execScan.c

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/*-------------------------------------------------------------------------
 *
 * execScan.c
 *    This code provides support for generalized relation scans. ExecScan
 *    is passed a node and a pointer to a function to "do the right thing"
 *    and return a tuple from the relation. ExecScan then does the tedious
 *    stuff - checking the qualification and projecting the tuple
 *    appropriately.
 *
 * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/execScan.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "executor/executor.h"
#include "miscadmin.h"
#include "utils/memutils.h"
 
 
 
/*
 * ExecScanFetch -- check interrupts & fetch next potential tuple
 *
 * This routine is concerned with substituting a test tuple if we are
 * inside an EvalPlanQual recheck.  If we aren't, just execute
 * the access method's next-tuple routine.
 */
static inline TupleTableSlot *
ExecScanFetch(ScanState *node,
              ExecScanAccessMtd accessMtd,
              ExecScanRecheckMtd recheckMtd)
{
    EState     *estate = node->ps.state;
 
    CHECK_FOR_INTERRUPTS();
 
    if (estate->es_epq_active != NULL)
    {
        EPQState   *epqstate = estate->es_epq_active;
 
        /*
         * We are inside an EvalPlanQual recheck.  Return the test tuple if
         * one is available, after rechecking any access-method-specific
         * conditions.
         */
        Index       scanrelid = ((Scan *) node->ps.plan)->scanrelid;
 
        if (scanrelid == 0)
        {
            /*
             * This is a ForeignScan or CustomScan which has pushed down a
             * join to the remote side.  The recheck method is responsible not
             * only for rechecking the scan/join quals but also for storing
             * the correct tuple in the slot.
             */
 
            TupleTableSlot *slot = node->ss_ScanTupleSlot;
 
            if (!(*recheckMtd) (node, slot))
                ExecClearTuple(slot);   /* would not be returned by scan */
            return slot;
        }
        else if (epqstate->relsubs_done[scanrelid - 1])
        {
            /*
             * Return empty slot, as either there is no EPQ tuple for this rel
             * or we already returned it.
             */
 
            TupleTableSlot *slot = node->ss_ScanTupleSlot;
 
            return ExecClearTuple(slot);
        }
        else if (epqstate->relsubs_slot[scanrelid - 1] != NULL)
        {
            /*
             * Return replacement tuple provided by the EPQ caller.
             */
 
            TupleTableSlot *slot = epqstate->relsubs_slot[scanrelid - 1];
 
            Assert(epqstate->relsubs_rowmark[scanrelid - 1] == NULL);
 
            /* Mark to remember that we shouldn't return it again */
            epqstate->relsubs_done[scanrelid - 1] = true;
 
            /* Return empty slot if we haven't got a test tuple */
            if (TupIsNull(slot))
                return NULL;
 
            /* Check if it meets the access-method conditions */
            if (!(*recheckMtd) (node, slot))
                return ExecClearTuple(slot);    /* would not be returned by
                                                 * scan */
            return slot;
        }
        else if (epqstate->relsubs_rowmark[scanrelid - 1] != NULL)
        {
            /*
             * Fetch and return replacement tuple using a non-locking rowmark.
             */
 
            TupleTableSlot *slot = node->ss_ScanTupleSlot;
 
            /* Mark to remember that we shouldn't return more */
            epqstate->relsubs_done[scanrelid - 1] = true;
 
            if (!EvalPlanQualFetchRowMark(epqstate, scanrelid, slot))
                return NULL;
 
            /* Return empty slot if we haven't got a test tuple */
            if (TupIsNull(slot))
                return NULL;
 
            /* Check if it meets the access-method conditions */
            if (!(*recheckMtd) (node, slot))
                return ExecClearTuple(slot);    /* would not be returned by
                                                 * scan */
            return slot;
        }
    }
 
    /*
     * Run the node-type-specific access method function to get the next tuple
     */
    return (*accessMtd) (node);
}
 
/* ----------------------------------------------------------------
 *      ExecScan
 *
 *      Scans the relation using the 'access method' indicated and
 *      returns the next qualifying tuple.
 *      The access method returns the next tuple and ExecScan() is
 *      responsible for checking the tuple returned against the qual-clause.
 *
 *      A 'recheck method' must also be provided that can check an
 *      arbitrary tuple of the relation against any qual conditions
 *      that are implemented internal to the access method.
 *
 *      Conditions:
 *        -- the "cursor" maintained by the AMI is positioned at the tuple
 *           returned previously.
 *
 *      Initial States:
 *        -- the relation indicated is opened for scanning so that the
 *           "cursor" is positioned before the first qualifying tuple.
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecScan(ScanState *node,
         ExecScanAccessMtd accessMtd,   /* function returning a tuple */
         ExecScanRecheckMtd recheckMtd)
{
    ExprContext *econtext;
    ExprState  *qual;
    ProjectionInfo *projInfo;
 
    /*
     * Fetch data from node
     */
    qual = node->ps.qual;
    projInfo = node->ps.ps_ProjInfo;
    econtext = node->ps.ps_ExprContext;
 
    /* interrupt checks are in ExecScanFetch */
 
    /*
     * If we have neither a qual to check nor a projection to do, just skip
     * all the overhead and return the raw scan tuple.
     */
    if (!qual && !projInfo)
    {
        ResetExprContext(econtext);
        return ExecScanFetch(node, accessMtd, recheckMtd);
    }
 
    /*
     * Reset per-tuple memory context to free any expression evaluation
     * storage allocated in the previous tuple cycle.
     */
    ResetExprContext(econtext);
 
    /*
     * get a tuple from the access method.  Loop until we obtain a tuple that
     * passes the qualification.
     */
    for (;;)
    {
        TupleTableSlot *slot;
 
        slot = ExecScanFetch(node, accessMtd, recheckMtd);
 
        /*
         * if the slot returned by the accessMtd contains NULL, then it means
         * there is nothing more to scan so we just return an empty slot,
         * being careful to use the projection result slot so it has correct
         * tupleDesc.
         */
        if (TupIsNull(slot))
        {
            if (projInfo)
                return ExecClearTuple(projInfo->pi_state.resultslot);
            else
                return slot;
        }
 
        /*
         * place the current tuple into the expr context
         */
        econtext->ecxt_scantuple = slot;
 
        /*
         * check that the current tuple satisfies the qual-clause
         *
         * check for non-null qual here to avoid a function call to ExecQual()
         * when the qual is null ... saves only a few cycles, but they add up
         * ...
         */
        if (qual == NULL || ExecQual(qual, econtext))
        {
            /*
             * Found a satisfactory scan tuple.
             */
            if (projInfo)
            {
                /*
                 * Form a projection tuple, store it in the result tuple slot
                 * and return it.
                 */
                return ExecProject(projInfo);
            }
            else
            {
                /*
                 * Here, we aren't projecting, so just return scan tuple.
                 */
                return slot;
            }
        }
        else
            InstrCountFiltered1(node, 1);
 
        /*
         * Tuple fails qual, so free per-tuple memory and try again.
         */
        ResetExprContext(econtext);
    }
}
 
/*
 * ExecAssignScanProjectionInfo
 *      Set up projection info for a scan node, if necessary.
 *
 * We can avoid a projection step if the requested tlist exactly matches
 * the underlying tuple type.  If so, we just set ps_ProjInfo to NULL.
 * Note that this case occurs not only for simple "SELECT * FROM ...", but
 * also in most cases where there are joins or other processing nodes above
 * the scan node, because the planner will preferentially generate a matching
 * tlist.
 *
 * The scan slot's descriptor must have been set already.
 */
void
ExecAssignScanProjectionInfo(ScanState *node)
{
    Scan       *scan = (Scan *) node->ps.plan;
    TupleDesc   tupdesc = node->ss_ScanTupleSlot->tts_tupleDescriptor;
 
    ExecConditionalAssignProjectionInfo(&node->ps, tupdesc, scan->scanrelid);
}
 
/*
 * ExecAssignScanProjectionInfoWithVarno
 *      As above, but caller can specify varno expected in Vars in the tlist.
 */
void
ExecAssignScanProjectionInfoWithVarno(ScanState *node, int varno)
{
    TupleDesc   tupdesc = node->ss_ScanTupleSlot->tts_tupleDescriptor;
 
    ExecConditionalAssignProjectionInfo(&node->ps, tupdesc, varno);
}
 
/*
 * ExecScanReScan
 *
 * This must be called within the ReScan function of any plan node type
 * that uses ExecScan().
 */
void
ExecScanReScan(ScanState *node)
{
    EState     *estate = node->ps.state;
 
    /*
     * We must clear the scan tuple so that observers (e.g., execCurrent.c)
     * can tell that this plan node is not positioned on a tuple.
     */
    ExecClearTuple(node->ss_ScanTupleSlot);
 
    /*
     * Rescan EvalPlanQual tuple(s) if we're inside an EvalPlanQual recheck.
     * But don't lose the "blocked" status of blocked target relations.
     */
    if (estate->es_epq_active != NULL)
    {
        EPQState   *epqstate = estate->es_epq_active;
        Index       scanrelid = ((Scan *) node->ps.plan)->scanrelid;
 
        if (scanrelid > 0)
            epqstate->relsubs_done[scanrelid - 1] =
                epqstate->relsubs_blocked[scanrelid - 1];
        else
        {
            Bitmapset  *relids;
            int         rtindex = -1;
 
            /*
             * If an FDW or custom scan provider has replaced the join with a
             * scan, there are multiple RTIs; reset the epqScanDone flag for
             * all of them.
             */
            if (IsA(node->ps.plan, ForeignScan))
                relids = ((ForeignScan *) node->ps.plan)->fs_base_relids;
            else if (IsA(node->ps.plan, CustomScan))
                relids = ((CustomScan *) node->ps.plan)->custom_relids;
            else
                elog(ERROR, "unexpected scan node: %d",
                     (int) nodeTag(node->ps.plan));
 
            while ((rtindex = bms_next_member(relids, rtindex)) >= 0)
            {
                Assert(rtindex > 0);
                epqstate->relsubs_done[rtindex - 1] =
                    epqstate->relsubs_blocked[rtindex - 1];
            }
        }
    }
}