nodeMergeAppend.c

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/*-------------------------------------------------------------------------
 *
 * nodeMergeAppend.c
 *    routines to handle MergeAppend nodes.
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeMergeAppend.c
 *
 *-------------------------------------------------------------------------
 */
/* INTERFACE ROUTINES
 *      ExecInitMergeAppend     - initialize the MergeAppend node
 *      ExecMergeAppend         - retrieve the next tuple from the node
 *      ExecEndMergeAppend      - shut down the MergeAppend node
 *      ExecReScanMergeAppend   - rescan the MergeAppend node
 *
 *   NOTES
 *      A MergeAppend node contains a list of one or more subplans.
 *      These are each expected to deliver tuples that are sorted according
 *      to a common sort key.  The MergeAppend node merges these streams
 *      to produce output sorted the same way.
 *
 *      MergeAppend nodes don't make use of their left and right
 *      subtrees, rather they maintain a list of subplans so
 *      a typical MergeAppend node looks like this in the plan tree:
 *
 *                 ...
 *                 /
 *              MergeAppend---+------+------+--- nil
 *              /   \         |      |      |
 *            nil   nil      ...    ...    ...
 *                               subplans
 */

#include "postgres.h"

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

#include "lib/binaryheap.h"

/*
 * We have one slot for each item in the heap array.  We use SlotNumber
 * to store slot indexes.  This doesn't actually provide any formal
 * type-safety, but it makes the code more self-documenting.
 */
typedef int32 SlotNumber;

static int  heap_compare_slots(Datum a, Datum b, void *arg);


/* ----------------------------------------------------------------
 *      ExecInitMergeAppend
 *
 *      Begin all of the subscans of the MergeAppend node.
 * ----------------------------------------------------------------
 */
MergeAppendState *
ExecInitMergeAppend(MergeAppend *node, EState *estate, int eflags)
{
    MergeAppendState *mergestate = makeNode(MergeAppendState);
    PlanState **mergeplanstates;
    int         nplans;
    int         i;
    ListCell   *lc;

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

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

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

    /*
     * create new MergeAppendState for our node
     */
    mergestate->ps.plan = (Plan *) node;
    mergestate->ps.state = estate;
    mergestate->mergeplans = mergeplanstates;
    mergestate->ms_nplans = nplans;

    mergestate->ms_slots = (TupleTableSlot **) palloc0(sizeof(TupleTableSlot *) * nplans);
    mergestate->ms_heap = binaryheap_allocate(nplans, heap_compare_slots,
                                              mergestate);

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

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

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

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

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

    /*
     * initialize sort-key information
     */
    mergestate->ms_nkeys = node->numCols;
    mergestate->ms_sortkeys = palloc0(sizeof(SortSupportData) * node->numCols);

    for (i = 0; i < node->numCols; i++)
    {
        SortSupport sortKey = mergestate->ms_sortkeys + i;

        sortKey->ssup_cxt = CurrentMemoryContext;
        sortKey->ssup_collation = node->collations[i];
        sortKey->ssup_nulls_first = node->nullsFirst[i];
        sortKey->ssup_attno = node->sortColIdx[i];

        PrepareSortSupportFromOrderingOp(node->sortOperators[i], sortKey);
    }

    /*
     * initialize to show we have not run the subplans yet
     */
    mergestate->ms_initialized = false;

    return mergestate;
}

/* ----------------------------------------------------------------
 *     ExecMergeAppend
 *
 *      Handles iteration over multiple subplans.
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecMergeAppend(MergeAppendState *node)
{
    TupleTableSlot *result;
    SlotNumber  i;

    if (!node->ms_initialized)
    {
        /*
         * First time through: pull the first tuple from each subplan, and set
         * up the heap.
         */
        for (i = 0; i < node->ms_nplans; i++)
        {
            node->ms_slots[i] = ExecProcNode(node->mergeplans[i]);
            if (!TupIsNull(node->ms_slots[i]))
                binaryheap_add_unordered(node->ms_heap, Int32GetDatum(i));
        }
        binaryheap_build(node->ms_heap);
        node->ms_initialized = true;
    }
    else
    {
        /*
         * Otherwise, pull the next tuple from whichever subplan we returned
         * from last time, and reinsert the subplan index into the heap,
         * because it might now compare differently against the existing
         * elements of the heap.  (We could perhaps simplify the logic a bit
         * by doing this before returning from the prior call, but it's better
         * to not pull tuples until necessary.)
         */
        i = DatumGetInt32(binaryheap_first(node->ms_heap));
        node->ms_slots[i] = ExecProcNode(node->mergeplans[i]);
        if (!TupIsNull(node->ms_slots[i]))
            binaryheap_replace_first(node->ms_heap, Int32GetDatum(i));
        else
            (void) binaryheap_remove_first(node->ms_heap);
    }

    if (binaryheap_empty(node->ms_heap))
    {
        /* All the subplans are exhausted, and so is the heap */
        result = ExecClearTuple(node->ps.ps_ResultTupleSlot);
    }
    else
    {
        i = DatumGetInt32(binaryheap_first(node->ms_heap));
        result = node->ms_slots[i];
    }

    return result;
}

/*
 * Compare the tuples in the two given slots.
 */
static int32
heap_compare_slots(Datum a, Datum b, void *arg)
{
    MergeAppendState *node = (MergeAppendState *) arg;
    SlotNumber  slot1 = DatumGetInt32(a);
    SlotNumber  slot2 = DatumGetInt32(b);

    TupleTableSlot *s1 = node->ms_slots[slot1];
    TupleTableSlot *s2 = node->ms_slots[slot2];
    int         nkey;

    Assert(!TupIsNull(s1));
    Assert(!TupIsNull(s2));

    for (nkey = 0; nkey < node->ms_nkeys; nkey++)
    {
        SortSupport sortKey = node->ms_sortkeys + nkey;
        AttrNumber  attno = sortKey->ssup_attno;
        Datum       datum1,
                    datum2;
        bool        isNull1,
                    isNull2;
        int         compare;

        datum1 = slot_getattr(s1, attno, &isNull1);
        datum2 = slot_getattr(s2, attno, &isNull2);

        compare = ApplySortComparator(datum1, isNull1,
                                      datum2, isNull2,
                                      sortKey);
        if (compare != 0)
            return -compare;
    }
    return 0;
}

/* ----------------------------------------------------------------
 *      ExecEndMergeAppend
 *
 *      Shuts down the subscans of the MergeAppend node.
 *
 *      Returns nothing of interest.
 * ----------------------------------------------------------------
 */
void
ExecEndMergeAppend(MergeAppendState *node)
{
    PlanState **mergeplans;
    int         nplans;
    int         i;

    /*
     * get information from the node
     */
    mergeplans = node->mergeplans;
    nplans = node->ms_nplans;

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

void
ExecReScanMergeAppend(MergeAppendState *node)
{
    int         i;

    for (i = 0; i < node->ms_nplans; i++)
    {
        PlanState  *subnode = node->mergeplans[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->ms_initialized = false;
}