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Further twiddling of nodeHash.c hashtable sizing calculation.

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On reflection, the submitted patch didn't really work to prevent the
request size from exceeding MaxAllocSize, because of the fact that we'd
happily round nbuckets up to the next power of 2 after we'd limited it to
max_pointers.  The simplest way to enforce the limit correctly is to
round max_pointers down to a power of 2 when it isn't one already.

(Note that the constraint to INT_MAX / 2, if it were doing anything useful
at all, is properly applied after that.)
This commit is contained in:
Tom Lane 2015-10-04 15:55:07 -04:00
parent 2647b245d2
commit d637a899ca
1 changed files with 7 additions and 1 deletions
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8
src/backend/executor/nodeHash.c
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@ -396,6 +396,7 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
long hash_table_bytes;
long skew_table_bytes;
long max_pointers;
long mppow2;
int nbatch;
int nbuckets;
int i;
@ -463,7 +464,12 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
*/
max_pointers = (work_mem * 1024L) / sizeof(HashJoinTuple);
max_pointers = Min(max_pointers, MaxAllocSize / sizeof(HashJoinTuple));
/* also ensure we avoid integer overflow in nbatch and nbuckets */
/* 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);