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Further twiddling of nodeHash.c hashtable sizing calculation.

Sun, 4 Oct 2015 19:55:07 +0000 (15:55 -0400)

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.)

src/backend/executor/nodeHash.c patch | blob | blame | history

diff --git a/src/backend/executor/nodeHash.c b/src/backend/executor/nodeHash.c

index a47dc328c4b07504489f9bf55e887dd5beaa839f..5e05ec3df37011fbeb2859310e638c1be8513385 100644 (file)

--- a/src/backend/executor/nodeHash.c

+++ b/src/backend/executor/nodeHash.c

@@ -415,6 +415,7 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,

long hash_table_bytes;

long skew_table_bytes;

long max_pointers;

+ long mppow2;

int nbatch = 1;

int nbuckets;

double dbuckets;

@@ -485,14 +486,20 @@ 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);

dbuckets = ceil(ntuples / NTUP_PER_BUCKET);

dbuckets = Min(dbuckets, max_pointers);

+ nbuckets = (int) dbuckets;

/* don't let nbuckets be really small, though ... */

- nbuckets = Max((int) dbuckets, 1024);

+ nbuckets = Max(nbuckets, 1024);

/* ... and force it to be a power of 2. */

nbuckets = 1 << my_log2(nbuckets);

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