Avoid misbehavior when hash_table_bytes < bucket_size.

It's possible to reach this case when work_mem is very small and tupsize
is (relatively) very large.  In that case ExecChooseHashTableSize would
get an assertion failure, or with asserts off it'd compute nbuckets = 0,
which'd likely cause misbehavior later (I've not checked).  To fix,
clamp the number of buckets to be at least 1.

This is due to faulty conversion of old my_log2() coding in 28d9360.
Back-patch to v13, as that was.

Zhang Mingli

Discussion: https://postgr.es/m/beb64ca0-91e2-44ac-bf4a-7ea36275ec02@Spark

Author: tglsfdc

src/backend/executor/nodeHash.c

@@ -832,7 +832,10 @@ ExecChooseHashTableSize(double ntuples, int tupwidth, bool useskew,
 		 * overhead for the hash code, pointer to the next tuple, etc.
 		 */
 		bucket_size = (tupsize * NTUP_PER_BUCKET + sizeof(HashJoinTuple));
-		sbuckets = pg_nextpower2_size_t(hash_table_bytes / bucket_size);
+		if (hash_table_bytes <= bucket_size)
+			sbuckets = 1;		/* avoid pg_nextpower2_size_t(0) */
+		else
+			sbuckets = pg_nextpower2_size_t(hash_table_bytes / bucket_size);
 		sbuckets = Min(sbuckets, max_pointers);
 		nbuckets = (int) sbuckets;
 		nbuckets = pg_nextpower2_32(nbuckets);