Prevent Parallel Hash Join for JOIN_UNIQUE_INNER.

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

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);