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Commit5dc692f

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Ensure proper alignment of tuples in HashMemoryChunkData buffers.
The previous coding relied (without any documentation) on the data[]member of HashMemoryChunkData being at a MAXALIGN'ed offset. If itwas not, the tuples would not be maxaligned either, leading to failureson alignment-picky machines. While there seems to be no live bug on anyplatform we support, this is clearly pretty fragile: any addition to orrearrangement of the fields in HashMemoryChunkData could break it.Let's remove the hazard by getting rid of the data[] member and insteadusing pointer arithmetic with an explicitly maxalign'ed offset.Discussion:https://postgr.es/m/14483.1514938129@sss.pgh.pa.us
1 parent54eff53 commit5dc692f

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2 files changed

+24
-22
lines changed

2 files changed

+24
-22
lines changed

‎src/backend/executor/nodeHash.c‎

Lines changed: 15 additions & 19 deletions
Original file line numberDiff line numberDiff line change
@@ -979,7 +979,7 @@ ExecHashIncreaseNumBatches(HashJoinTable hashtable)
979979
/* process all tuples stored in this chunk (and then free it) */
980980
while (idx<oldchunks->used)
981981
{
982-
HashJoinTuplehashTuple= (HashJoinTuple) (oldchunks->data+idx);
982+
HashJoinTuplehashTuple= (HashJoinTuple) (HASH_CHUNK_DATA(oldchunks)+idx);
983983
MinimalTupletuple=HJTUPLE_MINTUPLE(hashTuple);
984984
inthashTupleSize= (HJTUPLE_OVERHEAD+tuple->t_len);
985985
intbucketno;
@@ -1285,7 +1285,7 @@ ExecParallelHashRepartitionFirst(HashJoinTable hashtable)
12851285
/* Repartition all tuples in this chunk. */
12861286
while (idx<chunk->used)
12871287
{
1288-
HashJoinTuplehashTuple= (HashJoinTuple) (chunk->data+idx);
1288+
HashJoinTuplehashTuple= (HashJoinTuple) (HASH_CHUNK_DATA(chunk)+idx);
12891289
MinimalTupletuple=HJTUPLE_MINTUPLE(hashTuple);
12901290
HashJoinTuplecopyTuple;
12911291
dsa_pointershared;
@@ -1469,7 +1469,7 @@ ExecHashIncreaseNumBuckets(HashJoinTable hashtable)
14691469

14701470
while (idx<chunk->used)
14711471
{
1472-
HashJoinTuplehashTuple= (HashJoinTuple) (chunk->data+idx);
1472+
HashJoinTuplehashTuple= (HashJoinTuple) (HASH_CHUNK_DATA(chunk)+idx);
14731473
intbucketno;
14741474
intbatchno;
14751475

@@ -1552,7 +1552,7 @@ ExecParallelHashIncreaseNumBuckets(HashJoinTable hashtable)
15521552

15531553
while (idx<chunk->used)
15541554
{
1555-
HashJoinTuplehashTuple= (HashJoinTuple) (chunk->data+idx);
1555+
HashJoinTuplehashTuple= (HashJoinTuple) (HASH_CHUNK_DATA(chunk)+idx);
15561556
dsa_pointershared=chunk_s+HASH_CHUNK_HEADER_SIZE+idx;
15571557
intbucketno;
15581558
intbatchno;
@@ -2651,17 +2651,16 @@ dense_alloc(HashJoinTable hashtable, Size size)
26512651
size=MAXALIGN(size);
26522652

26532653
/*
2654-
* If tuple size is larger than of 1/4 of chunk size, allocate a separate
2655-
* chunk.
2654+
* If tuple size is larger than threshold, allocate a separate chunk.
26562655
*/
26572656
if (size>HASH_CHUNK_THRESHOLD)
26582657
{
26592658
/* allocate new chunk and put it at the beginning of the list */
26602659
newChunk= (HashMemoryChunk)MemoryContextAlloc(hashtable->batchCxt,
2661-
offsetof(HashMemoryChunkData,data)+size);
2660+
HASH_CHUNK_HEADER_SIZE+size);
26622661
newChunk->maxlen=size;
2663-
newChunk->used=0;
2664-
newChunk->ntuples=0;
2662+
newChunk->used=size;
2663+
newChunk->ntuples=1;
26652664

26662665
/*
26672666
* Add this chunk to the list after the first existing chunk, so that
@@ -2678,10 +2677,7 @@ dense_alloc(HashJoinTable hashtable, Size size)
26782677
hashtable->chunks=newChunk;
26792678
}
26802679

2681-
newChunk->used+=size;
2682-
newChunk->ntuples+=1;
2683-
2684-
returnnewChunk->data;
2680+
returnHASH_CHUNK_DATA(newChunk);
26852681
}
26862682

26872683
/*
@@ -2693,7 +2689,7 @@ dense_alloc(HashJoinTable hashtable, Size size)
26932689
{
26942690
/* allocate new chunk and put it at the beginning of the list */
26952691
newChunk= (HashMemoryChunk)MemoryContextAlloc(hashtable->batchCxt,
2696-
offsetof(HashMemoryChunkData,data)+HASH_CHUNK_SIZE);
2692+
HASH_CHUNK_HEADER_SIZE+HASH_CHUNK_SIZE);
26972693

26982694
newChunk->maxlen=HASH_CHUNK_SIZE;
26992695
newChunk->used=size;
@@ -2702,11 +2698,11 @@ dense_alloc(HashJoinTable hashtable, Size size)
27022698
newChunk->next.unshared=hashtable->chunks;
27032699
hashtable->chunks=newChunk;
27042700

2705-
returnnewChunk->data;
2701+
returnHASH_CHUNK_DATA(newChunk);
27062702
}
27072703

27082704
/* There is enough space in the current chunk, let's add the tuple */
2709-
ptr=hashtable->chunks->data+hashtable->chunks->used;
2705+
ptr=HASH_CHUNK_DATA(hashtable->chunks)+hashtable->chunks->used;
27102706
hashtable->chunks->used+=size;
27112707
hashtable->chunks->ntuples+=1;
27122708

@@ -2751,7 +2747,7 @@ ExecParallelHashTupleAlloc(HashJoinTable hashtable, size_t size,
27512747
chunk_shared=hashtable->current_chunk_shared;
27522748
Assert(chunk==dsa_get_address(hashtable->area,chunk_shared));
27532749
*shared=chunk_shared+HASH_CHUNK_HEADER_SIZE+chunk->used;
2754-
result= (HashJoinTuple) (chunk->data+chunk->used);
2750+
result= (HashJoinTuple) (HASH_CHUNK_DATA(chunk)+chunk->used);
27552751
chunk->used+=size;
27562752

27572753
Assert(chunk->used <=chunk->maxlen);
@@ -2859,8 +2855,8 @@ ExecParallelHashTupleAlloc(HashJoinTable hashtable, size_t size,
28592855
}
28602856
LWLockRelease(&pstate->lock);
28612857

2862-
Assert(chunk->data==dsa_get_address(hashtable->area,*shared));
2863-
result= (HashJoinTuple)chunk->data;
2858+
Assert(HASH_CHUNK_DATA(chunk)==dsa_get_address(hashtable->area,*shared));
2859+
result= (HashJoinTuple)HASH_CHUNK_DATA(chunk);
28642860

28652861
returnresult;
28662862
}

‎src/include/executor/hashjoin.h‎

Lines changed: 9 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -117,7 +117,7 @@ typedef struct HashSkewBucket
117117
typedefstructHashMemoryChunkData
118118
{
119119
intntuples;/* number of tuples stored in this chunk */
120-
size_tmaxlen;/* size of thebuffer holding the tuples */
120+
size_tmaxlen;/* size of thechunk's tuple buffer */
121121
size_tused;/* number of buffer bytes already used */
122122

123123
/* pointer to the next chunk (linked list) */
@@ -127,13 +127,19 @@ typedef struct HashMemoryChunkData
127127
dsa_pointershared;
128128
}next;
129129

130-
chardata[FLEXIBLE_ARRAY_MEMBER];/* buffer allocated at the end */
130+
/*
131+
* The chunk's tuple buffer starts after the HashMemoryChunkData struct,
132+
* at offset HASH_CHUNK_HEADER_SIZE (which must be maxaligned). Note that
133+
* that offset is not included in "maxlen" or "used".
134+
*/
131135
}HashMemoryChunkData;
132136

133137
typedefstructHashMemoryChunkData*HashMemoryChunk;
134138

135139
#defineHASH_CHUNK_SIZE(32 * 1024L)
136-
#defineHASH_CHUNK_HEADER_SIZE (offsetof(HashMemoryChunkData, data))
140+
#defineHASH_CHUNK_HEADER_SIZEMAXALIGN(sizeof(HashMemoryChunkData))
141+
#defineHASH_CHUNK_DATA(hc)(((char *) (hc)) + HASH_CHUNK_HEADER_SIZE)
142+
/* tuples exceeding HASH_CHUNK_THRESHOLD bytes are put in their own chunk */
137143
#defineHASH_CHUNK_THRESHOLD(HASH_CHUNK_SIZE / 4)
138144

139145
/*

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