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Clamp adjusted ndistinct to positive integer in estimate_hash_bucketsize().

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This avoids a possible divide-by-zero in the following calculation,
and rounding the number to an integer seems like saner behavior anyway.
Assuming IEEE math, the division would yield +Infinity which would get
replaced by 1.0 at the bottom of the function, so nothing really
interesting would ensue; but avoiding divide-by-zero seems like a
good idea on general principles.

Per report from Piotr Stefaniak.  No back-patch since this seems
mostly cosmetic.
This commit is contained in:
Tom Lane 2016-03-27 18:06:53 -04:00
parent 408f043853
commit fa09f89351
1 changed files with 3 additions and 0 deletions
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3
src/backend/utils/adt/selfuncs.c
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@ -3541,7 +3541,10 @@ estimate_hash_bucketsize(PlannerInfo *root, Node *hashkey, double nbuckets)
* selectivity of rel's restriction clauses that mention the target Var.
*/
if (vardata.rel)
{
ndistinct *= vardata.rel->rows / vardata.rel->tuples;
ndistinct = clamp_row_est(ndistinct);
}
/*
* Initial estimate of bucketsize fraction is 1/nbuckets as long as the