diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 1e42d75465..56722616a3 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -1085,7 +1085,8 @@ preprocess_expression(PlannerInfo *root, Node *expr, int kind)
 	/*
 	 * Simplify constant expressions.  For function RTEs, this was already
 	 * done by preprocess_function_rtes ... but we have to do it again if the
-	 * RTE is LATERAL and might have contained join alias variables.
+	 * RTE is LATERAL, as it might contain un-simplified subexpressions
+	 * inserted by flattening of subqueries or join alias variables.
 	 *
 	 * Note: an essential effect of this is to convert named-argument function
 	 * calls to positional notation and insert the current actual values of
@@ -1099,8 +1100,7 @@ preprocess_expression(PlannerInfo *root, Node *expr, int kind)
 	 * careful to maintain AND/OR flatness --- that is, do not generate a tree
 	 * with AND directly under AND, nor OR directly under OR.
 	 */
-	if (!(kind == EXPRKIND_RTFUNC ||
-		  (kind == EXPRKIND_RTFUNC_LATERAL && !root->hasJoinRTEs)))
+	if (kind != EXPRKIND_RTFUNC)
 		expr = eval_const_expressions(root, expr);
 
 	/*