Consider parallel awareness when removing single-child Appends

8edd0e79 added some code to remove Append and MergeAppend nodes when they
contained a single child node.  As it turned out, this was unsafe to do
when the Append/MergeAppend was parallel_aware and the child node was not.
Removing the Append/MergeAppend, in this case, could lead to the child plan
being called multiple times by parallel workers when it was unsafe to do
so.

Here we fix this by just not removing the Append/MergeAppend when the
parallel_aware flag of the parent and child node don't match.

Reported-by: Yura Sokolov
Bug: #17335
Discussion: https://postgr.es/m/b59605fecb20ba9ea94e70ab60098c237c870628.camel%40postgrespro.ru
Backpatch-through: 12, where 8edd0e79 was first introduced

src/backend/optimizer/plan/setrefs.c

@@ -1522,8 +1522,16 @@ set_append_references(PlannerInfo *root,
 		lfirst(l) = set_plan_refs(root, (Plan *) lfirst(l), rtoffset);
 	}
 
-	/* Now, if there's just one, forget the Append and return that child */
-	if (list_length(aplan->appendplans) == 1)
+	/*
+	 * See if it's safe to get rid of the Append entirely.  For this to be
+	 * safe, there must be only one child plan and that child plan's parallel
+	 * awareness must match that of the Append's.  The reason for the latter
+	 * is that the if the Append is parallel aware and the child is not then
+	 * the calling plan may execute the non-parallel aware child multiple
+	 * times.
+	 */
+	if (list_length(aplan->appendplans) == 1 &&
+		((Plan *) linitial(aplan->appendplans))->parallel_aware == aplan->plan.parallel_aware)
 		return clean_up_removed_plan_level((Plan *) aplan,
 										   (Plan *) linitial(aplan->appendplans));
 
@@ -1586,8 +1594,16 @@ set_mergeappend_references(PlannerInfo *root,
 		lfirst(l) = set_plan_refs(root, (Plan *) lfirst(l), rtoffset);
 	}
 
-	/* Now, if there's just one, forget the MergeAppend and return that child */
-	if (list_length(mplan->mergeplans) == 1)
+	/*
+	 * See if it's safe to get rid of the MergeAppend entirely.  For this to
+	 * be safe, there must be only one child plan and that child plan's
+	 * parallel awareness must match that of the MergeAppend's.  The reason
+	 * for the latter is that the if the MergeAppend is parallel aware and the
+	 * child is not then the calling plan may execute the non-parallel aware
+	 * child multiple times.
+	 */
+	if (list_length(mplan->mergeplans) == 1 &&
+		((Plan *) linitial(mplan->mergeplans))->parallel_aware == mplan->plan.parallel_aware)
 		return clean_up_removed_plan_level((Plan *) mplan,
 										   (Plan *) linitial(mplan->mergeplans));