nodeIncrementalSort.c

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/*-------------------------------------------------------------------------
 *
 * nodeIncrementalSort.c
 *    Routines to handle incremental sorting of relations.
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeIncrementalSort.c
 *
 * DESCRIPTION
 *
 *  Incremental sort is an optimized variant of multikey sort for cases
 *  when the input is already sorted by a prefix of the sort keys.  For
 *  example when a sort by (key1, key2 ... keyN) is requested, and the
 *  input is already sorted by (key1, key2 ... keyM), M < N, we can
 *  divide the input into groups where keys (key1, ... keyM) are equal,
 *  and only sort on the remaining columns.
 *
 *  Consider the following example.  We have input tuples consisting of
 *  two integers (X, Y) already presorted by X, while it's required to
 *  sort them by both X and Y.  Let input tuples be following.
 *
 *  (1, 5)
 *  (1, 2)
 *  (2, 9)
 *  (2, 1)
 *  (2, 5)
 *  (3, 3)
 *  (3, 7)
 *
 *  An incremental sort algorithm would split the input into the following
 *  groups, which have equal X, and then sort them by Y individually:
 *
 *      (1, 5) (1, 2)
 *      (2, 9) (2, 1) (2, 5)
 *      (3, 3) (3, 7)
 *
 *  After sorting these groups and putting them altogether, we would get
 *  the following result which is sorted by X and Y, as requested:
 *
 *  (1, 2)
 *  (1, 5)
 *  (2, 1)
 *  (2, 5)
 *  (2, 9)
 *  (3, 3)
 *  (3, 7)
 *
 *  Incremental sort may be more efficient than plain sort, particularly
 *  on large datasets, as it reduces the amount of data to sort at once,
 *  making it more likely it fits into work_mem (eliminating the need to
 *  spill to disk).  But the main advantage of incremental sort is that
 *  it can start producing rows early, before sorting the whole dataset,
 *  which is a significant benefit especially for queries with LIMIT.
 *
 *  The algorithm we've implemented here is modified from the theoretical
 *  base described above by operating in two different modes:
 *    - Fetching a minimum number of tuples without checking prefix key
 *      group membership and sorting on all columns when safe.
 *    - Fetching all tuples for a single prefix key group and sorting on
 *      solely the unsorted columns.
 *  We always begin in the first mode, and employ a heuristic to switch
 *  into the second mode if we believe it's beneficial.
 *
 *  Sorting incrementally can potentially use less memory, avoid fetching
 *  and sorting all tuples in the the dataset, and begin returning tuples
 *  before the entire result set is available.
 *
 *  The hybrid mode approach allows us to optimize for both very small
 *  groups (where the overhead of a new tuplesort is high) and very large
 *  groups (where we can lower cost by not having to sort on already sorted
 *  columns), albeit at some extra cost while switching between modes.
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/htup_details.h"
#include "executor/execdebug.h"
#include "executor/nodeIncrementalSort.h"
#include "miscadmin.h"
#include "utils/lsyscache.h"
#include "utils/tuplesort.h"

/*
 * We need to store the instrumentation information in either local node's sort
 * info or, for a parallel worker process, in the shared info (this avoids
 * having to additionally memcpy the info from local memory to shared memory
 * at each instrumentation call). This macro expands to choose the proper sort
 * state and group info.
 *
 * Arguments:
 * - node: type IncrementalSortState *
 * - groupName: the token fullsort or prefixsort
 */
#define INSTRUMENT_SORT_GROUP(node, groupName) \
    do { \
        if ((node)->ss.ps.instrument != NULL) \
        { \
            if ((node)->shared_info && (node)->am_worker) \
            { \
                Assert(IsParallelWorker()); \
                Assert(ParallelWorkerNumber <= (node)->shared_info->num_workers); \
                instrumentSortedGroup(&(node)->shared_info->sinfo[ParallelWorkerNumber].groupName##GroupInfo, \
                                      (node)->groupName##_state); \
            } \
            else \
            { \
                instrumentSortedGroup(&(node)->incsort_info.groupName##GroupInfo, \
                                      (node)->groupName##_state); \
            } \
        } \
    } while (0)


/* ----------------------------------------------------------------
 * instrumentSortedGroup
 *
 * Because incremental sort processes (potentially many) sort batches, we need
 * to capture tuplesort stats each time we finalize a sort state. This summary
 * data is later used for EXPLAIN ANALYZE output.
 * ----------------------------------------------------------------
 */
static void
instrumentSortedGroup(IncrementalSortGroupInfo *groupInfo,
                      Tuplesortstate *sortState)
{
    TuplesortInstrumentation sort_instr;

    groupInfo->groupCount++;

    tuplesort_get_stats(sortState, &sort_instr);

    /* Calculate total and maximum memory and disk space used. */
    switch (sort_instr.spaceType)
    {
        case SORT_SPACE_TYPE_DISK:
            groupInfo->totalDiskSpaceUsed += sort_instr.spaceUsed;
            if (sort_instr.spaceUsed > groupInfo->maxDiskSpaceUsed)
                groupInfo->maxDiskSpaceUsed = sort_instr.spaceUsed;

            break;
        case SORT_SPACE_TYPE_MEMORY:
            groupInfo->totalMemorySpaceUsed += sort_instr.spaceUsed;
            if (sort_instr.spaceUsed > groupInfo->maxMemorySpaceUsed)
                groupInfo->maxMemorySpaceUsed = sort_instr.spaceUsed;

            break;
    }

    /* Track each sort method we've used. */
    groupInfo->sortMethods |= sort_instr.sortMethod;
}

/* ----------------------------------------------------------------
 * preparePresortedCols
 *
 * Prepare information for presorted_keys comparisons.
 * ----------------------------------------------------------------
 */
static void
preparePresortedCols(IncrementalSortState *node)
{
    IncrementalSort *plannode = castNode(IncrementalSort, node->ss.ps.plan);

    node->presorted_keys =
        (PresortedKeyData *) palloc(plannode->nPresortedCols *
                                    sizeof(PresortedKeyData));

    /* Pre-cache comparison functions for each pre-sorted key. */
    for (int i = 0; i < plannode->nPresortedCols; i++)
    {
        Oid         equalityOp,
                    equalityFunc;
        PresortedKeyData *key;

        key = &node->presorted_keys[i];
        key->attno = plannode->sort.sortColIdx[i];

        equalityOp = get_equality_op_for_ordering_op(plannode->sort.sortOperators[i],
                                                     NULL);
        if (!OidIsValid(equalityOp))
            elog(ERROR, "missing equality operator for ordering operator %u",
                 plannode->sort.sortOperators[i]);

        equalityFunc = get_opcode(equalityOp);
        if (!OidIsValid(equalityFunc))
            elog(ERROR, "missing function for operator %u", equalityOp);

        /* Lookup the comparison function */
        fmgr_info_cxt(equalityFunc, &key->flinfo, CurrentMemoryContext);

        /* We can initialize the callinfo just once and re-use it */
        key->fcinfo = palloc0(SizeForFunctionCallInfo(2));
        InitFunctionCallInfoData(*key->fcinfo, &key->flinfo, 2,
                                 plannode->sort.collations[i], NULL, NULL);
        key->fcinfo->args[0].isnull = false;
        key->fcinfo->args[1].isnull = false;
    }
}

/* ----------------------------------------------------------------
 * isCurrentGroup
 *
 * Check whether a given tuple belongs to the current sort group by comparing
 * the presorted column values to the pivot tuple of the current group.
 * ----------------------------------------------------------------
 */
static bool
isCurrentGroup(IncrementalSortState *node, TupleTableSlot *pivot, TupleTableSlot *tuple)
{
    int         nPresortedCols;

    nPresortedCols = castNode(IncrementalSort, node->ss.ps.plan)->nPresortedCols;

    /*
     * That the input is sorted by keys * (0, ... n) implies that the tail
     * keys are more likely to change. Therefore we do our comparison starting
     * from the last pre-sorted column to optimize for early detection of
     * inequality and minimizing the number of function calls..
     */
    for (int i = nPresortedCols - 1; i >= 0; i--)
    {
        Datum       datumA,
                    datumB,
                    result;
        bool        isnullA,
                    isnullB;
        AttrNumber  attno = node->presorted_keys[i].attno;
        PresortedKeyData *key;

        datumA = slot_getattr(pivot, attno, &isnullA);
        datumB = slot_getattr(tuple, attno, &isnullB);

        /* Special case for NULL-vs-NULL, else use standard comparison */
        if (isnullA || isnullB)
        {
            if (isnullA == isnullB)
                continue;
            else
                return false;
        }

        key = &node->presorted_keys[i];

        key->fcinfo->args[0].value = datumA;
        key->fcinfo->args[1].value = datumB;

        /* just for paranoia's sake, we reset isnull each time */
        key->fcinfo->isnull = false;

        result = FunctionCallInvoke(key->fcinfo);

        /* Check for null result, since caller is clearly not expecting one */
        if (key->fcinfo->isnull)
            elog(ERROR, "function %u returned NULL", key->flinfo.fn_oid);

        if (!DatumGetBool(result))
            return false;
    }
    return true;
}

/* ----------------------------------------------------------------
 * switchToPresortedPrefixMode
 *
 * When we determine that we've likely encountered a large batch of tuples all
 * having the same presorted prefix values, we want to optimize tuplesort by
 * only sorting on unsorted suffix keys.
 *
 * The problem is that we've already accumulated several tuples in another
 * tuplesort configured to sort by all columns (assuming that there may be
 * more than one prefix key group). So to switch to presorted prefix mode we
 * have to go back and look at all the tuples we've already accumulated to
 * verify they're all part of the same prefix key group before sorting them
 * solely by unsorted suffix keys.
 *
 * While it's likely that all tuples already fetched are all part of a single
 * prefix group, we also have to handle the possibility that there is at least
 * one different prefix key group before the large prefix key group.
 * ----------------------------------------------------------------
 */
static void
switchToPresortedPrefixMode(PlanState *pstate)
{
    IncrementalSortState *node = castNode(IncrementalSortState, pstate);
    ScanDirection dir;
    int64       nTuples = 0;
    bool        lastTuple = false;
    bool        firstTuple = true;
    TupleDesc   tupDesc;
    PlanState  *outerNode;
    IncrementalSort *plannode = castNode(IncrementalSort, node->ss.ps.plan);

    dir = node->ss.ps.state->es_direction;
    outerNode = outerPlanState(node);
    tupDesc = ExecGetResultType(outerNode);

    /* Configure the prefix sort state the first time around. */
    if (node->prefixsort_state == NULL)
    {
        Tuplesortstate *prefixsort_state;
        int         nPresortedCols = plannode->nPresortedCols;

        /*
         * Optimize the sort by assuming the prefix columns are all equal and
         * thus we only need to sort by any remaining columns.
         */
        prefixsort_state = tuplesort_begin_heap(tupDesc,
                                                plannode->sort.numCols - nPresortedCols,
                                                &(plannode->sort.sortColIdx[nPresortedCols]),
                                                &(plannode->sort.sortOperators[nPresortedCols]),
                                                &(plannode->sort.collations[nPresortedCols]),
                                                &(plannode->sort.nullsFirst[nPresortedCols]),
                                                work_mem,
                                                NULL,
                                                false);
        node->prefixsort_state = prefixsort_state;
    }
    else
    {
        /* Next group of presorted data */
        tuplesort_reset(node->prefixsort_state);
    }

    /*
     * If the current node has a bound, then it's reasonably likely that a
     * large prefix key group will benefit from bounded sort, so configure the
     * tuplesort to allow for that optimization.
     */
    if (node->bounded)
    {
        SO1_printf("Setting bound on presorted prefix tuplesort to: %ld\n",
                   node->bound - node->bound_Done);
        tuplesort_set_bound(node->prefixsort_state,
                            node->bound - node->bound_Done);
    }

    /*
     * Copy as many tuples as we can (i.e., in the same prefix key group) from
     * the full sort state to the prefix sort state.
     */
    for (;;)
    {
        lastTuple = node->n_fullsort_remaining - nTuples == 1;

        /*
         * When we encounter multiple prefix key groups inside the full sort
         * tuplesort we have to carry over the last read tuple into the next
         * batch.
         */
        if (firstTuple && !TupIsNull(node->transfer_tuple))
        {
            tuplesort_puttupleslot(node->prefixsort_state, node->transfer_tuple);
            nTuples++;

            /* The carried over tuple is our new group pivot tuple. */
            ExecCopySlot(node->group_pivot, node->transfer_tuple);
        }
        else
        {
            tuplesort_gettupleslot(node->fullsort_state,
                                   ScanDirectionIsForward(dir),
                                   false, node->transfer_tuple, NULL);

            /*
             * If this is our first time through the loop, then we need to
             * save the first tuple we get as our new group pivot.
             */
            if (TupIsNull(node->group_pivot))
                ExecCopySlot(node->group_pivot, node->transfer_tuple);

            if (isCurrentGroup(node, node->group_pivot, node->transfer_tuple))
            {
                tuplesort_puttupleslot(node->prefixsort_state, node->transfer_tuple);
                nTuples++;
            }
            else
            {
                /*
                 * The tuple isn't part of the current batch so we need to
                 * carry it over into the next batch of tuples we transfer out
                 * of the full sort tuplesort into the presorted prefix
                 * tuplesort. We don't actually have to do anything special to
                 * save the tuple since we've already loaded it into the
                 * node->transfer_tuple slot, and, even though that slot
                 * points to memory inside the full sort tuplesort, we can't
                 * reset that tuplesort anyway until we've fully transferred
                 * out its tuples, so this reference is safe. We do need to
                 * reset the group pivot tuple though since we've finished the
                 * current prefix key group.
                 */
                ExecClearTuple(node->group_pivot);
                break;
            }
        }

        firstTuple = false;

        /*
         * If we've copied all of the tuples from the full sort state into the
         * prefix sort state, then we don't actually know that we've yet found
         * the last tuple in that prefix key group until we check the next
         * tuple from the outer plan node, so we retain the current group
         * pivot tuple prefix key group comparison.
         */
        if (lastTuple)
            break;
    }

    /*
     * Track how many tuples remain in the full sort batch so that we know if
     * we need to sort multiple prefix key groups before processing tuples
     * remaining in the large single prefix key group we think we've
     * encountered.
     */
    SO1_printf("Moving %ld tuples to presorted prefix tuplesort\n", nTuples);
    node->n_fullsort_remaining -= nTuples;
    SO1_printf("Setting n_fullsort_remaining to %ld\n", node->n_fullsort_remaining);

    if (lastTuple)
    {
        /*
         * We've confirmed that all tuples remaining in the full sort batch is
         * in the same prefix key group and moved all of those tuples into the
         * presorted prefix tuplesort. Now we can save our pivot comparison
         * tuple and continue fetching tuples from the outer execution node to
         * load into the presorted prefix tuplesort.
         */
        ExecCopySlot(node->group_pivot, node->transfer_tuple);
        SO_printf("Setting execution_status to INCSORT_LOADPREFIXSORT (switchToPresortedPrefixMode)\n");
        node->execution_status = INCSORT_LOADPREFIXSORT;

        /*
         * Make sure we clear the transfer tuple slot so that next time we
         * encounter a large prefix key group we don't incorrectly assume we
         * have a tuple carried over from the previous group.
         */
        ExecClearTuple(node->transfer_tuple);
    }
    else
    {
        /*
         * We finished a group but didn't consume all of the tuples from the
         * full sort state, so we'll sort this batch, let the outer node read
         * out all of those tuples, and then come back around to find another
         * batch.
         */
        SO1_printf("Sorting presorted prefix tuplesort with %ld tuples\n", nTuples);
        tuplesort_performsort(node->prefixsort_state);

        INSTRUMENT_SORT_GROUP(node, prefixsort);

        if (node->bounded)
        {
            /*
             * If the current node has a bound and we've already sorted n
             * tuples, then the functional bound remaining is (original bound
             * - n), so store the current number of processed tuples for use
             * in configuring sorting bound.
             */
            SO2_printf("Changing bound_Done from %ld to %ld\n",
                       Min(node->bound, node->bound_Done + nTuples), node->bound_Done);
            node->bound_Done = Min(node->bound, node->bound_Done + nTuples);
        }

        SO_printf("Setting execution_status to INCSORT_READPREFIXSORT  (switchToPresortedPrefixMode)\n");
        node->execution_status = INCSORT_READPREFIXSORT;
    }
}

/*
 * Sorting many small groups with tuplesort is inefficient. In order to
 * cope with this problem we don't start a new group until the current one
 * contains at least DEFAULT_MIN_GROUP_SIZE tuples (unfortunately this also
 * means we can't assume small groups of tuples all have the same prefix keys.)
 * When we have a bound that's less than DEFAULT_MIN_GROUP_SIZE we start looking
 * for the new group as soon as we've met our bound to avoid fetching more
 * tuples than we absolutely have to fetch.
 */
#define DEFAULT_MIN_GROUP_SIZE 32

/*
 * While we've optimized for small prefix key groups by not starting our prefix
 * key comparisons until we've reached a minimum number of tuples, we don't want
 * that optimization to cause us to lose out on the benefits of being able to
 * assume a large group of tuples is fully presorted by its prefix keys.
 * Therefore we use the DEFAULT_MAX_FULL_SORT_GROUP_SIZE cutoff as a heuristic
 * for determining when we believe we've encountered a large group, and, if we
 * get to that point without finding a new prefix key group we transition to
 * presorted prefix key mode.
 */
#define DEFAULT_MAX_FULL_SORT_GROUP_SIZE (2 * DEFAULT_MIN_GROUP_SIZE)

/* ----------------------------------------------------------------
 *      ExecIncrementalSort
 *
 *      Assuming that outer subtree returns tuple presorted by some prefix
 *      of target sort columns, performs incremental sort.
 *
 *      Conditions:
 *        -- none.
 *
 *      Initial States:
 *        -- the outer child is prepared to return the first tuple.
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecIncrementalSort(PlanState *pstate)
{
    IncrementalSortState *node = castNode(IncrementalSortState, pstate);
    EState     *estate;
    ScanDirection dir;
    Tuplesortstate *read_sortstate;
    Tuplesortstate *fullsort_state;
    TupleTableSlot *slot;
    IncrementalSort *plannode = (IncrementalSort *) node->ss.ps.plan;
    PlanState  *outerNode;
    TupleDesc   tupDesc;
    int64       nTuples = 0;
    int64       minGroupSize;

    CHECK_FOR_INTERRUPTS();

    estate = node->ss.ps.state;
    dir = estate->es_direction;
    fullsort_state = node->fullsort_state;

    /*
     * If a previous iteration has sorted a batch, then we need to check to
     * see if there are any remaining tuples in that batch that we can return
     * before moving on to other execution states.
     */
    if (node->execution_status == INCSORT_READFULLSORT
        || node->execution_status == INCSORT_READPREFIXSORT)
    {
        /*
         * Return next tuple from the current sorted group set if available.
         */
        read_sortstate = node->execution_status == INCSORT_READFULLSORT ?
            fullsort_state : node->prefixsort_state;
        slot = node->ss.ps.ps_ResultTupleSlot;

        /*
         * We have to populate the slot from the tuplesort before checking
         * outerNodeDone because it will set the slot to NULL if no more
         * tuples remain. If the tuplesort is empty, but we don't have any
         * more tuples available for sort from the outer node, then
         * outerNodeDone will have been set so we'll return that now-empty
         * slot to the caller.
         */
        if (tuplesort_gettupleslot(read_sortstate, ScanDirectionIsForward(dir),
                                   false, slot, NULL) || node->outerNodeDone)

            /*
             * Note: there isn't a good test case for the node->outerNodeDone
             * check directly, but we need it for any plan where the outer
             * node will fail when trying to fetch too many tuples.
             */
            return slot;
        else if (node->n_fullsort_remaining > 0)
        {
            /*
             * When we transition to presorted prefix mode, we might have
             * accumulated at least one additional prefix key group in the
             * full sort tuplesort. The first call to
             * switchToPresortedPrefixMode() will have pulled the first one of
             * those groups out, and we've returned those tuples to the parent
             * node, but if at this point we still have tuples remaining in
             * the full sort state (i.e., n_fullsort_remaining > 0), then we
             * need to re-execute the prefix mode transition function to pull
             * out the next prefix key group.
             */
            SO1_printf("Re-calling switchToPresortedPrefixMode() because n_fullsort_remaining is > 0 (%ld)\n",
                       node->n_fullsort_remaining);
            switchToPresortedPrefixMode(pstate);
        }
        else
        {
            /*
             * If we don't have any sorted tuples to read and we're not
             * currently transitioning into presorted prefix sort mode, then
             * it's time to start the process all over again by building a new
             * group in the full sort state.
             */
            SO_printf("Setting execution_status to INCSORT_LOADFULLSORT (n_fullsort_remaining > 0)\n");
            node->execution_status = INCSORT_LOADFULLSORT;
        }
    }

    /*
     * Scan the subplan in the forward direction while creating the sorted
     * data.
     */
    estate->es_direction = ForwardScanDirection;

    outerNode = outerPlanState(node);
    tupDesc = ExecGetResultType(outerNode);

    /* Load tuples into the full sort state. */
    if (node->execution_status == INCSORT_LOADFULLSORT)
    {
        /*
         * Initialize sorting structures.
         */
        if (fullsort_state == NULL)
        {
            /*
             * Initialize presorted column support structures for
             * isCurrentGroup(). It's correct to do this along with the
             * initial initialization for the full sort state (and not for the
             * prefix sort state) since we always load the full sort state
             * first.
             */
            preparePresortedCols(node);

            /*
             * Since we optimize small prefix key groups by accumulating a
             * minimum number of tuples before sorting, we can't assume that a
             * group of tuples all have the same prefix key values. Hence we
             * setup the full sort tuplesort to sort by all requested sort
             * keys.
             */
            fullsort_state = tuplesort_begin_heap(tupDesc,
                                                  plannode->sort.numCols,
                                                  plannode->sort.sortColIdx,
                                                  plannode->sort.sortOperators,
                                                  plannode->sort.collations,
                                                  plannode->sort.nullsFirst,
                                                  work_mem,
                                                  NULL,
                                                  false);
            node->fullsort_state = fullsort_state;
        }
        else
        {
            /* Reset sort for the next batch. */
            tuplesort_reset(fullsort_state);
        }

        /*
         * Calculate the remaining tuples left if bounded and configure both
         * bounded sort and the minimum group size accordingly.
         */
        if (node->bounded)
        {
            int64       currentBound = node->bound - node->bound_Done;

            /*
             * Bounded sort isn't likely to be a useful optimization for full
             * sort mode since we limit full sort mode to a relatively small
             * number of tuples and tuplesort doesn't switch over to top-n
             * heap sort anyway unless it hits (2 * bound) tuples.
             */
            if (currentBound < DEFAULT_MIN_GROUP_SIZE)
                tuplesort_set_bound(fullsort_state, currentBound);

            minGroupSize = Min(DEFAULT_MIN_GROUP_SIZE, currentBound);
        }
        else
            minGroupSize = DEFAULT_MIN_GROUP_SIZE;

        /*
         * Because we have to read the next tuple to find out that we've
         * encountered a new prefix key group, on subsequent groups we have to
         * carry over that extra tuple and add it to the new group's sort here
         * before we read any new tuples from the outer node.
         */
        if (!TupIsNull(node->group_pivot))
        {
            tuplesort_puttupleslot(fullsort_state, node->group_pivot);
            nTuples++;

            /*
             * We're in full sort mode accumulating a minimum number of tuples
             * and not checking for prefix key equality yet, so we can't
             * assume the group pivot tuple will reamin the same -- unless
             * we're using a minimum group size of 1, in which case the pivot
             * is obviously still the pviot.
             */
            if (nTuples != minGroupSize)
                ExecClearTuple(node->group_pivot);
        }


        /*
         * Pull as many tuples from the outer node as possible given our
         * current operating mode.
         */
        for (;;)
        {
            slot = ExecProcNode(outerNode);

            /*
             * If the outer node can't provide us any more tuples, then we can
             * sort the current group and return those tuples.
             */
            if (TupIsNull(slot))
            {
                /*
                 * We need to know later if the outer node has completed to be
                 * able to distinguish between being done with a batch and
                 * being done with the whole node.
                 */
                node->outerNodeDone = true;

                SO1_printf("Sorting fullsort with %ld tuples\n", nTuples);
                tuplesort_performsort(fullsort_state);

                INSTRUMENT_SORT_GROUP(node, fullsort);

                SO_printf("Setting execution_status to INCSORT_READFULLSORT (final tuple)\n");
                node->execution_status = INCSORT_READFULLSORT;
                break;
            }

            /* Accumulate the next group of presorted tuples. */
            if (nTuples < minGroupSize)
            {
                /*
                 * If we haven't yet hit our target minimum group size, then
                 * we don't need to bother checking for inclusion in the
                 * current prefix group since at this point we'll assume that
                 * we'll full sort this batch to avoid a large number of very
                 * tiny (and thus inefficient) sorts.
                 */
                tuplesort_puttupleslot(fullsort_state, slot);
                nTuples++;

                /*
                 * If we've reached our minimum group size, then we need to
                 * store the most recent tuple as a pivot.
                 */
                if (nTuples == minGroupSize)
                    ExecCopySlot(node->group_pivot, slot);
            }
            else
            {
                /*
                 * If we've already accumulated enough tuples to reach our
                 * minimum group size, then we need to compare any additional
                 * tuples to our pivot tuple to see if we reach the end of
                 * that prefix key group. Only after we find changed prefix
                 * keys can we guarantee sort stability of the tuples we've
                 * already accumulated.
                 */
                if (isCurrentGroup(node, node->group_pivot, slot))
                {
                    /*
                     * As long as the prefix keys match the pivot tuple then
                     * load the tuple into the tuplesort.
                     */
                    tuplesort_puttupleslot(fullsort_state, slot);
                    nTuples++;
                }
                else
                {
                    /*
                     * Since the tuple we fetched isn't part of the current
                     * prefix key group we don't want to sort it as part of
                     * the current batch. Instead we use the group_pivot slot
                     * to carry it over to the next batch (even though we
                     * won't actually treat it as a group pivot).
                     */
                    ExecCopySlot(node->group_pivot, slot);

                    if (node->bounded)
                    {
                        /*
                         * If the current node has a bound, and we've already
                         * sorted n tuples, then the functional bound
                         * remaining is (original bound - n), so store the
                         * current number of processed tuples for later use
                         * configuring the sort state's bound.
                         */
                        SO2_printf("Changing bound_Done from %ld to %ld\n",
                                   node->bound_Done,
                                   Min(node->bound, node->bound_Done + nTuples));
                        node->bound_Done = Min(node->bound, node->bound_Done + nTuples);
                    }

                    /*
                     * Once we find changed prefix keys we can complete the
                     * sort and transition modes to reading out the sorted
                     * tuples.
                     */
                    SO1_printf("Sorting fullsort tuplesort with %ld tuples\n",
                               nTuples);
                    tuplesort_performsort(fullsort_state);

                    INSTRUMENT_SORT_GROUP(node, fullsort);

                    SO_printf("Setting execution_status to INCSORT_READFULLSORT (found end of group)\n");
                    node->execution_status = INCSORT_READFULLSORT;
                    break;
                }
            }

            /*
             * Unless we've already transitioned modes to reading from the
             * full sort state, then we assume that having read at least
             * DEFAULT_MAX_FULL_SORT_GROUP_SIZE tuples means it's likely we're
             * processing a large group of tuples all having equal prefix keys
             * (but haven't yet found the final tuple in that prefix key
             * group), so we need to transition into presorted prefix mode.
             */
            if (nTuples > DEFAULT_MAX_FULL_SORT_GROUP_SIZE &&
                node->execution_status != INCSORT_READFULLSORT)
            {
                /*
                 * The group pivot we have stored has already been put into
                 * the tuplesort; we don't want to carry it over. Since we
                 * haven't yet found the end of the prefix key group, it might
                 * seem like we should keep this, but we don't actually know
                 * how many prefix key groups might be represented in the full
                 * sort state, so we'll let the mode transition function
                 * manage this state for us.
                 */
                ExecClearTuple(node->group_pivot);

                /*
                 * Unfortunately the tuplesort API doesn't include a way to
                 * retrieve tuples unless a sort has been performed, so we
                 * perform the sort even though we could just as easily rely
                 * on FIFO retrieval semantics when transferring them to the
                 * presorted prefix tuplesort.
                 */
                SO1_printf("Sorting fullsort tuplesort with %ld tuples\n", nTuples);
                tuplesort_performsort(fullsort_state);

                INSTRUMENT_SORT_GROUP(node, fullsort);

                /*
                 * If the full sort tuplesort happened to switch into top-n
                 * heapsort mode then we will only be able to retrieve
                 * currentBound tuples (since the tuplesort will have only
                 * retained the top-n tuples). This is safe even though we
                 * haven't yet completed fetching the current prefix key group
                 * because the tuples we've "lost" already sorted "below" the
                 * retained ones, and we're already contractually guaranteed
                 * to not need any more than the currentBound tuples.
                 */
                if (tuplesort_used_bound(node->fullsort_state))
                {
                    int64       currentBound = node->bound - node->bound_Done;

                    SO2_printf("Read %ld tuples, but setting to %ld because we used bounded sort\n",
                               nTuples, Min(currentBound, nTuples));
                    nTuples = Min(currentBound, nTuples);
                }

                SO1_printf("Setting n_fullsort_remaining to %ld and calling switchToPresortedPrefixMode()\n",
                           nTuples);

                /*
                 * We might have multiple prefix key groups in the full sort
                 * state, so the mode transition function needs to know that
                 * it needs to move from the fullsort to presorted prefix
                 * sort.
                 */
                node->n_fullsort_remaining = nTuples;

                /* Transition the tuples to the presorted prefix tuplesort. */
                switchToPresortedPrefixMode(pstate);

                /*
                 * Since we know we had tuples to move to the presorted prefix
                 * tuplesort, we know that unless that transition has verified
                 * that all tuples belonged to the same prefix key group (in
                 * which case we can go straight to continuing to load tuples
                 * into that tuplesort), we should have a tuple to return
                 * here.
                 *
                 * Either way, the appropriate execution status should have
                 * been set by switchToPresortedPrefixMode(), so we can drop
                 * out of the loop here and let the appropriate path kick in.
                 */
                break;
            }
        }
    }

    if (node->execution_status == INCSORT_LOADPREFIXSORT)
    {
        /*
         * We only enter this state after the mode transition function has
         * confirmed all remaining tuples from the full sort state have the
         * same prefix and moved those tuples to the prefix sort state. That
         * function has also set a group pivot tuple (which doesn't need to be
         * carried over; it's already been put into the prefix sort state).
         */
        Assert(!TupIsNull(node->group_pivot));

        /*
         * Read tuples from the outer node and load them into the prefix sort
         * state until we encounter a tuple whose prefix keys don't match the
         * current group_pivot tuple, since we can't guarantee sort stability
         * until we have all tuples matching those prefix keys.
         */
        for (;;)
        {
            slot = ExecProcNode(outerNode);

            /*
             * If we've exhausted tuples from the outer node we're done
             * loading the prefix sort state.
             */
            if (TupIsNull(slot))
            {
                /*
                 * We need to know later if the outer node has completed to be
                 * able to distinguish between being done with a batch and
                 * being done with the whole node.
                 */
                node->outerNodeDone = true;
                break;
            }

            /*
             * If the tuple's prefix keys match our pivot tuple, we're not
             * done yet and can load it into the prefix sort state. If not, we
             * don't want to sort it as part of the current batch. Instead we
             * use the group_pivot slot to carry it over to the next batch
             * (even though we won't actually treat it as a group pivot).
             */
            if (isCurrentGroup(node, node->group_pivot, slot))
            {
                tuplesort_puttupleslot(node->prefixsort_state, slot);
                nTuples++;
            }
            else
            {
                ExecCopySlot(node->group_pivot, slot);
                break;
            }
        }

        /*
         * Perform the sort and begin returning the tuples to the parent plan
         * node.
         */
        SO1_printf("Sorting presorted prefix tuplesort with >= %ld tuples\n", nTuples);
        tuplesort_performsort(node->prefixsort_state);

        INSTRUMENT_SORT_GROUP(node, prefixsort);

        SO_printf("Setting execution_status to INCSORT_READPREFIXSORT (found end of group)\n");
        node->execution_status = INCSORT_READPREFIXSORT;

        if (node->bounded)
        {
            /*
             * If the current node has a bound, and we've already sorted n
             * tuples, then the functional bound remaining is (original bound
             * - n), so store the current number of processed tuples for use
             * in configuring sorting bound.
             */
            SO2_printf("Changing bound_Done from %ld to %ld\n",
                       node->bound_Done,
                       Min(node->bound, node->bound_Done + nTuples));
            node->bound_Done = Min(node->bound, node->bound_Done + nTuples);
        }
    }

    /* Restore to user specified direction. */
    estate->es_direction = dir;

    /*
     * Get the first or next tuple from tuplesort. Returns NULL if no more
     * tuples.
     */
    read_sortstate = node->execution_status == INCSORT_READFULLSORT ?
        fullsort_state : node->prefixsort_state;
    slot = node->ss.ps.ps_ResultTupleSlot;
    (void) tuplesort_gettupleslot(read_sortstate, ScanDirectionIsForward(dir),
                                  false, slot, NULL);
    return slot;
}

/* ----------------------------------------------------------------
 *      ExecInitIncrementalSort
 *
 *      Creates the run-time state information for the sort node
 *      produced by the planner and initializes its outer subtree.
 * ----------------------------------------------------------------
 */
IncrementalSortState *
ExecInitIncrementalSort(IncrementalSort *node, EState *estate, int eflags)
{
    IncrementalSortState *incrsortstate;

    SO_printf("ExecInitIncrementalSort: initializing sort node\n");

    /*
     * Incremental sort can't be used with EXEC_FLAG_BACKWARD or
     * EXEC_FLAG_MARK, because the current sort state contains only one sort
     * batch rather than the full result set.
     */
    Assert((eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)) == 0);

    /* Initialize state structure. */
    incrsortstate = makeNode(IncrementalSortState);
    incrsortstate->ss.ps.plan = (Plan *) node;
    incrsortstate->ss.ps.state = estate;
    incrsortstate->ss.ps.ExecProcNode = ExecIncrementalSort;

    incrsortstate->execution_status = INCSORT_LOADFULLSORT;
    incrsortstate->bounded = false;
    incrsortstate->outerNodeDone = false;
    incrsortstate->bound_Done = 0;
    incrsortstate->fullsort_state = NULL;
    incrsortstate->prefixsort_state = NULL;
    incrsortstate->group_pivot = NULL;
    incrsortstate->transfer_tuple = NULL;
    incrsortstate->n_fullsort_remaining = 0;
    incrsortstate->presorted_keys = NULL;

    if (incrsortstate->ss.ps.instrument != NULL)
    {
        IncrementalSortGroupInfo *fullsortGroupInfo =
        &incrsortstate->incsort_info.fullsortGroupInfo;
        IncrementalSortGroupInfo *prefixsortGroupInfo =
        &incrsortstate->incsort_info.prefixsortGroupInfo;

        fullsortGroupInfo->groupCount = 0;
        fullsortGroupInfo->maxDiskSpaceUsed = 0;
        fullsortGroupInfo->totalDiskSpaceUsed = 0;
        fullsortGroupInfo->maxMemorySpaceUsed = 0;
        fullsortGroupInfo->totalMemorySpaceUsed = 0;
        fullsortGroupInfo->sortMethods = 0;
        prefixsortGroupInfo->groupCount = 0;
        prefixsortGroupInfo->maxDiskSpaceUsed = 0;
        prefixsortGroupInfo->totalDiskSpaceUsed = 0;
        prefixsortGroupInfo->maxMemorySpaceUsed = 0;
        prefixsortGroupInfo->totalMemorySpaceUsed = 0;
        prefixsortGroupInfo->sortMethods = 0;
    }

    /*
     * Miscellaneous initialization
     *
     * Sort nodes don't initialize their ExprContexts because they never call
     * ExecQual or ExecProject.
     */

    /*
     * Initialize child nodes.
     *
     * Incremental sort does not support backwards scans and mark/restore, so
     * we don't bother removing the flags from eflags here. We allow passing a
     * REWIND flag, because although incremental sort can't use it, the child
     * nodes may be able to do something more useful.
     */
    outerPlanState(incrsortstate) = ExecInitNode(outerPlan(node), estate, eflags);

    /*
     * Initialize scan slot and type.
     */
    ExecCreateScanSlotFromOuterPlan(estate, &incrsortstate->ss, &TTSOpsMinimalTuple);

    /*
     * Initialize return slot and type. No need to initialize projection info
     * because we don't do any projections.
     */
    ExecInitResultTupleSlotTL(&incrsortstate->ss.ps, &TTSOpsMinimalTuple);
    incrsortstate->ss.ps.ps_ProjInfo = NULL;

    /*
     * Initialize standalone slots to store a tuple for pivot prefix keys and
     * for carrying over a tuple from one batch to the next.
     */
    incrsortstate->group_pivot =
        MakeSingleTupleTableSlot(ExecGetResultType(outerPlanState(incrsortstate)),
                                 &TTSOpsMinimalTuple);
    incrsortstate->transfer_tuple =
        MakeSingleTupleTableSlot(ExecGetResultType(outerPlanState(incrsortstate)),
                                 &TTSOpsMinimalTuple);

    SO_printf("ExecInitIncrementalSort: sort node initialized\n");

    return incrsortstate;
}

/* ----------------------------------------------------------------
 *      ExecEndIncrementalSort(node)
 * ----------------------------------------------------------------
 */
void
ExecEndIncrementalSort(IncrementalSortState *node)
{
    SO_printf("ExecEndIncrementalSort: shutting down sort node\n");

    /* clean out the scan tuple */
    ExecClearTuple(node->ss.ss_ScanTupleSlot);
    /* must drop pointer to sort result tuple */
    ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);
    /* must drop stanalone tuple slots from outer node */
    ExecDropSingleTupleTableSlot(node->group_pivot);
    ExecDropSingleTupleTableSlot(node->transfer_tuple);

    /*
     * Release tuplesort resources.
     */
    if (node->fullsort_state != NULL)
    {
        tuplesort_end(node->fullsort_state);
        node->fullsort_state = NULL;
    }
    if (node->prefixsort_state != NULL)
    {
        tuplesort_end(node->prefixsort_state);
        node->prefixsort_state = NULL;
    }

    /*
     * Shut down the subplan.
     */
    ExecEndNode(outerPlanState(node));

    SO_printf("ExecEndIncrementalSort: sort node shutdown\n");
}

void
ExecReScanIncrementalSort(IncrementalSortState *node)
{
    PlanState  *outerPlan = outerPlanState(node);

    /*
     * Incremental sort doesn't support efficient rescan even when parameters
     * haven't changed (e.g., rewind) because unlike regular sort we don't
     * store all tuples at once for the full sort.
     *
     * So even if EXEC_FLAG_REWIND is set we just reset all of our state and
     * re-execute the sort along with the child node. Incremental sort itself
     * can't do anything smarter, but maybe the child nodes can.
     *
     * In theory if we've only filled the full sort with one batch (and
     * haven't reset it for a new batch yet) then we could efficiently rewind,
     * but that seems a narrow enough case that it's not worth handling
     * specially at this time.
     */

    /* must drop pointer to sort result tuple */
    ExecClearTuple(node->ss.ps.ps_ResultTupleSlot);

    if (node->group_pivot != NULL)
        ExecClearTuple(node->group_pivot);
    if (node->transfer_tuple != NULL)
        ExecClearTuple(node->transfer_tuple);

    node->outerNodeDone = false;
    node->n_fullsort_remaining = 0;
    node->bound_Done = 0;
    node->presorted_keys = NULL;

    node->execution_status = INCSORT_LOADFULLSORT;

    /*
     * If we've set up either of the sort states yet, we need to reset them.
     * We could end them and null out the pointers, but there's no reason to
     * repay the setup cost, and because ExecIncrementalSort guards presorted
     * column functions by checking to see if the full sort state has been
     * initialized yet, setting the sort states to null here might actually
     * cause a leak.
     */
    if (node->fullsort_state != NULL)
    {
        tuplesort_reset(node->fullsort_state);
        node->fullsort_state = NULL;
    }
    if (node->prefixsort_state != NULL)
    {
        tuplesort_reset(node->prefixsort_state);
        node->prefixsort_state = NULL;
    }

    /*
     * If chgParam of subnode is not null, theni the plan will be re-scanned
     * by the first ExecProcNode.
     */
    if (outerPlan->chgParam == NULL)
        ExecReScan(outerPlan);
}

/* ----------------------------------------------------------------
 *                      Parallel Query Support
 * ----------------------------------------------------------------
 */

/* ----------------------------------------------------------------
 *      ExecSortEstimate
 *
 *      Estimate space required to propagate sort statistics.
 * ----------------------------------------------------------------
 */
void
ExecIncrementalSortEstimate(IncrementalSortState *node, ParallelContext *pcxt)
{
    Size        size;

    /* don't need this if not instrumenting or no workers */
    if (!node->ss.ps.instrument || pcxt->nworkers == 0)
        return;

    size = mul_size(pcxt->nworkers, sizeof(IncrementalSortInfo));
    size = add_size(size, offsetof(SharedIncrementalSortInfo, sinfo));
    shm_toc_estimate_chunk(&pcxt->estimator, size);
    shm_toc_estimate_keys(&pcxt->estimator, 1);
}

/* ----------------------------------------------------------------
 *      ExecSortInitializeDSM
 *
 *      Initialize DSM space for sort statistics.
 * ----------------------------------------------------------------
 */
void
ExecIncrementalSortInitializeDSM(IncrementalSortState *node, ParallelContext *pcxt)
{
    Size        size;

    /* don't need this if not instrumenting or no workers */
    if (!node->ss.ps.instrument || pcxt->nworkers == 0)
        return;

    size = offsetof(SharedIncrementalSortInfo, sinfo)
        + pcxt->nworkers * sizeof(IncrementalSortInfo);
    node->shared_info = shm_toc_allocate(pcxt->toc, size);
    /* ensure any unfilled slots will contain zeroes */
    memset(node->shared_info, 0, size);
    node->shared_info->num_workers = pcxt->nworkers;
    shm_toc_insert(pcxt->toc, node->ss.ps.plan->plan_node_id,
                   node->shared_info);
}

/* ----------------------------------------------------------------
 *      ExecSortInitializeWorker
 *
 *      Attach worker to DSM space for sort statistics.
 * ----------------------------------------------------------------
 */
void
ExecIncrementalSortInitializeWorker(IncrementalSortState *node, ParallelWorkerContext *pwcxt)
{
    node->shared_info =
        shm_toc_lookup(pwcxt->toc, node->ss.ps.plan->plan_node_id, true);
    node->am_worker = true;
}

/* ----------------------------------------------------------------
 *      ExecSortRetrieveInstrumentation
 *
 *      Transfer sort statistics from DSM to private memory.
 * ----------------------------------------------------------------
 */
void
ExecIncrementalSortRetrieveInstrumentation(IncrementalSortState *node)
{
    Size        size;
    SharedIncrementalSortInfo *si;

    if (node->shared_info == NULL)
        return;

    size = offsetof(SharedIncrementalSortInfo, sinfo)
        + node->shared_info->num_workers * sizeof(IncrementalSortInfo);
    si = palloc(size);
    memcpy(si, node->shared_info, size);
    node->shared_info = si;
}