Division by zero in selfuncs.c:estimate_hash_bucketsize()

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.