diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index 1c84a2cb28..4bdfe73d29 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -1964,7 +1964,8 @@ adjust_appendrel_attrs(PlannerInfo *root, Node *node, int nappinfos,
 		newnode = query_tree_mutator((Query *) node,
 									 adjust_appendrel_attrs_mutator,
 									 (void *) &context,
-									 QTW_IGNORE_RC_SUBQUERIES);
+									 QTW_IGNORE_RC_SUBQUERIES |
+									 QTW_IGNORE_JOINALIASES);
 		for (cnt = 0; cnt < nappinfos; cnt++)
 		{
 			AppendRelInfo *appinfo = appinfos[cnt];