Guard against zero vardata.rel->tuples in estimate_hash_bucketsize().

tglsfdc · tglsfdc · commit d65b665d524a · 2016-03-27T18:21:03.000-04:00
If the referenced rel was proven empty, we'd compute 0/0 here, which
results in the function returning NaN.  That's a bit more serious
than the other zero-divide case.  Still, it only seems to be possible
in HEAD, so no back-patch.

Per report from Piotr Stefaniak.  I looked through the rest of selfuncs.c
and found no other likely trouble spots.
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
@@ -3540,7 +3540,7 @@ estimate_hash_bucketsize(PlannerInfo *root, Node *hashkey, double nbuckets)
 	 * XXX Possibly better way, but much more expensive: multiply by
 	 * selectivity of rel's restriction clauses that mention the target Var.
 	 */
-	if (vardata.rel)
+	if (vardata.rel && vardata.rel->tuples > 0)
 	{
 		ndistinct *= vardata.rel->rows / vardata.rel->tuples;
 		ndistinct = clamp_row_est(ndistinct);

Original file line number	Diff line number	Diff line change
`@@ -3540,7 +3540,7 @@ estimate_hash_bucketsize(PlannerInfo root, Node hashkey, double nbuckets)`
`3540`	`3540`	`* XXX Possibly better way, but much more expensive: multiply by`
`3541`	`3541`	`* selectivity of rel's restriction clauses that mention the target Var.`
`3542`	`3542`	`*/`
`3543`		`- if (vardata.rel)`
	`3543`	`+ if (vardata.rel && vardata.rel->tuples > 0)`
`3544`	`3544`	`{`
`3545`	`3545`	`ndistinct *= vardata.rel->rows / vardata.rel->tuples;`
`3546`	`3546`	`ndistinct = clamp_row_est(ndistinct);`