From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sun, 27 Mar 2016 22:21:03 +0000 (-0400)
Subject: Guard against zero vardata.rel->tuples in estimate_hash_bucketsize().
X-Git-Tag: REL9_6_BETA1~380
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=d65b665d524a67273b075f468bf3d60ce31f4040;p=postgresql

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

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
index 30751fc8ac..a6555e91b7 100644
--- 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);