nodeGatherMerge.c

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/*-------------------------------------------------------------------------
 *
 * nodeGatherMerge.c
 *      Scan a plan in multiple workers, and do order-preserving merge.
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeGatherMerge.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/htup_details.h"
#include "executor/executor.h"
#include "executor/execParallel.h"
#include "executor/nodeGatherMerge.h"
#include "executor/tqueue.h"
#include "lib/binaryheap.h"
#include "miscadmin.h"
#include "optimizer/optimizer.h"
#include "utils/sortsupport.h"
 
/*
 * When we read tuples from workers, it's a good idea to read several at once
 * for efficiency when possible: this minimizes context-switching overhead.
 * But reading too many at a time wastes memory without improving performance.
 * We'll read up to MAX_TUPLE_STORE tuples (in addition to the first one).
 */
#define MAX_TUPLE_STORE 10
 
/*
 * Pending-tuple array for each worker.  This holds additional tuples that
 * we were able to fetch from the worker, but can't process yet.  In addition,
 * this struct holds the "done" flag indicating the worker is known to have
 * no more tuples.  (We do not use this struct for the leader; we don't keep
 * any pending tuples for the leader, and the need_to_scan_locally flag serves
 * as its "done" indicator.)
 */
typedef struct GMReaderTupleBuffer
{
    MinimalTuple *tuple;        /* array of length MAX_TUPLE_STORE */
    int         nTuples;        /* number of tuples currently stored */
    int         readCounter;    /* index of next tuple to extract */
    bool        done;           /* true if reader is known exhausted */
} GMReaderTupleBuffer;
 
static TupleTableSlot *ExecGatherMerge(PlanState *pstate);
static int32 heap_compare_slots(Datum a, Datum b, void *arg);
static TupleTableSlot *gather_merge_getnext(GatherMergeState *gm_state);
static MinimalTuple gm_readnext_tuple(GatherMergeState *gm_state, int nreader,
                                      bool nowait, bool *done);
static void ExecShutdownGatherMergeWorkers(GatherMergeState *node);
static void gather_merge_setup(GatherMergeState *gm_state);
static void gather_merge_init(GatherMergeState *gm_state);
static void gather_merge_clear_tuples(GatherMergeState *gm_state);
static bool gather_merge_readnext(GatherMergeState *gm_state, int reader,
                                  bool nowait);
static void load_tuple_array(GatherMergeState *gm_state, int reader);
 
/* ----------------------------------------------------------------
 *      ExecInitGather
 * ----------------------------------------------------------------
 */
GatherMergeState *
ExecInitGatherMerge(GatherMerge *node, EState *estate, int eflags)
{
    GatherMergeState *gm_state;
    Plan       *outerNode;
    TupleDesc   tupDesc;
 
    /* Gather merge node doesn't have innerPlan node. */
    Assert(innerPlan(node) == NULL);
 
    /*
     * create state structure
     */
    gm_state = makeNode(GatherMergeState);
    gm_state->ps.plan = (Plan *) node;
    gm_state->ps.state = estate;
    gm_state->ps.ExecProcNode = ExecGatherMerge;
 
    gm_state->initialized = false;
    gm_state->gm_initialized = false;
    gm_state->tuples_needed = -1;
 
    /*
     * Miscellaneous initialization
     *
     * create expression context for node
     */
    ExecAssignExprContext(estate, &gm_state->ps);
 
    /*
     * GatherMerge doesn't support checking a qual (it's always more efficient
     * to do it in the child node).
     */
    Assert(!node->plan.qual);
 
    /*
     * now initialize outer plan
     */
    outerNode = outerPlan(node);
    outerPlanState(gm_state) = ExecInitNode(outerNode, estate, eflags);
 
    /*
     * Leader may access ExecProcNode result directly (if
     * need_to_scan_locally), or from workers via tuple queue.  So we can't
     * trivially rely on the slot type being fixed for expressions evaluated
     * within this node.
     */
    gm_state->ps.outeropsset = true;
    gm_state->ps.outeropsfixed = false;
 
    /*
     * Store the tuple descriptor into gather merge state, so we can use it
     * while initializing the gather merge slots.
     */
    tupDesc = ExecGetResultType(outerPlanState(gm_state));
    gm_state->tupDesc = tupDesc;
 
    /*
     * Initialize result type and projection.
     */
    ExecInitResultTypeTL(&gm_state->ps);
    ExecConditionalAssignProjectionInfo(&gm_state->ps, tupDesc, OUTER_VAR);
 
    /*
     * Without projections result slot type is not trivially known, see
     * comment above.
     */
    if (gm_state->ps.ps_ProjInfo == NULL)
    {
        gm_state->ps.resultopsset = true;
        gm_state->ps.resultopsfixed = false;
    }
 
    /*
     * initialize sort-key information
     */
    if (node->numCols)
    {
        int         i;
 
        gm_state->gm_nkeys = node->numCols;
        gm_state->gm_sortkeys = palloc0_array(SortSupportData, node->numCols);
 
        for (i = 0; i < node->numCols; i++)
        {
            SortSupport sortKey = gm_state->gm_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];
 
            /*
             * We don't perform abbreviated key conversion here, for the same
             * reasons that it isn't used in MergeAppend
             */
            sortKey->abbreviate = false;
 
            PrepareSortSupportFromOrderingOp(node->sortOperators[i], sortKey);
        }
    }
 
    /* Now allocate the workspace for gather merge */
    gather_merge_setup(gm_state);
 
    return gm_state;
}
 
/* ----------------------------------------------------------------
 *      ExecGatherMerge(node)
 *
 *      Scans the relation via multiple workers and returns
 *      the next qualifying tuple.
 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecGatherMerge(PlanState *pstate)
{
    GatherMergeState *node = castNode(GatherMergeState, pstate);
    TupleTableSlot *slot;
    ExprContext *econtext;
 
    CHECK_FOR_INTERRUPTS();
 
    /*
     * As with Gather, we don't launch workers until this node is actually
     * executed.
     */
    if (!node->initialized)
    {
        EState     *estate = node->ps.state;
        GatherMerge *gm = castNode(GatherMerge, node->ps.plan);
 
        /*
         * Sometimes we might have to run without parallelism; but if parallel
         * mode is active then we can try to fire up some workers.
         */
        if (gm->num_workers > 0 && estate->es_use_parallel_mode)
        {
            ParallelContext *pcxt;
 
            /* Initialize, or re-initialize, shared state needed by workers. */
            if (!node->pei)
                node->pei = ExecInitParallelPlan(outerPlanState(node),
                                                 estate,
                                                 gm->initParam,
                                                 gm->num_workers,
                                                 node->tuples_needed);
            else
                ExecParallelReinitialize(outerPlanState(node),
                                         node->pei,
                                         gm->initParam);
 
            /* Try to launch workers. */
            pcxt = node->pei->pcxt;
            LaunchParallelWorkers(pcxt);
            /* We save # workers launched for the benefit of EXPLAIN */
            node->nworkers_launched = pcxt->nworkers_launched;
 
            /*
             * Count number of workers originally wanted and actually
             * launched.
             */
            estate->es_parallel_workers_to_launch += pcxt->nworkers_to_launch;
            estate->es_parallel_workers_launched += pcxt->nworkers_launched;
 
            /* Set up tuple queue readers to read the results. */
            if (pcxt->nworkers_launched > 0)
            {
                ExecParallelCreateReaders(node->pei);
                /* Make a working array showing the active readers */
                node->nreaders = pcxt->nworkers_launched;
                node->reader = (TupleQueueReader **)
                    palloc(node->nreaders * sizeof(TupleQueueReader *));
                memcpy(node->reader, node->pei->reader,
                       node->nreaders * sizeof(TupleQueueReader *));
            }
            else
            {
                /* No workers?  Then never mind. */
                node->nreaders = 0;
                node->reader = NULL;
            }
        }
 
        /* allow leader to participate if enabled or no choice */
        if (parallel_leader_participation || node->nreaders == 0)
            node->need_to_scan_locally = true;
        node->initialized = true;
    }
 
    /*
     * Reset per-tuple memory context to free any expression evaluation
     * storage allocated in the previous tuple cycle.
     */
    econtext = node->ps.ps_ExprContext;
    ResetExprContext(econtext);
 
    /*
     * Get next tuple, either from one of our workers, or by running the plan
     * ourselves.
     */
    slot = gather_merge_getnext(node);
    if (TupIsNull(slot))
        return NULL;
 
    /* If no projection is required, we're done. */
    if (node->ps.ps_ProjInfo == NULL)
        return slot;
 
    /*
     * Form the result tuple using ExecProject(), and return it.
     */
    econtext->ecxt_outertuple = slot;
    return ExecProject(node->ps.ps_ProjInfo);
}
 
/* ----------------------------------------------------------------
 *      ExecEndGatherMerge
 *
 *      frees any storage allocated through C routines.
 * ----------------------------------------------------------------
 */
void
ExecEndGatherMerge(GatherMergeState *node)
{
    ExecEndNode(outerPlanState(node));  /* let children clean up first */
    ExecShutdownGatherMerge(node);
}
 
/* ----------------------------------------------------------------
 *      ExecShutdownGatherMerge
 *
 *      Destroy the setup for parallel workers including parallel context.
 * ----------------------------------------------------------------
 */
void
ExecShutdownGatherMerge(GatherMergeState *node)
{
    ExecShutdownGatherMergeWorkers(node);
 
    /* Now destroy the parallel context. */
    if (node->pei != NULL)
    {
        ExecParallelCleanup(node->pei);
        node->pei = NULL;
    }
}
 
/* ----------------------------------------------------------------
 *      ExecShutdownGatherMergeWorkers
 *
 *      Stop all the parallel workers.
 * ----------------------------------------------------------------
 */
static void
ExecShutdownGatherMergeWorkers(GatherMergeState *node)
{
    if (node->pei != NULL)
        ExecParallelFinish(node->pei);
 
    /* Flush local copy of reader array */
    if (node->reader)
        pfree(node->reader);
    node->reader = NULL;
}
 
/* ----------------------------------------------------------------
 *      ExecReScanGatherMerge
 *
 *      Prepare to re-scan the result of a GatherMerge.
 * ----------------------------------------------------------------
 */
void
ExecReScanGatherMerge(GatherMergeState *node)
{
    GatherMerge *gm = (GatherMerge *) node->ps.plan;
    PlanState  *outerPlan = outerPlanState(node);
 
    /* Make sure any existing workers are gracefully shut down */
    ExecShutdownGatherMergeWorkers(node);
 
    /* Free any unused tuples, so we don't leak memory across rescans */
    gather_merge_clear_tuples(node);
 
    /* Mark node so that shared state will be rebuilt at next call */
    node->initialized = false;
    node->gm_initialized = false;
 
    /*
     * Set child node's chgParam to tell it that the next scan might deliver a
     * different set of rows within the leader process.  (The overall rowset
     * shouldn't change, but the leader process's subset might; hence nodes
     * between here and the parallel table scan node mustn't optimize on the
     * assumption of an unchanging rowset.)
     */
    if (gm->rescan_param >= 0)
        outerPlan->chgParam = bms_add_member(outerPlan->chgParam,
                                             gm->rescan_param);
 
    /*
     * If chgParam of subnode is not null then plan will be re-scanned by
     * first ExecProcNode.  Note: because this does nothing if we have a
     * rescan_param, it's currently guaranteed that parallel-aware child nodes
     * will not see a ReScan call until after they get a ReInitializeDSM call.
     * That ordering might not be something to rely on, though.  A good rule
     * of thumb is that ReInitializeDSM should reset only shared state, ReScan
     * should reset only local state, and anything that depends on both of
     * those steps being finished must wait until the first ExecProcNode call.
     */
    if (outerPlan->chgParam == NULL)
        ExecReScan(outerPlan);
}
 
/*
 * Set up the data structures that we'll need for Gather Merge.
 *
 * We allocate these once on the basis of gm->num_workers, which is an
 * upper bound for the number of workers we'll actually have.  During
 * a rescan, we reset the structures to empty.  This approach simplifies
 * not leaking memory across rescans.
 *
 * In the gm_slots[] array, index 0 is for the leader, and indexes 1 to n
 * are for workers.  The values placed into gm_heap correspond to indexes
 * in gm_slots[].  The gm_tuple_buffers[] array, however, is indexed from
 * 0 to n-1; it has no entry for the leader.
 */
static void
gather_merge_setup(GatherMergeState *gm_state)
{
    GatherMerge *gm = castNode(GatherMerge, gm_state->ps.plan);
    int         nreaders = gm->num_workers;
    int         i;
 
    /*
     * Allocate gm_slots for the number of workers + one more slot for leader.
     * Slot 0 is always for the leader.  Leader always calls ExecProcNode() to
     * read the tuple, and then stores it directly into its gm_slots entry.
     * For other slots, code below will call ExecInitExtraTupleSlot() to
     * create a slot for the worker's results.  Note that during any single
     * scan, we might have fewer than num_workers available workers, in which
     * case the extra array entries go unused.
     */
    gm_state->gm_slots = (TupleTableSlot **)
        palloc0((nreaders + 1) * sizeof(TupleTableSlot *));
 
    /* Allocate the tuple slot and tuple array for each worker */
    gm_state->gm_tuple_buffers = (GMReaderTupleBuffer *)
        palloc0(nreaders * sizeof(GMReaderTupleBuffer));
 
    for (i = 0; i < nreaders; i++)
    {
        /* Allocate the tuple array with length MAX_TUPLE_STORE */
        gm_state->gm_tuple_buffers[i].tuple = palloc0_array(MinimalTuple, MAX_TUPLE_STORE);
 
        /* Initialize tuple slot for worker */
        gm_state->gm_slots[i + 1] =
            ExecInitExtraTupleSlot(gm_state->ps.state, gm_state->tupDesc,
                                   &TTSOpsMinimalTuple);
    }
 
    /* Allocate the resources for the merge */
    gm_state->gm_heap = binaryheap_allocate(nreaders + 1,
                                            heap_compare_slots,
                                            gm_state);
}
 
/*
 * Initialize the Gather Merge.
 *
 * Reset data structures to ensure they're empty.  Then pull at least one
 * tuple from leader + each worker (or set its "done" indicator), and set up
 * the heap.
 */
static void
gather_merge_init(GatherMergeState *gm_state)
{
    int         nreaders = gm_state->nreaders;
    bool        nowait = true;
    int         i;
 
    /* Assert that gather_merge_setup made enough space */
    Assert(nreaders <= castNode(GatherMerge, gm_state->ps.plan)->num_workers);
 
    /* Reset leader's tuple slot to empty */
    gm_state->gm_slots[0] = NULL;
 
    /* Reset the tuple slot and tuple array for each worker */
    for (i = 0; i < nreaders; i++)
    {
        /* Reset tuple array to empty */
        gm_state->gm_tuple_buffers[i].nTuples = 0;
        gm_state->gm_tuple_buffers[i].readCounter = 0;
        /* Reset done flag to not-done */
        gm_state->gm_tuple_buffers[i].done = false;
        /* Ensure output slot is empty */
        ExecClearTuple(gm_state->gm_slots[i + 1]);
    }
 
    /* Reset binary heap to empty */
    binaryheap_reset(gm_state->gm_heap);
 
    /*
     * First, try to read a tuple from each worker (including leader) in
     * nowait mode.  After this, if not all workers were able to produce a
     * tuple (or a "done" indication), then re-read from remaining workers,
     * this time using wait mode.  Add all live readers (those producing at
     * least one tuple) to the heap.
     */
reread:
    for (i = 0; i <= nreaders; i++)
    {
        CHECK_FOR_INTERRUPTS();
 
        /* skip this source if already known done */
        if ((i == 0) ? gm_state->need_to_scan_locally :
            !gm_state->gm_tuple_buffers[i - 1].done)
        {
            if (TupIsNull(gm_state->gm_slots[i]))
            {
                /* Don't have a tuple yet, try to get one */
                if (gather_merge_readnext(gm_state, i, nowait))
                    binaryheap_add_unordered(gm_state->gm_heap,
                                             Int32GetDatum(i));
            }
            else
            {
                /*
                 * We already got at least one tuple from this worker, but
                 * might as well see if it has any more ready by now.
                 */
                load_tuple_array(gm_state, i);
            }
        }
    }
 
    /* need not recheck leader, since nowait doesn't matter for it */
    for (i = 1; i <= nreaders; i++)
    {
        if (!gm_state->gm_tuple_buffers[i - 1].done &&
            TupIsNull(gm_state->gm_slots[i]))
        {
            nowait = false;
            goto reread;
        }
    }
 
    /* Now heapify the heap. */
    binaryheap_build(gm_state->gm_heap);
 
    gm_state->gm_initialized = true;
}
 
/*
 * Clear out the tuple table slot, and any unused pending tuples,
 * for each gather merge input.
 */
static void
gather_merge_clear_tuples(GatherMergeState *gm_state)
{
    int         i;
 
    for (i = 0; i < gm_state->nreaders; i++)
    {
        GMReaderTupleBuffer *tuple_buffer = &gm_state->gm_tuple_buffers[i];
 
        while (tuple_buffer->readCounter < tuple_buffer->nTuples)
            pfree(tuple_buffer->tuple[tuple_buffer->readCounter++]);
 
        ExecClearTuple(gm_state->gm_slots[i + 1]);
    }
}
 
/*
 * Read the next tuple for gather merge.
 *
 * Fetch the sorted tuple out of the heap.
 */
static TupleTableSlot *
gather_merge_getnext(GatherMergeState *gm_state)
{
    int         i;
 
    if (!gm_state->gm_initialized)
    {
        /*
         * First time through: pull the first tuple from each participant, and
         * set up the heap.
         */
        gather_merge_init(gm_state);
    }
    else
    {
        /*
         * Otherwise, pull the next tuple from whichever participant we
         * returned from last time, and reinsert that participant's index into
         * the heap, because it might now compare differently against the
         * other elements of the heap.
         */
        i = DatumGetInt32(binaryheap_first(gm_state->gm_heap));
 
        if (gather_merge_readnext(gm_state, i, false))
            binaryheap_replace_first(gm_state->gm_heap, Int32GetDatum(i));
        else
        {
            /* reader exhausted, remove it from heap */
            (void) binaryheap_remove_first(gm_state->gm_heap);
        }
    }
 
    if (binaryheap_empty(gm_state->gm_heap))
    {
        /* All the queues are exhausted, and so is the heap */
        gather_merge_clear_tuples(gm_state);
        return NULL;
    }
    else
    {
        /* Return next tuple from whichever participant has the leading one */
        i = DatumGetInt32(binaryheap_first(gm_state->gm_heap));
        return gm_state->gm_slots[i];
    }
}
 
/*
 * Read tuple(s) for given reader in nowait mode, and load into its tuple
 * array, until we have MAX_TUPLE_STORE of them or would have to block.
 */
static void
load_tuple_array(GatherMergeState *gm_state, int reader)
{
    GMReaderTupleBuffer *tuple_buffer;
    int         i;
 
    /* Don't do anything if this is the leader. */
    if (reader == 0)
        return;
 
    tuple_buffer = &gm_state->gm_tuple_buffers[reader - 1];
 
    /* If there's nothing in the array, reset the counters to zero. */
    if (tuple_buffer->nTuples == tuple_buffer->readCounter)
        tuple_buffer->nTuples = tuple_buffer->readCounter = 0;
 
    /* Try to fill additional slots in the array. */
    for (i = tuple_buffer->nTuples; i < MAX_TUPLE_STORE; i++)
    {
        MinimalTuple tuple;
 
        tuple = gm_readnext_tuple(gm_state,
                                  reader,
                                  true,
                                  &tuple_buffer->done);
        if (!tuple)
            break;
        tuple_buffer->tuple[i] = tuple;
        tuple_buffer->nTuples++;
    }
}
 
/*
 * Store the next tuple for a given reader into the appropriate slot.
 *
 * Returns true if successful, false if not (either reader is exhausted,
 * or we didn't want to wait for a tuple).  Sets done flag if reader
 * is found to be exhausted.
 */
static bool
gather_merge_readnext(GatherMergeState *gm_state, int reader, bool nowait)
{
    GMReaderTupleBuffer *tuple_buffer;
    MinimalTuple tup;
 
    /*
     * If we're being asked to generate a tuple from the leader, then we just
     * call ExecProcNode as normal to produce one.
     */
    if (reader == 0)
    {
        if (gm_state->need_to_scan_locally)
        {
            PlanState  *outerPlan = outerPlanState(gm_state);
            TupleTableSlot *outerTupleSlot;
            EState     *estate = gm_state->ps.state;
 
            /* Install our DSA area while executing the plan. */
            estate->es_query_dsa = gm_state->pei ? gm_state->pei->area : NULL;
            outerTupleSlot = ExecProcNode(outerPlan);
            estate->es_query_dsa = NULL;
 
            if (!TupIsNull(outerTupleSlot))
            {
                gm_state->gm_slots[0] = outerTupleSlot;
                return true;
            }
            /* need_to_scan_locally serves as "done" flag for leader */
            gm_state->need_to_scan_locally = false;
        }
        return false;
    }
 
    /* Otherwise, check the state of the relevant tuple buffer. */
    tuple_buffer = &gm_state->gm_tuple_buffers[reader - 1];
 
    if (tuple_buffer->nTuples > tuple_buffer->readCounter)
    {
        /* Return any tuple previously read that is still buffered. */
        tup = tuple_buffer->tuple[tuple_buffer->readCounter++];
    }
    else if (tuple_buffer->done)
    {
        /* Reader is known to be exhausted. */
        return false;
    }
    else
    {
        /* Read and buffer next tuple. */
        tup = gm_readnext_tuple(gm_state,
                                reader,
                                nowait,
                                &tuple_buffer->done);
        if (!tup)
            return false;
 
        /*
         * Attempt to read more tuples in nowait mode and store them in the
         * pending-tuple array for the reader.
         */
        load_tuple_array(gm_state, reader);
    }
 
    Assert(tup);
 
    /* Build the TupleTableSlot for the given tuple */
    ExecStoreMinimalTuple(tup,  /* tuple to store */
                          gm_state->gm_slots[reader],   /* slot in which to
                                                         * store the tuple */
                          true);    /* pfree tuple when done with it */
 
    return true;
}
 
/*
 * Attempt to read a tuple from given worker.
 */
static MinimalTuple
gm_readnext_tuple(GatherMergeState *gm_state, int nreader, bool nowait,
                  bool *done)
{
    TupleQueueReader *reader;
    MinimalTuple tup;
 
    /* Check for async events, particularly messages from workers. */
    CHECK_FOR_INTERRUPTS();
 
    /*
     * Attempt to read a tuple.
     *
     * Note that TupleQueueReaderNext will just return NULL for a worker which
     * fails to initialize.  We'll treat that worker as having produced no
     * tuples; WaitForParallelWorkersToFinish will error out when we get
     * there.
     */
    reader = gm_state->reader[nreader - 1];
    tup = TupleQueueReaderNext(reader, nowait, done);
 
    /*
     * Since we'll be buffering these across multiple calls, we need to make a
     * copy.
     */
    return tup ? heap_copy_minimal_tuple(tup, 0) : NULL;
}
 
/*
 * 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;
 
/*
 * Compare the tuples in the two given slots.
 */
static int32
heap_compare_slots(Datum a, Datum b, void *arg)
{
    GatherMergeState *node = (GatherMergeState *) arg;
    SlotNumber  slot1 = DatumGetInt32(a);
    SlotNumber  slot2 = DatumGetInt32(b);
 
    TupleTableSlot *s1 = node->gm_slots[slot1];
    TupleTableSlot *s2 = node->gm_slots[slot2];
    int         nkey;
 
    Assert(!TupIsNull(s1));
    Assert(!TupIsNull(s2));
 
    for (nkey = 0; nkey < node->gm_nkeys; nkey++)
    {
        SortSupport sortKey = node->gm_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)
        {
            INVERT_COMPARE_RESULT(compare);
            return compare;
        }
    }
    return 0;
}