Prevent Parallel Hash Join for JOIN_UNIQUE_INNER.

macdice · macdice · commit aca127c105aa · 2019-06-19T01:25:57.000+12:00
WHERE EXISTS (...) queries cannot be executed by Parallel Hash Join with jointype JOIN_UNIQUE_INNER, because there is no way to make a partial plan totally unique. The consequence of allowing such plans was duplicate results from some EXISTS queries. Back-patch to 11. Bug #15857. Author: Thomas Munro Reviewed-by: Tom Lane Reported-by: Vladimir Kriukov Discussion: https://postgr.es/m/15857-d1ba2a64bce0795e%40postgresql.org
diff --git a/src/backend/optimizer/path/joinpath.c b/src/backend/optimizer/path/joinpath.c
@@ -1867,9 +1867,12 @@ hash_inner_and_outer(PlannerInfo *root,
 
 			/*
 			 * Can we use a partial inner plan too, so that we can build a
-			 * shared hash table in parallel?
+			 * shared hash table in parallel?  We can't handle
+			 * JOIN_UNIQUE_INNER because we can't guarantee uniqueness.
 			 */
-			if (innerrel->partial_pathlist != NIL && enable_parallel_hash)
+			if (innerrel->partial_pathlist != NIL &&
+				save_jointype != JOIN_UNIQUE_INNER &&
+				enable_parallel_hash)
 			{
 				cheapest_partial_inner =
 					(Path *) linitial(innerrel->partial_pathlist);
