Division by zero in selfuncs.c:estimate_hash_bucketsize()

From: Piotr Stefaniak <postgres(at)piotr-stefaniak(dot)me>
To: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Division by zero in selfuncs.c:estimate_hash_bucketsize()
Date: 2015-07-30 05:28:24
Message-ID: BLU436-SMTP6793A5675100602B2012D6F28B0@phx.gbl
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Hello,

the two asserts below will fail with this query (ran against the
regression db):

SELECT 1
FROM (SELECT age FROM public.person LIMIT 1) s
INNER JOIN public.person USING (age);

diff --git a/src/backend/utils/adt/selfuncs.c
b/src/backend/utils/adt/selfuncs.c
index 64b6ae4..56c65b3 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -3482,6 +3482,7 @@ estimate_hash_bucketsize(PlannerInfo *root, Node
*hashkey, double nbuckets)
stanullfrac = 0.0;

/* Compute avg freq of all distinct data values in raw relation */
+ Assert(ndistinct != 0);
avgfreq = (1.0 - stanullfrac) / ndistinct;

/*
@@ -3502,8 +3503,10 @@ estimate_hash_bucketsize(PlannerInfo *root, Node
*hashkey, double nbuckets)
*/
if (ndistinct > nbuckets)
estfract = 1.0 / nbuckets;
- else
+ else {
+ Assert(ndistinct != 0);
estfract = 1.0 / ndistinct;
+ }

/*
* Look up the frequency of the most common value, if available.

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