*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.117 2009/01/01 17:23:41 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/executor/nodeHash.c,v 1.118 2009/03/21 00:04:38 tgl Exp $
*
*-------------------------------------------------------------------------
*/
#include <math.h>
#include <limits.h>
+#include "catalog/pg_statistic.h"
#include "commands/tablespace.h"
#include "executor/execdebug.h"
#include "executor/hashjoin.h"
#include "utils/dynahash.h"
#include "utils/memutils.h"
#include "utils/lsyscache.h"
+#include "utils/syscache.h"
static void ExecHashIncreaseNumBatches(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);
/* ----------------------------------------------------------------
if (ExecHashGetHashValue(hashtable, econtext, hashkeys, false, false,
&hashvalue))
{
- ExecHashTableInsert(hashtable, slot, 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);
+ }
+ else
+ {
+ /* Not subject to skew optimization, so insert normally */
+ ExecHashTableInsert(hashtable, slot, hashvalue);
+ }
hashtable->totalTuples += 1;
}
}
Plan *outerNode;
int nbuckets;
int nbatch;
+ int num_skew_mcvs;
int log2_nbuckets;
int nkeys;
int i;
outerNode = outerPlan(node);
ExecChooseHashTableSize(outerNode->plan_rows, outerNode->plan_width,
- &nbuckets, &nbatch);
+ OidIsValid(node->skewTable),
+ &nbuckets, &nbatch, &num_skew_mcvs);
#ifdef HJDEBUG
printf("nbatch = %d, nbuckets = %d\n", nbatch, nbuckets);
hashtable->nbuckets = nbuckets;
hashtable->log2_nbuckets = log2_nbuckets;
hashtable->buckets = NULL;
+ 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->outerBatchFile = NULL;
hashtable->spaceUsed = 0;
hashtable->spaceAllowed = work_mem * 1024L;
+ hashtable->spaceUsedSkew = 0;
+ hashtable->spaceAllowedSkew =
+ hashtable->spaceAllowed * SKEW_WORK_MEM_PERCENT / 100;
/*
* Get info about the hash functions to be used for each hash key. Also
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;
#define NTUP_PER_BUCKET 10
void
-ExecChooseHashTableSize(double ntuples, int tupwidth,
+ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
int *numbuckets,
- int *numbatches)
+ int *numbatches,
+ int *num_skew_mcvs)
{
int tupsize;
double inner_rel_bytes;
long hash_table_bytes;
+ long skew_table_bytes;
int nbatch;
int nbuckets;
int i;
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;
+
+ *num_skew_mcvs = skew_table_bytes / (
+ /* size of a hash tuple */
+ tupsize +
+ /* worst-case size of skewBucket[] per MCV */
+ (8 * sizeof(HashSkewBucket *)) +
+ /* size of skewBucketNums[] entry */
+ sizeof(int) +
+ /* size of skew bucket struct itself */
+ 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. Set nbatch to the smallest power of 2 that appears
* sufficient.
uint32 hashvalue = hjstate->hj_CurHashValue;
/*
- * hj_CurTuple is NULL to start scanning a new bucket, or the address of
- * the last tuple returned from the current bucket.
+ * 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 = hashtable->buckets[hjstate->hj_CurBucketNo];
- else
+ 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 (((PlanState *) node)->lefttree->chgParam == NULL)
ExecReScan(((PlanState *) node)->lefttree, exprCtxt);
}
+
+
+/*
+ * 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 = SearchSysCache(STATRELATT,
+ ObjectIdGetDatum(node->skewTable),
+ Int16GetDatum(node->skewColumn),
+ 0, 0);
+ if (!HeapTupleIsValid(statsTuple))
+ return;
+
+ if (get_attstatsslot(statsTuple, node->skewColType, node->skewColTypmod,
+ STATISTIC_KIND_MCV, InvalidOid,
+ &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, 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);
+
+ /*
+ * 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;
+ }
+
+ 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);
+
+ /* 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;
+ 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 */
+ hashTuple->next = hashtable->buckets[bucketno];
+ hashtable->buckets[bucketno] = hashTuple;
+ /* 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;
+ }
+}
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