nodeAppend.c

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/*-------------------------------------------------------------------------
 *
 * nodeAppend.c
 *    routines to handle append nodes.
 *
 * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeAppend.c
 *
 *-------------------------------------------------------------------------
 */
/* INTERFACE ROUTINES
 *      ExecInitAppend  - initialize the append node
 *      ExecAppend      - retrieve the next tuple from the node
 *      ExecEndAppend   - shut down the append node
 *      ExecReScanAppend - rescan the append node
 *
 *   NOTES
 *      Each append node contains a list of one or more subplans which
 *      must be iteratively processed (forwards or backwards).
 *      Tuples are retrieved by executing the 'whichplan'th subplan
 *      until the subplan stops returning tuples, at which point that
 *      plan is shut down and the next started up.
 *
 *      Append nodes don't make use of their left and right
 *      subtrees, rather they maintain a list of subplans so
 *      a typical append node looks like this in the plan tree:
 *
 *                 ...
 *                 /
 *              Append -------+------+------+--- nil
 *              /   \         |      |      |
 *            nil   nil      ...    ...    ...
 *                               subplans
 *
 *      Append nodes are currently used for unions, and to support
 *      inheritance queries, where several relations need to be scanned.
 *      For example, in our standard person/student/employee/student-emp
 *      example, where student and employee inherit from person
 *      and student-emp inherits from student and employee, the
 *      query:
 *
 *              select name from person
 *
 *      generates the plan:
 *
 *                |
 *              Append -------+-------+--------+--------+
 *              /   \         |       |        |        |
 *            nil   nil      Scan    Scan     Scan     Scan
 *                            |       |        |        |
 *                          person employee student student-emp
 */

#include "postgres.h"

#include "executor/execdebug.h"
#include "executor/nodeAppend.h"

static bool exec_append_initialize_next(AppendState *appendstate);


/* ----------------------------------------------------------------
 *      exec_append_initialize_next
 *
 *      Sets up the append state node for the "next" scan.
 *
 *      Returns t iff there is a "next" scan to process.
 * ----------------------------------------------------------------
 */
static bool
exec_append_initialize_next(AppendState *appendstate)
{
    int         whichplan;

    /*
     * get information from the append node
     */
    whichplan = appendstate->as_whichplan;

    if (whichplan < 0)
    {
        /*
         * if scanning in reverse, we start at the last scan in the list and
         * then proceed back to the first.. in any case we inform ExecAppend
         * that we are at the end of the line by returning FALSE
         */
        appendstate->as_whichplan = 0;
        return FALSE;
    }
    else if (whichplan >= appendstate->as_nplans)
    {
        /*
         * as above, end the scan if we go beyond the last scan in our list..
         */
        appendstate->as_whichplan = appendstate->as_nplans - 1;
        return FALSE;
    }
    else
    {
        return TRUE;
    }
}

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

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

    /*
     * Set up empty vector of subplan states
     */
    nplans = list_length(node->appendplans);

    appendplanstates = (PlanState **) palloc0(nplans * sizeof(PlanState *));

    /*
     * create new AppendState for our append node
     */
    appendstate->ps.plan = (Plan *) node;
    appendstate->ps.state = estate;
    appendstate->appendplans = appendplanstates;
    appendstate->as_nplans = nplans;

    /*
     * Miscellaneous initialization
     *
     * Append plans don't have expression contexts because they never call
     * ExecQual or ExecProject.
     */

    /*
     * append nodes still have Result slots, which hold pointers to tuples, so
     * we have to initialize them.
     */
    ExecInitResultTupleSlot(estate, &appendstate->ps);

    /*
     * call ExecInitNode on each of the plans to be executed and save the
     * results into the array "appendplans".
     */
    i = 0;
    foreach(lc, node->appendplans)
    {
        Plan       *initNode = (Plan *) lfirst(lc);

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

    /*
     * initialize output tuple type
     */
    ExecAssignResultTypeFromTL(&appendstate->ps);
    appendstate->ps.ps_ProjInfo = NULL;

    /*
     * initialize to scan first subplan
     */
    appendstate->as_whichplan = 0;
    exec_append_initialize_next(appendstate);

    return appendstate;
}

/* ----------------------------------------------------------------
 *     ExecAppend
 *
 *      Handles iteration over multiple subplans.
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecAppend(AppendState *node)
{
    for (;;)
    {
        PlanState  *subnode;
        TupleTableSlot *result;

        /*
         * figure out which subplan we are currently processing
         */
        subnode = node->appendplans[node->as_whichplan];

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

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

        /*
         * Go on to the "next" subplan in the appropriate direction. If no
         * more subplans, return the empty slot set up for us by
         * ExecInitAppend.
         */
        if (ScanDirectionIsForward(node->ps.state->es_direction))
            node->as_whichplan++;
        else
            node->as_whichplan--;
        if (!exec_append_initialize_next(node))
            return ExecClearTuple(node->ps.ps_ResultTupleSlot);

        /* Else loop back and try to get a tuple from the new subplan */
    }
}

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

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

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

void
ExecReScanAppend(AppendState *node)
{
    int         i;

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

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

        /*
         * If chgParam of subnode is not null then plan will be re-scanned by
         * first ExecProcNode.
         */
        if (subnode->chgParam == NULL)
            ExecReScan(subnode);
    }
    node->as_whichplan = 0;
    exec_append_initialize_next(node);
}