if (node->leader == node)
    {
        tuplestore_end(node->cte_table);
        node->cte_table = NULL;
    }
}
 
/* ----------------------------------------------------------------
 *      ExecReScanCteScan
 *
 *      Rescans the relation.
 * ----------------------------------------------------------------
 */
void
ExecReScanCteScan(CteScanState *node)
{
    Tuplestorestate *tuplestorestate = node->leader->cte_table;
 
    if (node->ss.ps.ps_ResultTupleSlot)
        ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
 
    ExecScanReScan(&node->ss);
 
    /*
     * Clear the tuplestore if a new scan of the underlying CTE is required.
     * This implicitly resets all the tuplestore's read pointers.  Note that
     * multiple CTE nodes might redundantly clear the tuplestore; that's OK,
     * and not unduly expensive.  We'll stop taking this path as soon as
     * somebody has attempted to read something from the underlying CTE
     * (thereby causing its chgParam to be cleared).
     */
    if (node->leader->cteplanstate->chgParam != NULL)
    {
        tuplestore_clear(tuplestorestate);
        node->leader->eof_cte = false;
    }
    else
    {
        /*
         * Else, just rewind my own pointer.  Either the underlying CTE
         * doesn't need a rescan (and we can re-read what's in the tuplestore
         * now), or somebody else already took care of it.
         */
        tuplestore_select_read_pointer(tuplestorestate, node->readptr);
        tuplestore_rescan(tuplestorestate);
    }
}