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Postgres FD Implementation
Commit 596efd27 authored by Tom Lane's avatar Tom Lane
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Optimize multi-batch hash joins when the outer relation has a nonuniform 

distribution, by creating a special fast path for the (first few) most common
values of the outer relation.  Tuples having hashvalues matching the MCVs
are effectively forced to be in the first batch, so that we never write
them out to the batch temp files.

Bryce Cutt and Ramon Lawrence, with some editorialization by me.
parent 249d936b
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Showing with 604 additions and 34 deletions
src/backend/executor/nodeHash.c
View file @ 596efd27
......@@ -8,7 +8,7 @@
*
*
* 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 $
*
*-------------------------------------------------------------------------
*/
......@@ -24,6 +24,7 @@
#include <math.h>
#include <limits.h>
#include "catalog/pg_statistic.h"
#include "commands/tablespace.h"
#include "executor/execdebug.h"
#include "executor/hashjoin.h"
......@@ -35,9 +36,17 @@
#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);
/* ----------------------------------------------------------------
......@@ -99,7 +108,20 @@ MultiExecHash(HashState *node)
if (ExecHashGetHashValue(hashtable, econtext, hashkeys, false, false,
&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;
}
}
......@@ -225,6 +247,7 @@ ExecHashTableCreate(Hash *node, List *hashOperators)
Plan *outerNode;
int nbuckets;
int nbatch;
int num_skew_mcvs;
int log2_nbuckets;
int nkeys;
int i;
......@@ -239,7 +262,8 @@ ExecHashTableCreate(Hash *node, List *hashOperators)
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);
......@@ -259,6 +283,11 @@ ExecHashTableCreate(Hash *node, List *hashOperators)
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;
......@@ -269,6 +298,9 @@ ExecHashTableCreate(Hash *node, List *hashOperators)
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
......@@ -339,6 +371,13 @@ ExecHashTableCreate(Hash *node, List *hashOperators)
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;
......@@ -356,13 +395,15 @@ ExecHashTableCreate(Hash *node, List *hashOperators)
#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;
......@@ -386,6 +427,41 @@ ExecChooseHashTableSize(double ntuples, int tupwidth,
*/
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
......@@ -813,13 +889,18 @@ ExecScanHashBucket(HashJoinState *hjstate,
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)
{
......@@ -889,3 +970,347 @@ ExecReScanHash(HashState *node, ExprContext *exprCtxt)
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;
}
}
src/backend/executor/nodeHashjoin.c
View file @ 596efd27
......@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/executor/nodeHashjoin.c,v 1.97 2009/01/01 17:23:41 momjian Exp $
* $PostgreSQL: pgsql/src/backend/executor/nodeHashjoin.c,v 1.98 2009/03/21 00:04:38 tgl Exp $
*
*-------------------------------------------------------------------------
*/
......@@ -198,19 +198,23 @@ ExecHashJoin(HashJoinState *node)
node->hj_MatchedOuter = false;
/*
* now we have an outer tuple, find the corresponding bucket for
* this tuple from the hash table
* Now we have an outer tuple; find the corresponding bucket for
* this tuple in the main hash table or skew hash table.
*/
node->hj_CurHashValue = hashvalue;
ExecHashGetBucketAndBatch(hashtable, hashvalue,
&node->hj_CurBucketNo, &batchno);
node->hj_CurSkewBucketNo = ExecHashGetSkewBucket(hashtable,
hashvalue);
node->hj_CurTuple = NULL;
/*
* Now we've got an outer tuple and the corresponding hash bucket,
* but this tuple may not belong to the current batch.
* but it might not belong to the current batch, or it might
* match a skew bucket.
*/
if (batchno != hashtable->curbatch)
if (batchno != hashtable->curbatch &&
node->hj_CurSkewBucketNo == INVALID_SKEW_BUCKET_NO)
{
/*
* Need to postpone this outer tuple to a later batch. Save it
......@@ -452,6 +456,7 @@ ExecInitHashJoin(HashJoin *node, EState *estate, int eflags)
hjstate->hj_CurHashValue = 0;
hjstate->hj_CurBucketNo = 0;
hjstate->hj_CurSkewBucketNo = INVALID_SKEW_BUCKET_NO;
hjstate->hj_CurTuple = NULL;
/*
......@@ -651,6 +656,19 @@ start_over:
BufFileClose(hashtable->outerBatchFile[curbatch]);
hashtable->outerBatchFile[curbatch] = NULL;
}
else /* we just finished the first batch */
{
/*
* Reset some of the skew optimization state variables, since we
* no longer need to consider skew tuples after the first batch.
* The memory context reset we are about to do will release the
* skew hashtable itself.
*/
hashtable->skewEnabled = false;
hashtable->skewBucket = NULL;
hashtable->skewBucketNums = NULL;
hashtable->spaceUsedSkew = 0;
}
/*
* We can always skip over any batches that are completely empty on both
......@@ -880,6 +898,7 @@ ExecReScanHashJoin(HashJoinState *node, ExprContext *exprCtxt)
/* Always reset intra-tuple state */
node->hj_CurHashValue = 0;
node->hj_CurBucketNo = 0;
node->hj_CurSkewBucketNo = INVALID_SKEW_BUCKET_NO;
node->hj_CurTuple = NULL;
node->js.ps.ps_TupFromTlist = false;
......
src/backend/nodes/copyfuncs.c
View file @ 596efd27
......@@ -15,7 +15,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/nodes/copyfuncs.c,v 1.426 2009/03/10 22:09:25 tgl Exp $
* $PostgreSQL: pgsql/src/backend/nodes/copyfuncs.c,v 1.427 2009/03/21 00:04:39 tgl Exp $
*
*-------------------------------------------------------------------------
*/
......@@ -735,6 +735,10 @@ _copyHash(Hash *from)
/*
* copy remainder of node
*/
COPY_SCALAR_FIELD(skewTable);
COPY_SCALAR_FIELD(skewColumn);
COPY_SCALAR_FIELD(skewColType);
COPY_SCALAR_FIELD(skewColTypmod);
return newnode;
}
......
src/backend/nodes/outfuncs.c
View file @ 596efd27
......@@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/nodes/outfuncs.c,v 1.354 2009/03/10 22:09:25 tgl Exp $
* $PostgreSQL: pgsql/src/backend/nodes/outfuncs.c,v 1.355 2009/03/21 00:04:39 tgl Exp $
*
* NOTES
* Every node type that can appear in stored rules' parsetrees *must*
......@@ -675,6 +675,11 @@ _outHash(StringInfo str, Hash *node)
WRITE_NODE_TYPE("HASH");
_outPlanInfo(str, (Plan *) node);
WRITE_OID_FIELD(skewTable);
WRITE_INT_FIELD(skewColumn);
WRITE_OID_FIELD(skewColType);
WRITE_INT_FIELD(skewColTypmod);
}
static void
......
src/backend/optimizer/path/costsize.c
View file @ 596efd27
......@@ -54,7 +54,7 @@
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.204 2009/02/06 23:43:23 tgl Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.205 2009/03/21 00:04:39 tgl Exp $
*
*-------------------------------------------------------------------------
*/
......@@ -1821,6 +1821,7 @@ cost_hashjoin(HashPath *path, PlannerInfo *root, SpecialJoinInfo *sjinfo)
int num_hashclauses = list_length(hashclauses);
int numbuckets;
int numbatches;
int num_skew_mcvs;
double virtualbuckets;
Selectivity innerbucketsize;
ListCell *hcl;
......@@ -1862,11 +1863,22 @@ cost_hashjoin(HashPath *path, PlannerInfo *root, SpecialJoinInfo *sjinfo)
* inner_path_rows;
run_cost += cpu_operator_cost * num_hashclauses * outer_path_rows;
/* Get hash table size that executor would use for inner relation */
/*
* Get hash table size that executor would use for inner relation.
*
* XXX for the moment, always assume that skew optimization will be
* performed. As long as SKEW_WORK_MEM_PERCENT is small, it's not worth
* trying to determine that for sure.
*
* XXX at some point it might be interesting to try to account for skew
* optimization in the cost estimate, but for now, we don't.
*/
ExecChooseHashTableSize(inner_path_rows,
inner_path->parent->width,
true, /* useskew */
&numbuckets,
&numbatches);
&numbatches,
&num_skew_mcvs);
virtualbuckets = (double) numbuckets *(double) numbatches;
/*
......
src/backend/optimizer/plan/createplan.c
View file @ 596efd27
......@@ -10,7 +10,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.255 2009/01/01 17:23:44 momjian Exp $
* $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.256 2009/03/21 00:04:39 tgl Exp $
*
*-------------------------------------------------------------------------
*/
......@@ -112,7 +112,11 @@ static HashJoin *make_hashjoin(List *tlist,
List *hashclauses,
Plan *lefttree, Plan *righttree,
JoinType jointype);
static Hash *make_hash(Plan *lefttree);
static Hash *make_hash(Plan *lefttree,
Oid skewTable,
AttrNumber skewColumn,
Oid skewColType,
int32 skewColTypmod);
static MergeJoin *make_mergejoin(List *tlist,
List *joinclauses, List *otherclauses,
List *mergeclauses,
......@@ -1864,6 +1868,10 @@ create_hashjoin_plan(PlannerInfo *root,
List *joinclauses;
List *otherclauses;
List *hashclauses;
Oid skewTable = InvalidOid;
AttrNumber skewColumn = InvalidAttrNumber;
Oid skewColType = InvalidOid;
int32 skewColTypmod = -1;
HashJoin *join_plan;
Hash *hash_plan;
......@@ -1902,10 +1910,47 @@ create_hashjoin_plan(PlannerInfo *root,
/* We don't want any excess columns in the hashed tuples */
disuse_physical_tlist(inner_plan, best_path->jpath.innerjoinpath);
/*
* If there is a single join clause and we can identify the outer
* variable as a simple column reference, supply its identity for
* possible use in skew optimization. (Note: in principle we could
* do skew optimization with multiple join clauses, but we'd have to
* be able to determine the most common combinations of outer values,
* which we don't currently have enough stats for.)
*/
if (list_length(hashclauses) == 1)
{
OpExpr *clause = (OpExpr *) linitial(hashclauses);
Node *node;
Assert(is_opclause(clause));
node = (Node *) linitial(clause->args);
if (IsA(node, RelabelType))
node = (Node *) ((RelabelType *) node)->arg;
if (IsA(node, Var))
{
Var *var = (Var *) node;
RangeTblEntry *rte;
rte = root->simple_rte_array[var->varno];
if (rte->rtekind == RTE_RELATION)
{
skewTable = rte->relid;
skewColumn = var->varattno;
skewColType = var->vartype;
skewColTypmod = var->vartypmod;
}
}
}
/*
* Build the hash node and hash join node.
*/
hash_plan = make_hash(inner_plan);
hash_plan = make_hash(inner_plan,
skewTable,
skewColumn,
skewColType,
skewColTypmod);
join_plan = make_hashjoin(tlist,
joinclauses,
otherclauses,
......@@ -2713,7 +2758,11 @@ make_hashjoin(List *tlist,
}
static Hash *
make_hash(Plan *lefttree)
make_hash(Plan *lefttree,
Oid skewTable,
AttrNumber skewColumn,
Oid skewColType,
int32 skewColTypmod)
{
Hash *node = makeNode(Hash);
Plan *plan = &node->plan;
......@@ -2730,6 +2779,11 @@ make_hash(Plan *lefttree)
plan->lefttree = lefttree;
plan->righttree = NULL;
node->skewTable = skewTable;
node->skewColumn = skewColumn;
node->skewColType = skewColType;
node->skewColTypmod = skewColTypmod;
return node;
}
......
src/include/executor/hashjoin.h
View file @ 596efd27
......@@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/executor/hashjoin.h,v 1.49 2009/01/01 17:23:59 momjian Exp $
* $PostgreSQL: pgsql/src/include/executor/hashjoin.h,v 1.50 2009/03/21 00:04:40 tgl Exp $
*
*-------------------------------------------------------------------------
*/
......@@ -72,6 +72,36 @@ typedef struct HashJoinTupleData
#define HJTUPLE_MINTUPLE(hjtup) \
((MinimalTuple) ((char *) (hjtup) + HJTUPLE_OVERHEAD))
/*
* If the outer relation's distribution is sufficiently nonuniform, we attempt
* to optimize the join by treating the hash values corresponding to the outer
* relation's MCVs specially. Inner relation tuples matching these hash
* values go into the "skew" hashtable instead of the main hashtable, and
* outer relation tuples with these hash values are matched against that
* table instead of the main one. Thus, tuples with these hash values are
* effectively handled as part of the first batch and will never go to disk.
* The skew hashtable is limited to SKEW_WORK_MEM_PERCENT of the total memory
* allowed for the join; while building the hashtables, we decrease the number
* of MCVs being specially treated if needed to stay under this limit.
*
* Note: you might wonder why we look at the outer relation stats for this,
* rather than the inner. One reason is that the outer relation is typically
* bigger, so we get more I/O savings by optimizing for its most common values.
* Also, for similarly-sized relations, the planner prefers to put the more
* uniformly distributed relation on the inside, so we're more likely to find
* interesting skew in the outer relation.
*/
typedef struct HashSkewBucket
{
uint32 hashvalue; /* common hash value */
HashJoinTuple tuples; /* linked list of inner-relation tuples */
} HashSkewBucket;
#define SKEW_BUCKET_OVERHEAD MAXALIGN(sizeof(HashSkewBucket))
#define INVALID_SKEW_BUCKET_NO (-1)
#define SKEW_WORK_MEM_PERCENT 2
#define SKEW_MIN_OUTER_FRACTION 0.01
typedef struct HashJoinTableData
{
......@@ -82,6 +112,12 @@ typedef struct HashJoinTableData
struct HashJoinTupleData **buckets;
/* buckets array is per-batch storage, as are all the tuples */
bool skewEnabled; /* are we using skew optimization? */
HashSkewBucket **skewBucket; /* hashtable of skew buckets */
int skewBucketLen; /* size of skewBucket array (a power of 2!) */
int nSkewBuckets; /* number of active skew buckets */
int *skewBucketNums; /* array indexes of active skew buckets */
int nbatch; /* number of batches */
int curbatch; /* current batch #; 0 during 1st pass */
......@@ -113,6 +149,8 @@ typedef struct HashJoinTableData
Size spaceUsed; /* memory space currently used by tuples */
Size spaceAllowed; /* upper limit for space used */
Size spaceUsedSkew; /* skew hash table's current space usage */
Size spaceAllowedSkew; /* upper limit for skew hashtable */
MemoryContext hashCxt; /* context for whole-hash-join storage */
MemoryContext batchCxt; /* context for this-batch-only storage */
......
src/include/executor/nodeHash.h
View file @ 596efd27
......@@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/executor/nodeHash.h,v 1.46 2009/01/01 17:23:59 momjian Exp $
* $PostgreSQL: pgsql/src/include/executor/nodeHash.h,v 1.47 2009/03/21 00:04:40 tgl Exp $
*
*-------------------------------------------------------------------------
*/
......@@ -41,8 +41,10 @@ extern void ExecHashGetBucketAndBatch(HashJoinTable hashtable,
extern HashJoinTuple ExecScanHashBucket(HashJoinState *hjstate,
ExprContext *econtext);
extern void ExecHashTableReset(HashJoinTable hashtable);
extern void ExecChooseHashTableSize(double ntuples, int tupwidth,
extern void ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
int *numbuckets,
int *numbatches);
int *numbatches,
int *num_skew_mcvs);
extern int ExecHashGetSkewBucket(HashJoinTable hashtable, uint32 hashvalue);
#endif /* NODEHASH_H */
src/include/nodes/execnodes.h
View file @ 596efd27
......@@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.201 2009/01/12 05:10:45 tgl Exp $
* $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.202 2009/03/21 00:04:40 tgl Exp $
*
*-------------------------------------------------------------------------
*/
......@@ -1374,11 +1374,12 @@ typedef struct MergeJoinState
* hj_HashTable hash table for the hashjoin
* (NULL if table not built yet)
* hj_CurHashValue hash value for current outer tuple
* hj_CurBucketNo bucket# for current outer tuple
* hj_CurBucketNo regular bucket# for current outer tuple
* hj_CurSkewBucketNo skew bucket# for current outer tuple
* hj_CurTuple last inner tuple matched to current outer
* tuple, or NULL if starting search
* (CurHashValue, CurBucketNo and CurTuple are
* undefined if OuterTupleSlot is empty!)
* (hj_CurXXX variables are undefined if
* OuterTupleSlot is empty!)
* hj_OuterHashKeys the outer hash keys in the hashjoin condition
* hj_InnerHashKeys the inner hash keys in the hashjoin condition
* hj_HashOperators the join operators in the hashjoin condition
......@@ -1403,6 +1404,7 @@ typedef struct HashJoinState
HashJoinTable hj_HashTable;
uint32 hj_CurHashValue;
int hj_CurBucketNo;
int hj_CurSkewBucketNo;
HashJoinTuple hj_CurTuple;
List *hj_OuterHashKeys; /* list of ExprState nodes */
List *hj_InnerHashKeys; /* list of ExprState nodes */
......
src/include/nodes/plannodes.h
View file @ 596efd27
......@@ -7,7 +7,7 @@
* Portions Copyright (c) 1996-2009, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* $PostgreSQL: pgsql/src/include/nodes/plannodes.h,v 1.108 2009/01/01 17:24:00 momjian Exp $
* $PostgreSQL: pgsql/src/include/nodes/plannodes.h,v 1.109 2009/03/21 00:04:40 tgl Exp $
*
*-------------------------------------------------------------------------
*/
......@@ -460,7 +460,7 @@ typedef struct MergeJoin
} MergeJoin;
/* ----------------
* hash join (probe) node
* hash join node
* ----------------
*/
typedef struct HashJoin
......@@ -567,11 +567,20 @@ typedef struct Unique
/* ----------------
* hash build node
*
* If the executor is supposed to try to apply skew join optimization, then
* skewTable/skewColumn identify the outer relation's join key column, from
* which the relevant MCV statistics can be fetched. Also, its type
* information is provided to save a lookup.
* ----------------
*/
typedef struct Hash
{
Plan plan;
Oid skewTable; /* outer join key's table OID, or InvalidOid */
AttrNumber skewColumn; /* outer join key's column #, or zero */
Oid skewColType; /* datatype of the outer key column */
int32 skewColTypmod; /* typmod of the outer key column */
/* all other info is in the parent HashJoin node */
} Hash;
......
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