nodeHash.c

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/*-------------------------------------------------------------------------
 *
 * nodeHash.c
 *    Routines to hash relations for hashjoin
 *
 * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/executor/nodeHash.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * INTERFACE ROUTINES
 *      MultiExecHash   - generate an in-memory hash table of the relation
 *      ExecInitHash    - initialize node and subnodes
 *      ExecEndHash     - shutdown node and subnodes
 */

#include "postgres.h"

#include <math.h>
#include <limits.h>

#include "access/htup_details.h"
#include "catalog/pg_statistic.h"
#include "commands/tablespace.h"
#include "executor/execdebug.h"
#include "executor/hashjoin.h"
#include "executor/nodeHash.h"
#include "executor/nodeHashjoin.h"
#include "miscadmin.h"
#include "utils/dynahash.h"
#include "utils/memutils.h"
#include "utils/lsyscache.h"
#include "utils/syscache.h"


static void ExecHashIncreaseNumBatches(HashJoinTable hashtable);
static void ExecHashIncreaseNumBuckets(HashJoinTable hashtable);
static void ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node,
                      int mcvsToUse);
static void ExecHashSkewTableInsert(HashJoinTable hashtable,
                        TupleTableSlot *slot,
                        uint32 hashvalue,
                        int bucketNumber);
static void ExecHashRemoveNextSkewBucket(HashJoinTable hashtable);

static void *dense_alloc(HashJoinTable hashtable, Size size);

/* ----------------------------------------------------------------
 *      ExecHash
 *
 *      stub for pro forma compliance
 * ----------------------------------------------------------------
 */
TupleTableSlot *
ExecHash(HashState *node)
{
    elog(ERROR, "Hash node does not support ExecProcNode call convention");
    return NULL;
}

/* ----------------------------------------------------------------
 *      MultiExecHash
 *
 *      build hash table for hashjoin, doing partitioning if more
 *      than one batch is required.
 * ----------------------------------------------------------------
 */
Node *
MultiExecHash(HashState *node)
{
    PlanState  *outerNode;
    List       *hashkeys;
    HashJoinTable hashtable;
    TupleTableSlot *slot;
    ExprContext *econtext;
    uint32      hashvalue;

    /* must provide our own instrumentation support */
    if (node->ps.instrument)
        InstrStartNode(node->ps.instrument);

    /*
     * get state info from node
     */
    outerNode = outerPlanState(node);
    hashtable = node->hashtable;

    /*
     * set expression context
     */
    hashkeys = node->hashkeys;
    econtext = node->ps.ps_ExprContext;

    /*
     * get all inner tuples and insert into the hash table (or temp files)
     */
    for (;;)
    {
        slot = ExecProcNode(outerNode);
        if (TupIsNull(slot))
            break;
        /* We have to compute the hash value */
        econtext->ecxt_innertuple = slot;
        if (ExecHashGetHashValue(hashtable, econtext, hashkeys,
                                 false, hashtable->keepNulls,
                                 &hashvalue))
        {
            int         bucketNumber;

            bucketNumber = ExecHashGetSkewBucket(hashtable, hashvalue);
            if (bucketNumber != INVALID_SKEW_BUCKET_NO)
            {
                /* It's a skew tuple, so put it into that hash table */
                ExecHashSkewTableInsert(hashtable, slot, hashvalue,
                                        bucketNumber);
                hashtable->skewTuples += 1;
            }
            else
            {
                /* Not subject to skew optimization, so insert normally */
                ExecHashTableInsert(hashtable, slot, hashvalue);
            }
            hashtable->totalTuples += 1;
        }
    }

    /* resize the hash table if needed (NTUP_PER_BUCKET exceeded) */
    if (hashtable->nbuckets != hashtable->nbuckets_optimal)
        ExecHashIncreaseNumBuckets(hashtable);

    /* Account for the buckets in spaceUsed (reported in EXPLAIN ANALYZE) */
    hashtable->spaceUsed += hashtable->nbuckets * sizeof(HashJoinTuple);
    if (hashtable->spaceUsed > hashtable->spacePeak)
        hashtable->spacePeak = hashtable->spaceUsed;

    /* must provide our own instrumentation support */
    if (node->ps.instrument)
        InstrStopNode(node->ps.instrument, hashtable->totalTuples);

    /*
     * We do not return the hash table directly because it's not a subtype of
     * Node, and so would violate the MultiExecProcNode API.  Instead, our
     * parent Hashjoin node is expected to know how to fish it out of our node
     * state.  Ugly but not really worth cleaning up, since Hashjoin knows
     * quite a bit more about Hash besides that.
     */
    return NULL;
}

/* ----------------------------------------------------------------
 *      ExecInitHash
 *
 *      Init routine for Hash node
 * ----------------------------------------------------------------
 */
HashState *
ExecInitHash(Hash *node, EState *estate, int eflags)
{
    HashState  *hashstate;

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

    /*
     * create state structure
     */
    hashstate = makeNode(HashState);
    hashstate->ps.plan = (Plan *) node;
    hashstate->ps.state = estate;
    hashstate->hashtable = NULL;
    hashstate->hashkeys = NIL;  /* will be set by parent HashJoin */

    /*
     * Miscellaneous initialization
     *
     * create expression context for node
     */
    ExecAssignExprContext(estate, &hashstate->ps);

    /*
     * initialize our result slot
     */
    ExecInitResultTupleSlot(estate, &hashstate->ps);

    /*
     * initialize child expressions
     */
    hashstate->ps.targetlist = (List *)
        ExecInitExpr((Expr *) node->plan.targetlist,
                     (PlanState *) hashstate);
    hashstate->ps.qual = (List *)
        ExecInitExpr((Expr *) node->plan.qual,
                     (PlanState *) hashstate);

    /*
     * initialize child nodes
     */
    outerPlanState(hashstate) = ExecInitNode(outerPlan(node), estate, eflags);

    /*
     * initialize tuple type. no need to initialize projection info because
     * this node doesn't do projections
     */
    ExecAssignResultTypeFromTL(&hashstate->ps);
    hashstate->ps.ps_ProjInfo = NULL;

    return hashstate;
}

/* ---------------------------------------------------------------
 *      ExecEndHash
 *
 *      clean up routine for Hash node
 * ----------------------------------------------------------------
 */
void
ExecEndHash(HashState *node)
{
    PlanState  *outerPlan;

    /*
     * free exprcontext
     */
    ExecFreeExprContext(&node->ps);

    /*
     * shut down the subplan
     */
    outerPlan = outerPlanState(node);
    ExecEndNode(outerPlan);
}


/* ----------------------------------------------------------------
 *      ExecHashTableCreate
 *
 *      create an empty hashtable data structure for hashjoin.
 * ----------------------------------------------------------------
 */
HashJoinTable
ExecHashTableCreate(Hash *node, List *hashOperators, bool keepNulls)
{
    HashJoinTable hashtable;
    Plan       *outerNode;
    int         nbuckets;
    int         nbatch;
    int         num_skew_mcvs;
    int         log2_nbuckets;
    int         nkeys;
    int         i;
    ListCell   *ho;
    MemoryContext oldcxt;

    /*
     * Get information about the size of the relation to be hashed (it's the
     * "outer" subtree of this node, but the inner relation of the hashjoin).
     * Compute the appropriate size of the hash table.
     */
    outerNode = outerPlan(node);

    ExecChooseHashTableSize(outerNode->plan_rows, outerNode->plan_width,
                            OidIsValid(node->skewTable),
                            &nbuckets, &nbatch, &num_skew_mcvs);

    /* nbuckets must be a power of 2 */
    log2_nbuckets = my_log2(nbuckets);
    Assert(nbuckets == (1 << log2_nbuckets));

    /*
     * Initialize the hash table control block.
     *
     * The hashtable control block is just palloc'd from the executor's
     * per-query memory context.
     */
    hashtable = (HashJoinTable) palloc(sizeof(HashJoinTableData));
    hashtable->nbuckets = nbuckets;
    hashtable->nbuckets_original = nbuckets;
    hashtable->nbuckets_optimal = nbuckets;
    hashtable->log2_nbuckets = log2_nbuckets;
    hashtable->log2_nbuckets_optimal = log2_nbuckets;
    hashtable->buckets = NULL;
    hashtable->keepNulls = keepNulls;
    hashtable->skewEnabled = false;
    hashtable->skewBucket = NULL;
    hashtable->skewBucketLen = 0;
    hashtable->nSkewBuckets = 0;
    hashtable->skewBucketNums = NULL;
    hashtable->nbatch = nbatch;
    hashtable->curbatch = 0;
    hashtable->nbatch_original = nbatch;
    hashtable->nbatch_outstart = nbatch;
    hashtable->growEnabled = true;
    hashtable->totalTuples = 0;
    hashtable->skewTuples = 0;
    hashtable->innerBatchFile = NULL;
    hashtable->outerBatchFile = NULL;
    hashtable->spaceUsed = 0;
    hashtable->spacePeak = 0;
    hashtable->spaceAllowed = work_mem * 1024L;
    hashtable->spaceUsedSkew = 0;
    hashtable->spaceAllowedSkew =
        hashtable->spaceAllowed * SKEW_WORK_MEM_PERCENT / 100;
    hashtable->chunks = NULL;

#ifdef HJDEBUG
    printf("Hashjoin %p: initial nbatch = %d, nbuckets = %d\n",
           hashtable, nbatch, nbuckets);
#endif

    /*
     * Get info about the hash functions to be used for each hash key. Also
     * remember whether the join operators are strict.
     */
    nkeys = list_length(hashOperators);
    hashtable->outer_hashfunctions =
        (FmgrInfo *) palloc(nkeys * sizeof(FmgrInfo));
    hashtable->inner_hashfunctions =
        (FmgrInfo *) palloc(nkeys * sizeof(FmgrInfo));
    hashtable->hashStrict = (bool *) palloc(nkeys * sizeof(bool));
    i = 0;
    foreach(ho, hashOperators)
    {
        Oid         hashop = lfirst_oid(ho);
        Oid         left_hashfn;
        Oid         right_hashfn;

        if (!get_op_hash_functions(hashop, &left_hashfn, &right_hashfn))
            elog(ERROR, "could not find hash function for hash operator %u",
                 hashop);
        fmgr_info(left_hashfn, &hashtable->outer_hashfunctions[i]);
        fmgr_info(right_hashfn, &hashtable->inner_hashfunctions[i]);
        hashtable->hashStrict[i] = op_strict(hashop);
        i++;
    }

    /*
     * Create temporary memory contexts in which to keep the hashtable working
     * storage.  See notes in executor/hashjoin.h.
     */
    hashtable->hashCxt = AllocSetContextCreate(CurrentMemoryContext,
                                               "HashTableContext",
                                               ALLOCSET_DEFAULT_MINSIZE,
                                               ALLOCSET_DEFAULT_INITSIZE,
                                               ALLOCSET_DEFAULT_MAXSIZE);

    hashtable->batchCxt = AllocSetContextCreate(hashtable->hashCxt,
                                                "HashBatchContext",
                                                ALLOCSET_DEFAULT_MINSIZE,
                                                ALLOCSET_DEFAULT_INITSIZE,
                                                ALLOCSET_DEFAULT_MAXSIZE);

    /* Allocate data that will live for the life of the hashjoin */

    oldcxt = MemoryContextSwitchTo(hashtable->hashCxt);

    if (nbatch > 1)
    {
        /*
         * allocate and initialize the file arrays in hashCxt
         */
        hashtable->innerBatchFile = (BufFile **)
            palloc0(nbatch * sizeof(BufFile *));
        hashtable->outerBatchFile = (BufFile **)
            palloc0(nbatch * sizeof(BufFile *));
        /* The files will not be opened until needed... */
        /* ... but make sure we have temp tablespaces established for them */
        PrepareTempTablespaces();
    }

    /*
     * Prepare context for the first-scan space allocations; allocate the
     * hashbucket array therein, and set each bucket "empty".
     */
    MemoryContextSwitchTo(hashtable->batchCxt);

    hashtable->buckets = (HashJoinTuple *)
        palloc0(nbuckets * sizeof(HashJoinTuple));

    /*
     * Set up for skew optimization, if possible and there's a need for more
     * than one batch.  (In a one-batch join, there's no point in it.)
     */
    if (nbatch > 1)
        ExecHashBuildSkewHash(hashtable, node, num_skew_mcvs);

    MemoryContextSwitchTo(oldcxt);

    return hashtable;
}


/*
 * Compute appropriate size for hashtable given the estimated size of the
 * relation to be hashed (number of rows and average row width).
 *
 * This is exported so that the planner's costsize.c can use it.
 */

/* Target bucket loading (tuples per bucket) */
#define NTUP_PER_BUCKET         1

void
ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
                        int *numbuckets,
                        int *numbatches,
                        int *num_skew_mcvs)
{
    int         tupsize;
    double      inner_rel_bytes;
    long        bucket_bytes;
    long        hash_table_bytes;
    long        skew_table_bytes;
    long        max_pointers;
    long        mppow2;
    int         nbatch = 1;
    int         nbuckets;
    double      dbuckets;

    /* Force a plausible relation size if no info */
    if (ntuples <= 0.0)
        ntuples = 1000.0;

    /*
     * Estimate tupsize based on footprint of tuple in hashtable... note this
     * does not allow for any palloc overhead.  The manipulations of spaceUsed
     * don't count palloc overhead either.
     */
    tupsize = HJTUPLE_OVERHEAD +
        MAXALIGN(SizeofMinimalTupleHeader) +
        MAXALIGN(tupwidth);
    inner_rel_bytes = ntuples * tupsize;

    /*
     * Target in-memory hashtable size is work_mem kilobytes.
     */
    hash_table_bytes = work_mem * 1024L;

    /*
     * If skew optimization is possible, estimate the number of skew buckets
     * that will fit in the memory allowed, and decrement the assumed space
     * available for the main hash table accordingly.
     *
     * We make the optimistic assumption that each skew bucket will contain
     * one inner-relation tuple.  If that turns out to be low, we will recover
     * at runtime by reducing the number of skew buckets.
     *
     * hashtable->skewBucket will have up to 8 times as many HashSkewBucket
     * pointers as the number of MCVs we allow, since ExecHashBuildSkewHash
     * will round up to the next power of 2 and then multiply by 4 to reduce
     * collisions.
     */
    if (useskew)
    {
        skew_table_bytes = hash_table_bytes * SKEW_WORK_MEM_PERCENT / 100;

        /*----------
         * Divisor is:
         * size of a hash tuple +
         * worst-case size of skewBucket[] per MCV +
         * size of skewBucketNums[] entry +
         * size of skew bucket struct itself
         *----------
         */
        *num_skew_mcvs = skew_table_bytes / (tupsize +
                                             (8 * sizeof(HashSkewBucket *)) +
                                             sizeof(int) +
                                             SKEW_BUCKET_OVERHEAD);
        if (*num_skew_mcvs > 0)
            hash_table_bytes -= skew_table_bytes;
    }
    else
        *num_skew_mcvs = 0;

    /*
     * Set nbuckets to achieve an average bucket load of NTUP_PER_BUCKET when
     * memory is filled, assuming a single batch; but limit the value so that
     * the pointer arrays we'll try to allocate do not exceed work_mem nor
     * MaxAllocSize.
     *
     * Note that both nbuckets and nbatch must be powers of 2 to make
     * ExecHashGetBucketAndBatch fast.
     */
    max_pointers = (work_mem * 1024L) / sizeof(HashJoinTuple);
    max_pointers = Min(max_pointers, MaxAllocSize / sizeof(HashJoinTuple));
    /* If max_pointers isn't a power of 2, must round it down to one */
    mppow2 = 1L << my_log2(max_pointers);
    if (max_pointers != mppow2)
        max_pointers = mppow2 / 2;

    /* Also ensure we avoid integer overflow in nbatch and nbuckets */
    /* (this step is redundant given the current value of MaxAllocSize) */
    max_pointers = Min(max_pointers, INT_MAX / 2);

    dbuckets = ceil(ntuples / NTUP_PER_BUCKET);
    dbuckets = Min(dbuckets, max_pointers);
    nbuckets = (int) dbuckets;
    /* don't let nbuckets be really small, though ... */
    nbuckets = Max(nbuckets, 1024);
    /* ... and force it to be a power of 2. */
    nbuckets = 1 << my_log2(nbuckets);

    /*
     * If there's not enough space to store the projected number of tuples and
     * the required bucket headers, we will need multiple batches.
     */
    bucket_bytes = sizeof(HashJoinTuple) * nbuckets;
    if (inner_rel_bytes + bucket_bytes > hash_table_bytes)
    {
        /* We'll need multiple batches */
        long        lbuckets;
        double      dbatch;
        int         minbatch;
        long        bucket_size;

        /*
         * Estimate the number of buckets we'll want to have when work_mem is
         * entirely full.  Each bucket will contain a bucket pointer plus
         * NTUP_PER_BUCKET tuples, whose projected size already includes
         * overhead for the hash code, pointer to the next tuple, etc.
         */
        bucket_size = (tupsize * NTUP_PER_BUCKET + sizeof(HashJoinTuple));
        lbuckets = 1L << my_log2(hash_table_bytes / bucket_size);
        lbuckets = Min(lbuckets, max_pointers);
        nbuckets = (int) lbuckets;
        nbuckets = 1 << my_log2(nbuckets);
        bucket_bytes = nbuckets * sizeof(HashJoinTuple);

        /*
         * Buckets are simple pointers to hashjoin tuples, while tupsize
         * includes the pointer, hash code, and MinimalTupleData.  So buckets
         * should never really exceed 25% of work_mem (even for
         * NTUP_PER_BUCKET=1); except maybe for work_mem values that are not
         * 2^N bytes, where we might get more because of doubling. So let's
         * look for 50% here.
         */
        Assert(bucket_bytes <= hash_table_bytes / 2);

        /* Calculate required number of batches. */
        dbatch = ceil(inner_rel_bytes / (hash_table_bytes - bucket_bytes));
        dbatch = Min(dbatch, max_pointers);
        minbatch = (int) dbatch;
        nbatch = 2;
        while (nbatch < minbatch)
            nbatch <<= 1;
    }

    Assert(nbuckets > 0);
    Assert(nbatch > 0);

    *numbuckets = nbuckets;
    *numbatches = nbatch;
}


/* ----------------------------------------------------------------
 *      ExecHashTableDestroy
 *
 *      destroy a hash table
 * ----------------------------------------------------------------
 */
void
ExecHashTableDestroy(HashJoinTable hashtable)
{
    int         i;

    /*
     * Make sure all the temp files are closed.  We skip batch 0, since it
     * can't have any temp files (and the arrays might not even exist if
     * nbatch is only 1).
     */
    for (i = 1; i < hashtable->nbatch; i++)
    {
        if (hashtable->innerBatchFile[i])
            BufFileClose(hashtable->innerBatchFile[i]);
        if (hashtable->outerBatchFile[i])
            BufFileClose(hashtable->outerBatchFile[i]);
    }

    /* Release working memory (batchCxt is a child, so it goes away too) */
    MemoryContextDelete(hashtable->hashCxt);

    /* And drop the control block */
    pfree(hashtable);
}

/*
 * ExecHashIncreaseNumBatches
 *      increase the original number of batches in order to reduce
 *      current memory consumption
 */
static void
ExecHashIncreaseNumBatches(HashJoinTable hashtable)
{
    int         oldnbatch = hashtable->nbatch;
    int         curbatch = hashtable->curbatch;
    int         nbatch;
    MemoryContext oldcxt;
    long        ninmemory;
    long        nfreed;
    HashMemoryChunk oldchunks;

    /* do nothing if we've decided to shut off growth */
    if (!hashtable->growEnabled)
        return;

    /* safety check to avoid overflow */
    if (oldnbatch > Min(INT_MAX / 2, MaxAllocSize / (sizeof(void *) * 2)))
        return;

    nbatch = oldnbatch * 2;
    Assert(nbatch > 1);

#ifdef HJDEBUG
    printf("Hashjoin %p: increasing nbatch to %d because space = %zu\n",
           hashtable, nbatch, hashtable->spaceUsed);
#endif

    oldcxt = MemoryContextSwitchTo(hashtable->hashCxt);

    if (hashtable->innerBatchFile == NULL)
    {
        /* we had no file arrays before */
        hashtable->innerBatchFile = (BufFile **)
            palloc0(nbatch * sizeof(BufFile *));
        hashtable->outerBatchFile = (BufFile **)
            palloc0(nbatch * sizeof(BufFile *));
        /* time to establish the temp tablespaces, too */
        PrepareTempTablespaces();
    }
    else
    {
        /* enlarge arrays and zero out added entries */
        hashtable->innerBatchFile = (BufFile **)
            repalloc(hashtable->innerBatchFile, nbatch * sizeof(BufFile *));
        hashtable->outerBatchFile = (BufFile **)
            repalloc(hashtable->outerBatchFile, nbatch * sizeof(BufFile *));
        MemSet(hashtable->innerBatchFile + oldnbatch, 0,
               (nbatch - oldnbatch) * sizeof(BufFile *));
        MemSet(hashtable->outerBatchFile + oldnbatch, 0,
               (nbatch - oldnbatch) * sizeof(BufFile *));
    }

    MemoryContextSwitchTo(oldcxt);

    hashtable->nbatch = nbatch;

    /*
     * Scan through the existing hash table entries and dump out any that are
     * no longer of the current batch.
     */
    ninmemory = nfreed = 0;

    /* If know we need to resize nbuckets, we can do it while rebatching. */
    if (hashtable->nbuckets_optimal != hashtable->nbuckets)
    {
        /* we never decrease the number of buckets */
        Assert(hashtable->nbuckets_optimal > hashtable->nbuckets);

        hashtable->nbuckets = hashtable->nbuckets_optimal;
        hashtable->log2_nbuckets = hashtable->log2_nbuckets_optimal;

        hashtable->buckets = repalloc(hashtable->buckets,
                                sizeof(HashJoinTuple) * hashtable->nbuckets);
    }

    /*
     * We will scan through the chunks directly, so that we can reset the
     * buckets now and not have to keep track which tuples in the buckets have
     * already been processed. We will free the old chunks as we go.
     */
    memset(hashtable->buckets, 0, sizeof(HashJoinTuple) * hashtable->nbuckets);
    oldchunks = hashtable->chunks;
    hashtable->chunks = NULL;

    /* so, let's scan through the old chunks, and all tuples in each chunk */
    while (oldchunks != NULL)
    {
        HashMemoryChunk nextchunk = oldchunks->next;

        /* position within the buffer (up to oldchunks->used) */
        size_t      idx = 0;

        /* process all tuples stored in this chunk (and then free it) */
        while (idx < oldchunks->used)
        {
            HashJoinTuple hashTuple = (HashJoinTuple) (oldchunks->data + idx);
            MinimalTuple tuple = HJTUPLE_MINTUPLE(hashTuple);
            int         hashTupleSize = (HJTUPLE_OVERHEAD + tuple->t_len);
            int         bucketno;
            int         batchno;

            ninmemory++;
            ExecHashGetBucketAndBatch(hashtable, hashTuple->hashvalue,
                                      &bucketno, &batchno);

            if (batchno == curbatch)
            {
                /* keep tuple in memory - copy it into the new chunk */
                HashJoinTuple copyTuple;

                copyTuple = (HashJoinTuple) dense_alloc(hashtable, hashTupleSize);
                memcpy(copyTuple, hashTuple, hashTupleSize);

                /* and add it back to the appropriate bucket */
                copyTuple->next = hashtable->buckets[bucketno];
                hashtable->buckets[bucketno] = copyTuple;
            }
            else
            {
                /* dump it out */
                Assert(batchno > curbatch);
                ExecHashJoinSaveTuple(HJTUPLE_MINTUPLE(hashTuple),
                                      hashTuple->hashvalue,
                                      &hashtable->innerBatchFile[batchno]);

                hashtable->spaceUsed -= hashTupleSize;
                nfreed++;
            }

            /* next tuple in this chunk */
            idx += MAXALIGN(hashTupleSize);
        }

        /* we're done with this chunk - free it and proceed to the next one */
        pfree(oldchunks);
        oldchunks = nextchunk;
    }

#ifdef HJDEBUG
    printf("Hashjoin %p: freed %ld of %ld tuples, space now %zu\n",
           hashtable, nfreed, ninmemory, hashtable->spaceUsed);
#endif

    /*
     * If we dumped out either all or none of the tuples in the table, disable
     * further expansion of nbatch.  This situation implies that we have
     * enough tuples of identical hashvalues to overflow spaceAllowed.
     * Increasing nbatch will not fix it since there's no way to subdivide the
     * group any more finely. We have to just gut it out and hope the server
     * has enough RAM.
     */
    if (nfreed == 0 || nfreed == ninmemory)
    {
        hashtable->growEnabled = false;
#ifdef HJDEBUG
        printf("Hashjoin %p: disabling further increase of nbatch\n",
               hashtable);
#endif
    }
}

/*
 * ExecHashIncreaseNumBuckets
 *      increase the original number of buckets in order to reduce
 *      number of tuples per bucket
 */
static void
ExecHashIncreaseNumBuckets(HashJoinTable hashtable)
{
    HashMemoryChunk chunk;

    /* do nothing if not an increase (it's called increase for a reason) */
    if (hashtable->nbuckets >= hashtable->nbuckets_optimal)
        return;

#ifdef HJDEBUG
    printf("Hashjoin %p: increasing nbuckets %d => %d\n",
           hashtable, hashtable->nbuckets, hashtable->nbuckets_optimal);
#endif

    hashtable->nbuckets = hashtable->nbuckets_optimal;
    hashtable->log2_nbuckets = hashtable->log2_nbuckets_optimal;

    Assert(hashtable->nbuckets > 1);
    Assert(hashtable->nbuckets <= (INT_MAX / 2));
    Assert(hashtable->nbuckets == (1 << hashtable->log2_nbuckets));

    /*
     * Just reallocate the proper number of buckets - we don't need to walk
     * through them - we can walk the dense-allocated chunks (just like in
     * ExecHashIncreaseNumBatches, but without all the copying into new
     * chunks)
     */
    hashtable->buckets =
        (HashJoinTuple *) repalloc(hashtable->buckets,
                                hashtable->nbuckets * sizeof(HashJoinTuple));

    memset(hashtable->buckets, 0, hashtable->nbuckets * sizeof(HashJoinTuple));

    /* scan through all tuples in all chunks to rebuild the hash table */
    for (chunk = hashtable->chunks; chunk != NULL; chunk = chunk->next)
    {
        /* process all tuples stored in this chunk */
        size_t      idx = 0;

        while (idx < chunk->used)
        {
            HashJoinTuple hashTuple = (HashJoinTuple) (chunk->data + idx);
            int         bucketno;
            int         batchno;

            ExecHashGetBucketAndBatch(hashtable, hashTuple->hashvalue,
                                      &bucketno, &batchno);

            /* add the tuple to the proper bucket */
            hashTuple->next = hashtable->buckets[bucketno];
            hashtable->buckets[bucketno] = hashTuple;

            /* advance index past the tuple */
            idx += MAXALIGN(HJTUPLE_OVERHEAD +
                            HJTUPLE_MINTUPLE(hashTuple)->t_len);
        }
    }
}


/*
 * ExecHashTableInsert
 *      insert a tuple into the hash table depending on the hash value
 *      it may just go to a temp file for later batches
 *
 * Note: the passed TupleTableSlot may contain a regular, minimal, or virtual
 * tuple; the minimal case in particular is certain to happen while reloading
 * tuples from batch files.  We could save some cycles in the regular-tuple
 * case by not forcing the slot contents into minimal form; not clear if it's
 * worth the messiness required.
 */
void
ExecHashTableInsert(HashJoinTable hashtable,
                    TupleTableSlot *slot,
                    uint32 hashvalue)
{
    MinimalTuple tuple = ExecFetchSlotMinimalTuple(slot);
    int         bucketno;
    int         batchno;

    ExecHashGetBucketAndBatch(hashtable, hashvalue,
                              &bucketno, &batchno);

    /*
     * decide whether to put the tuple in the hash table or a temp file
     */
    if (batchno == hashtable->curbatch)
    {
        /*
         * put the tuple in hash table
         */
        HashJoinTuple hashTuple;
        int         hashTupleSize;
        double      ntuples = (hashtable->totalTuples - hashtable->skewTuples);

        /* Create the HashJoinTuple */
        hashTupleSize = HJTUPLE_OVERHEAD + tuple->t_len;
        hashTuple = (HashJoinTuple) dense_alloc(hashtable, hashTupleSize);

        hashTuple->hashvalue = hashvalue;
        memcpy(HJTUPLE_MINTUPLE(hashTuple), tuple, tuple->t_len);

        /*
         * We always reset the tuple-matched flag on insertion.  This is okay
         * even when reloading a tuple from a batch file, since the tuple
         * could not possibly have been matched to an outer tuple before it
         * went into the batch file.
         */
        HeapTupleHeaderClearMatch(HJTUPLE_MINTUPLE(hashTuple));

        /* Push it onto the front of the bucket's list */
        hashTuple->next = hashtable->buckets[bucketno];
        hashtable->buckets[bucketno] = hashTuple;

        /*
         * Increase the (optimal) number of buckets if we just exceeded the
         * NTUP_PER_BUCKET threshold, but only when there's still a single
         * batch.
         */
        if (hashtable->nbatch == 1 &&
            ntuples > (hashtable->nbuckets_optimal * NTUP_PER_BUCKET))
        {
            /* Guard against integer overflow and alloc size overflow */
            if (hashtable->nbuckets_optimal <= INT_MAX / 2 &&
                hashtable->nbuckets_optimal * 2 <= MaxAllocSize / sizeof(HashJoinTuple))
            {
                hashtable->nbuckets_optimal *= 2;
                hashtable->log2_nbuckets_optimal += 1;
            }
        }

        /* Account for space used, and back off if we've used too much */
        hashtable->spaceUsed += hashTupleSize;
        if (hashtable->spaceUsed > hashtable->spacePeak)
            hashtable->spacePeak = hashtable->spaceUsed;
        if (hashtable->spaceUsed +
            hashtable->nbuckets_optimal * sizeof(HashJoinTuple)
            > hashtable->spaceAllowed)
            ExecHashIncreaseNumBatches(hashtable);
    }
    else
    {
        /*
         * put the tuple into a temp file for later batches
         */
        Assert(batchno > hashtable->curbatch);
        ExecHashJoinSaveTuple(tuple,
                              hashvalue,
                              &hashtable->innerBatchFile[batchno]);
    }
}

/*
 * ExecHashGetHashValue
 *      Compute the hash value for a tuple
 *
 * The tuple to be tested must be in either econtext->ecxt_outertuple or
 * econtext->ecxt_innertuple.  Vars in the hashkeys expressions should have
 * varno either OUTER_VAR or INNER_VAR.
 *
 * A TRUE result means the tuple's hash value has been successfully computed
 * and stored at *hashvalue.  A FALSE result means the tuple cannot match
 * because it contains a null attribute, and hence it should be discarded
 * immediately.  (If keep_nulls is true then FALSE is never returned.)
 */
bool
ExecHashGetHashValue(HashJoinTable hashtable,
                     ExprContext *econtext,
                     List *hashkeys,
                     bool outer_tuple,
                     bool keep_nulls,
                     uint32 *hashvalue)
{
    uint32      hashkey = 0;
    FmgrInfo   *hashfunctions;
    ListCell   *hk;
    int         i = 0;
    MemoryContext oldContext;

    /*
     * We reset the eval context each time to reclaim any memory leaked in the
     * hashkey expressions.
     */
    ResetExprContext(econtext);

    oldContext = MemoryContextSwitchTo(econtext->ecxt_per_tuple_memory);

    if (outer_tuple)
        hashfunctions = hashtable->outer_hashfunctions;
    else
        hashfunctions = hashtable->inner_hashfunctions;

    foreach(hk, hashkeys)
    {
        ExprState  *keyexpr = (ExprState *) lfirst(hk);
        Datum       keyval;
        bool        isNull;

        /* rotate hashkey left 1 bit at each step */
        hashkey = (hashkey << 1) | ((hashkey & 0x80000000) ? 1 : 0);

        /*
         * Get the join attribute value of the tuple
         */
        keyval = ExecEvalExpr(keyexpr, econtext, &isNull, NULL);

        /*
         * If the attribute is NULL, and the join operator is strict, then
         * this tuple cannot pass the join qual so we can reject it
         * immediately (unless we're scanning the outside of an outer join, in
         * which case we must not reject it).  Otherwise we act like the
         * hashcode of NULL is zero (this will support operators that act like
         * IS NOT DISTINCT, though not any more-random behavior).  We treat
         * the hash support function as strict even if the operator is not.
         *
         * Note: currently, all hashjoinable operators must be strict since
         * the hash index AM assumes that.  However, it takes so little extra
         * code here to allow non-strict that we may as well do it.
         */
        if (isNull)
        {
            if (hashtable->hashStrict[i] && !keep_nulls)
            {
                MemoryContextSwitchTo(oldContext);
                return false;   /* cannot match */
            }
            /* else, leave hashkey unmodified, equivalent to hashcode 0 */
        }
        else
        {
            /* Compute the hash function */
            uint32      hkey;

            hkey = DatumGetUInt32(FunctionCall1(&hashfunctions[i], keyval));
            hashkey ^= hkey;
        }

        i++;
    }

    MemoryContextSwitchTo(oldContext);

    *hashvalue = hashkey;
    return true;
}

/*
 * ExecHashGetBucketAndBatch
 *      Determine the bucket number and batch number for a hash value
 *
 * Note: on-the-fly increases of nbatch must not change the bucket number
 * for a given hash code (since we don't move tuples to different hash
 * chains), and must only cause the batch number to remain the same or
 * increase.  Our algorithm is
 *      bucketno = hashvalue MOD nbuckets
 *      batchno = (hashvalue DIV nbuckets) MOD nbatch
 * where nbuckets and nbatch are both expected to be powers of 2, so we can
 * do the computations by shifting and masking.  (This assumes that all hash
 * functions are good about randomizing all their output bits, else we are
 * likely to have very skewed bucket or batch occupancy.)
 *
 * nbuckets and log2_nbuckets may change while nbatch == 1 because of dynamic
 * bucket count growth.  Once we start batching, the value is fixed and does
 * not change over the course of the join (making it possible to compute batch
 * number the way we do here).
 *
 * nbatch is always a power of 2; we increase it only by doubling it.  This
 * effectively adds one more bit to the top of the batchno.
 */
void
ExecHashGetBucketAndBatch(HashJoinTable hashtable,
                          uint32 hashvalue,
                          int *bucketno,
                          int *batchno)
{
    uint32      nbuckets = (uint32) hashtable->nbuckets;
    uint32      nbatch = (uint32) hashtable->nbatch;

    if (nbatch > 1)
    {
        /* we can do MOD by masking, DIV by shifting */
        *bucketno = hashvalue & (nbuckets - 1);
        *batchno = (hashvalue >> hashtable->log2_nbuckets) & (nbatch - 1);
    }
    else
    {
        *bucketno = hashvalue & (nbuckets - 1);
        *batchno = 0;
    }
}

/*
 * ExecScanHashBucket
 *      scan a hash bucket for matches to the current outer tuple
 *
 * The current outer tuple must be stored in econtext->ecxt_outertuple.
 *
 * On success, the inner tuple is stored into hjstate->hj_CurTuple and
 * econtext->ecxt_innertuple, using hjstate->hj_HashTupleSlot as the slot
 * for the latter.
 */
bool
ExecScanHashBucket(HashJoinState *hjstate,
                   ExprContext *econtext)
{
    List       *hjclauses = hjstate->hashclauses;
    HashJoinTable hashtable = hjstate->hj_HashTable;
    HashJoinTuple hashTuple = hjstate->hj_CurTuple;
    uint32      hashvalue = hjstate->hj_CurHashValue;

    /*
     * hj_CurTuple is the address of the tuple last returned from the current
     * bucket, or NULL if it's time to start scanning a new bucket.
     *
     * If the tuple hashed to a skew bucket then scan the skew bucket
     * otherwise scan the standard hashtable bucket.
     */
    if (hashTuple != NULL)
        hashTuple = hashTuple->next;
    else if (hjstate->hj_CurSkewBucketNo != INVALID_SKEW_BUCKET_NO)
        hashTuple = hashtable->skewBucket[hjstate->hj_CurSkewBucketNo]->tuples;
    else
        hashTuple = hashtable->buckets[hjstate->hj_CurBucketNo];

    while (hashTuple != NULL)
    {
        if (hashTuple->hashvalue == hashvalue)
        {
            TupleTableSlot *inntuple;

            /* insert hashtable's tuple into exec slot so ExecQual sees it */
            inntuple = ExecStoreMinimalTuple(HJTUPLE_MINTUPLE(hashTuple),
                                             hjstate->hj_HashTupleSlot,
                                             false);    /* do not pfree */
            econtext->ecxt_innertuple = inntuple;

            /* reset temp memory each time to avoid leaks from qual expr */
            ResetExprContext(econtext);

            if (ExecQual(hjclauses, econtext, false))
            {
                hjstate->hj_CurTuple = hashTuple;
                return true;
            }
        }

        hashTuple = hashTuple->next;
    }

    /*
     * no match
     */
    return false;
}

/*
 * ExecPrepHashTableForUnmatched
 *      set up for a series of ExecScanHashTableForUnmatched calls
 */
void
ExecPrepHashTableForUnmatched(HashJoinState *hjstate)
{
    /*----------
     * During this scan we use the HashJoinState fields as follows:
     *
     * hj_CurBucketNo: next regular bucket to scan
     * hj_CurSkewBucketNo: next skew bucket (an index into skewBucketNums)
     * hj_CurTuple: last tuple returned, or NULL to start next bucket
     *----------
     */
    hjstate->hj_CurBucketNo = 0;
    hjstate->hj_CurSkewBucketNo = 0;
    hjstate->hj_CurTuple = NULL;
}

/*
 * ExecScanHashTableForUnmatched
 *      scan the hash table for unmatched inner tuples
 *
 * On success, the inner tuple is stored into hjstate->hj_CurTuple and
 * econtext->ecxt_innertuple, using hjstate->hj_HashTupleSlot as the slot
 * for the latter.
 */
bool
ExecScanHashTableForUnmatched(HashJoinState *hjstate, ExprContext *econtext)
{
    HashJoinTable hashtable = hjstate->hj_HashTable;
    HashJoinTuple hashTuple = hjstate->hj_CurTuple;

    for (;;)
    {
        /*
         * hj_CurTuple is the address of the tuple last returned from the
         * current bucket, or NULL if it's time to start scanning a new
         * bucket.
         */
        if (hashTuple != NULL)
            hashTuple = hashTuple->next;
        else if (hjstate->hj_CurBucketNo < hashtable->nbuckets)
        {
            hashTuple = hashtable->buckets[hjstate->hj_CurBucketNo];
            hjstate->hj_CurBucketNo++;
        }
        else if (hjstate->hj_CurSkewBucketNo < hashtable->nSkewBuckets)
        {
            int         j = hashtable->skewBucketNums[hjstate->hj_CurSkewBucketNo];

            hashTuple = hashtable->skewBucket[j]->tuples;
            hjstate->hj_CurSkewBucketNo++;
        }
        else
            break;              /* finished all buckets */

        while (hashTuple != NULL)
        {
            if (!HeapTupleHeaderHasMatch(HJTUPLE_MINTUPLE(hashTuple)))
            {
                TupleTableSlot *inntuple;

                /* insert hashtable's tuple into exec slot */
                inntuple = ExecStoreMinimalTuple(HJTUPLE_MINTUPLE(hashTuple),
                                                 hjstate->hj_HashTupleSlot,
                                                 false);        /* do not pfree */
                econtext->ecxt_innertuple = inntuple;

                /*
                 * Reset temp memory each time; although this function doesn't
                 * do any qual eval, the caller will, so let's keep it
                 * parallel to ExecScanHashBucket.
                 */
                ResetExprContext(econtext);

                hjstate->hj_CurTuple = hashTuple;
                return true;
            }

            hashTuple = hashTuple->next;
        }
    }

    /*
     * no more unmatched tuples
     */
    return false;
}

/*
 * ExecHashTableReset
 *
 *      reset hash table header for new batch
 */
void
ExecHashTableReset(HashJoinTable hashtable)
{
    MemoryContext oldcxt;
    int         nbuckets = hashtable->nbuckets;

    /*
     * Release all the hash buckets and tuples acquired in the prior pass, and
     * reinitialize the context for a new pass.
     */
    MemoryContextReset(hashtable->batchCxt);
    oldcxt = MemoryContextSwitchTo(hashtable->batchCxt);

    /* Reallocate and reinitialize the hash bucket headers. */
    hashtable->buckets = (HashJoinTuple *)
        palloc0(nbuckets * sizeof(HashJoinTuple));

    hashtable->spaceUsed = 0;

    MemoryContextSwitchTo(oldcxt);

    /* Forget the chunks (the memory was freed by the context reset above). */
    hashtable->chunks = NULL;
}

/*
 * ExecHashTableResetMatchFlags
 *      Clear all the HeapTupleHeaderHasMatch flags in the table
 */
void
ExecHashTableResetMatchFlags(HashJoinTable hashtable)
{
    HashJoinTuple tuple;
    int         i;

    /* Reset all flags in the main table ... */
    for (i = 0; i < hashtable->nbuckets; i++)
    {
        for (tuple = hashtable->buckets[i]; tuple != NULL; tuple = tuple->next)
            HeapTupleHeaderClearMatch(HJTUPLE_MINTUPLE(tuple));
    }

    /* ... and the same for the skew buckets, if any */
    for (i = 0; i < hashtable->nSkewBuckets; i++)
    {
        int         j = hashtable->skewBucketNums[i];
        HashSkewBucket *skewBucket = hashtable->skewBucket[j];

        for (tuple = skewBucket->tuples; tuple != NULL; tuple = tuple->next)
            HeapTupleHeaderClearMatch(HJTUPLE_MINTUPLE(tuple));
    }
}


void
ExecReScanHash(HashState *node)
{
    /*
     * if chgParam of subnode is not null then plan will be re-scanned by
     * first ExecProcNode.
     */
    if (node->ps.lefttree->chgParam == NULL)
        ExecReScan(node->ps.lefttree);
}


/*
 * ExecHashBuildSkewHash
 *
 *      Set up for skew optimization if we can identify the most common values
 *      (MCVs) of the outer relation's join key.  We make a skew hash bucket
 *      for the hash value of each MCV, up to the number of slots allowed
 *      based on available memory.
 */
static void
ExecHashBuildSkewHash(HashJoinTable hashtable, Hash *node, int mcvsToUse)
{
    HeapTupleData *statsTuple;
    Datum      *values;
    int         nvalues;
    float4     *numbers;
    int         nnumbers;

    /* Do nothing if planner didn't identify the outer relation's join key */
    if (!OidIsValid(node->skewTable))
        return;
    /* Also, do nothing if we don't have room for at least one skew bucket */
    if (mcvsToUse <= 0)
        return;

    /*
     * Try to find the MCV statistics for the outer relation's join key.
     */
    statsTuple = SearchSysCache3(STATRELATTINH,
                                 ObjectIdGetDatum(node->skewTable),
                                 Int16GetDatum(node->skewColumn),
                                 BoolGetDatum(node->skewInherit));
    if (!HeapTupleIsValid(statsTuple))
        return;

    if (get_attstatsslot(statsTuple, node->skewColType, node->skewColTypmod,
                         STATISTIC_KIND_MCV, InvalidOid,
                         NULL,
                         &values, &nvalues,
                         &numbers, &nnumbers))
    {
        double      frac;
        int         nbuckets;
        FmgrInfo   *hashfunctions;
        int         i;

        if (mcvsToUse > nvalues)
            mcvsToUse = nvalues;

        /*
         * Calculate the expected fraction of outer relation that will
         * participate in the skew optimization.  If this isn't at least
         * SKEW_MIN_OUTER_FRACTION, don't use skew optimization.
         */
        frac = 0;
        for (i = 0; i < mcvsToUse; i++)
            frac += numbers[i];
        if (frac < SKEW_MIN_OUTER_FRACTION)
        {
            free_attstatsslot(node->skewColType,
                              values, nvalues, numbers, nnumbers);
            ReleaseSysCache(statsTuple);
            return;
        }

        /*
         * Okay, set up the skew hashtable.
         *
         * skewBucket[] is an open addressing hashtable with a power of 2 size
         * that is greater than the number of MCV values.  (This ensures there
         * will be at least one null entry, so searches will always
         * terminate.)
         *
         * Note: this code could fail if mcvsToUse exceeds INT_MAX/8 or
         * MaxAllocSize/sizeof(void *)/8, but that is not currently possible
         * since we limit pg_statistic entries to much less than that.
         */
        nbuckets = 2;
        while (nbuckets <= mcvsToUse)
            nbuckets <<= 1;
        /* use two more bits just to help avoid collisions */
        nbuckets <<= 2;

        hashtable->skewEnabled = true;
        hashtable->skewBucketLen = nbuckets;

        /*
         * We allocate the bucket memory in the hashtable's batch context. It
         * is only needed during the first batch, and this ensures it will be
         * automatically removed once the first batch is done.
         */
        hashtable->skewBucket = (HashSkewBucket **)
            MemoryContextAllocZero(hashtable->batchCxt,
                                   nbuckets * sizeof(HashSkewBucket *));
        hashtable->skewBucketNums = (int *)
            MemoryContextAllocZero(hashtable->batchCxt,
                                   mcvsToUse * sizeof(int));

        hashtable->spaceUsed += nbuckets * sizeof(HashSkewBucket *)
            + mcvsToUse * sizeof(int);
        hashtable->spaceUsedSkew += nbuckets * sizeof(HashSkewBucket *)
            + mcvsToUse * sizeof(int);
        if (hashtable->spaceUsed > hashtable->spacePeak)
            hashtable->spacePeak = hashtable->spaceUsed;

        /*
         * Create a skew bucket for each MCV hash value.
         *
         * Note: it is very important that we create the buckets in order of
         * decreasing MCV frequency.  If we have to remove some buckets, they
         * must be removed in reverse order of creation (see notes in
         * ExecHashRemoveNextSkewBucket) and we want the least common MCVs to
         * be removed first.
         */
        hashfunctions = hashtable->outer_hashfunctions;

        for (i = 0; i < mcvsToUse; i++)
        {
            uint32      hashvalue;
            int         bucket;

            hashvalue = DatumGetUInt32(FunctionCall1(&hashfunctions[0],
                                                     values[i]));

            /*
             * While we have not hit a hole in the hashtable and have not hit
             * the desired bucket, we have collided with some previous hash
             * value, so try the next bucket location.  NB: this code must
             * match ExecHashGetSkewBucket.
             */
            bucket = hashvalue & (nbuckets - 1);
            while (hashtable->skewBucket[bucket] != NULL &&
                   hashtable->skewBucket[bucket]->hashvalue != hashvalue)
                bucket = (bucket + 1) & (nbuckets - 1);

            /*
             * If we found an existing bucket with the same hashvalue, leave
             * it alone.  It's okay for two MCVs to share a hashvalue.
             */
            if (hashtable->skewBucket[bucket] != NULL)
                continue;

            /* Okay, create a new skew bucket for this hashvalue. */
            hashtable->skewBucket[bucket] = (HashSkewBucket *)
                MemoryContextAlloc(hashtable->batchCxt,
                                   sizeof(HashSkewBucket));
            hashtable->skewBucket[bucket]->hashvalue = hashvalue;
            hashtable->skewBucket[bucket]->tuples = NULL;
            hashtable->skewBucketNums[hashtable->nSkewBuckets] = bucket;
            hashtable->nSkewBuckets++;
            hashtable->spaceUsed += SKEW_BUCKET_OVERHEAD;
            hashtable->spaceUsedSkew += SKEW_BUCKET_OVERHEAD;
            if (hashtable->spaceUsed > hashtable->spacePeak)
                hashtable->spacePeak = hashtable->spaceUsed;
        }

        free_attstatsslot(node->skewColType,
                          values, nvalues, numbers, nnumbers);
    }

    ReleaseSysCache(statsTuple);
}

/*
 * ExecHashGetSkewBucket
 *
 *      Returns the index of the skew bucket for this hashvalue,
 *      or INVALID_SKEW_BUCKET_NO if the hashvalue is not
 *      associated with any active skew bucket.
 */
int
ExecHashGetSkewBucket(HashJoinTable hashtable, uint32 hashvalue)
{
    int         bucket;

    /*
     * Always return INVALID_SKEW_BUCKET_NO if not doing skew optimization (in
     * particular, this happens after the initial batch is done).
     */
    if (!hashtable->skewEnabled)
        return INVALID_SKEW_BUCKET_NO;

    /*
     * Since skewBucketLen is a power of 2, we can do a modulo by ANDing.
     */
    bucket = hashvalue & (hashtable->skewBucketLen - 1);

    /*
     * While we have not hit a hole in the hashtable and have not hit the
     * desired bucket, we have collided with some other hash value, so try the
     * next bucket location.
     */
    while (hashtable->skewBucket[bucket] != NULL &&
           hashtable->skewBucket[bucket]->hashvalue != hashvalue)
        bucket = (bucket + 1) & (hashtable->skewBucketLen - 1);

    /*
     * Found the desired bucket?
     */
    if (hashtable->skewBucket[bucket] != NULL)
        return bucket;

    /*
     * There must not be any hashtable entry for this hash value.
     */
    return INVALID_SKEW_BUCKET_NO;
}

/*
 * ExecHashSkewTableInsert
 *
 *      Insert a tuple into the skew hashtable.
 *
 * This should generally match up with the current-batch case in
 * ExecHashTableInsert.
 */
static void
ExecHashSkewTableInsert(HashJoinTable hashtable,
                        TupleTableSlot *slot,
                        uint32 hashvalue,
                        int bucketNumber)
{
    MinimalTuple tuple = ExecFetchSlotMinimalTuple(slot);
    HashJoinTuple hashTuple;
    int         hashTupleSize;

    /* Create the HashJoinTuple */
    hashTupleSize = HJTUPLE_OVERHEAD + tuple->t_len;
    hashTuple = (HashJoinTuple) MemoryContextAlloc(hashtable->batchCxt,
                                                   hashTupleSize);
    hashTuple->hashvalue = hashvalue;
    memcpy(HJTUPLE_MINTUPLE(hashTuple), tuple, tuple->t_len);
    HeapTupleHeaderClearMatch(HJTUPLE_MINTUPLE(hashTuple));

    /* Push it onto the front of the skew bucket's list */
    hashTuple->next = hashtable->skewBucket[bucketNumber]->tuples;
    hashtable->skewBucket[bucketNumber]->tuples = hashTuple;

    /* Account for space used, and back off if we've used too much */
    hashtable->spaceUsed += hashTupleSize;
    hashtable->spaceUsedSkew += hashTupleSize;
    if (hashtable->spaceUsed > hashtable->spacePeak)
        hashtable->spacePeak = hashtable->spaceUsed;
    while (hashtable->spaceUsedSkew > hashtable->spaceAllowedSkew)
        ExecHashRemoveNextSkewBucket(hashtable);

    /* Check we are not over the total spaceAllowed, either */
    if (hashtable->spaceUsed > hashtable->spaceAllowed)
        ExecHashIncreaseNumBatches(hashtable);
}

/*
 *      ExecHashRemoveNextSkewBucket
 *
 *      Remove the least valuable skew bucket by pushing its tuples into
 *      the main hash table.
 */
static void
ExecHashRemoveNextSkewBucket(HashJoinTable hashtable)
{
    int         bucketToRemove;
    HashSkewBucket *bucket;
    uint32      hashvalue;
    int         bucketno;
    int         batchno;
    HashJoinTuple hashTuple;

    /* Locate the bucket to remove */
    bucketToRemove = hashtable->skewBucketNums[hashtable->nSkewBuckets - 1];
    bucket = hashtable->skewBucket[bucketToRemove];

    /*
     * Calculate which bucket and batch the tuples belong to in the main
     * hashtable.  They all have the same hash value, so it's the same for all
     * of them.  Also note that it's not possible for nbatch to increase while
     * we are processing the tuples.
     */
    hashvalue = bucket->hashvalue;
    ExecHashGetBucketAndBatch(hashtable, hashvalue, &bucketno, &batchno);

    /* Process all tuples in the bucket */
    hashTuple = bucket->tuples;
    while (hashTuple != NULL)
    {
        HashJoinTuple nextHashTuple = hashTuple->next;
        MinimalTuple tuple;
        Size        tupleSize;

        /*
         * This code must agree with ExecHashTableInsert.  We do not use
         * ExecHashTableInsert directly as ExecHashTableInsert expects a
         * TupleTableSlot while we already have HashJoinTuples.
         */
        tuple = HJTUPLE_MINTUPLE(hashTuple);
        tupleSize = HJTUPLE_OVERHEAD + tuple->t_len;

        /* Decide whether to put the tuple in the hash table or a temp file */
        if (batchno == hashtable->curbatch)
        {
            /* Move the tuple to the main hash table */
            HashJoinTuple copyTuple;

            /*
             * We must copy the tuple into the dense storage, else it will not
             * be found by, eg, ExecHashIncreaseNumBatches.
             */
            copyTuple = (HashJoinTuple) dense_alloc(hashtable, tupleSize);
            memcpy(copyTuple, hashTuple, tupleSize);
            pfree(hashTuple);

            copyTuple->next = hashtable->buckets[bucketno];
            hashtable->buckets[bucketno] = copyTuple;

            /* We have reduced skew space, but overall space doesn't change */
            hashtable->spaceUsedSkew -= tupleSize;
        }
        else
        {
            /* Put the tuple into a temp file for later batches */
            Assert(batchno > hashtable->curbatch);
            ExecHashJoinSaveTuple(tuple, hashvalue,
                                  &hashtable->innerBatchFile[batchno]);
            pfree(hashTuple);
            hashtable->spaceUsed -= tupleSize;
            hashtable->spaceUsedSkew -= tupleSize;
        }

        hashTuple = nextHashTuple;
    }

    /*
     * Free the bucket struct itself and reset the hashtable entry to NULL.
     *
     * NOTE: this is not nearly as simple as it looks on the surface, because
     * of the possibility of collisions in the hashtable.  Suppose that hash
     * values A and B collide at a particular hashtable entry, and that A was
     * entered first so B gets shifted to a different table entry.  If we were
     * to remove A first then ExecHashGetSkewBucket would mistakenly start
     * reporting that B is not in the hashtable, because it would hit the NULL
     * before finding B.  However, we always remove entries in the reverse
     * order of creation, so this failure cannot happen.
     */
    hashtable->skewBucket[bucketToRemove] = NULL;
    hashtable->nSkewBuckets--;
    pfree(bucket);
    hashtable->spaceUsed -= SKEW_BUCKET_OVERHEAD;
    hashtable->spaceUsedSkew -= SKEW_BUCKET_OVERHEAD;

    /*
     * If we have removed all skew buckets then give up on skew optimization.
     * Release the arrays since they aren't useful any more.
     */
    if (hashtable->nSkewBuckets == 0)
    {
        hashtable->skewEnabled = false;
        pfree(hashtable->skewBucket);
        pfree(hashtable->skewBucketNums);
        hashtable->skewBucket = NULL;
        hashtable->skewBucketNums = NULL;
        hashtable->spaceUsed -= hashtable->spaceUsedSkew;
        hashtable->spaceUsedSkew = 0;
    }
}

/*
 * Allocate 'size' bytes from the currently active HashMemoryChunk
 */
static void *
dense_alloc(HashJoinTable hashtable, Size size)
{
    HashMemoryChunk newChunk;
    char       *ptr;

    /* just in case the size is not already aligned properly */
    size = MAXALIGN(size);

    /*
     * If tuple size is larger than of 1/4 of chunk size, allocate a separate
     * chunk.
     */
    if (size > HASH_CHUNK_THRESHOLD)
    {
        /* allocate new chunk and put it at the beginning of the list */
        newChunk = (HashMemoryChunk) MemoryContextAlloc(hashtable->batchCxt,
                                 offsetof(HashMemoryChunkData, data) + size);
        newChunk->maxlen = size;
        newChunk->used = 0;
        newChunk->ntuples = 0;

        /*
         * Add this chunk to the list after the first existing chunk, so that
         * we don't lose the remaining space in the "current" chunk.
         */
        if (hashtable->chunks != NULL)
        {
            newChunk->next = hashtable->chunks->next;
            hashtable->chunks->next = newChunk;
        }
        else
        {
            newChunk->next = hashtable->chunks;
            hashtable->chunks = newChunk;
        }

        newChunk->used += size;
        newChunk->ntuples += 1;

        return newChunk->data;
    }

    /*
     * See if we have enough space for it in the current chunk (if any). If
     * not, allocate a fresh chunk.
     */
    if ((hashtable->chunks == NULL) ||
        (hashtable->chunks->maxlen - hashtable->chunks->used) < size)
    {
        /* allocate new chunk and put it at the beginning of the list */
        newChunk = (HashMemoryChunk) MemoryContextAlloc(hashtable->batchCxt,
                      offsetof(HashMemoryChunkData, data) + HASH_CHUNK_SIZE);

        newChunk->maxlen = HASH_CHUNK_SIZE;
        newChunk->used = size;
        newChunk->ntuples = 1;

        newChunk->next = hashtable->chunks;
        hashtable->chunks = newChunk;

        return newChunk->data;
    }

    /* There is enough space in the current chunk, let's add the tuple */
    ptr = hashtable->chunks->data + hashtable->chunks->used;
    hashtable->chunks->used += size;
    hashtable->chunks->ntuples += 1;

    /* return pointer to the start of the tuple memory */
    return ptr;
}