diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 4c5af4b83c..8cf5385561 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -10563,6 +10563,16 @@ SELECT a,
SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;
+
+
+
+ As described in , functions and
+ operators marked IMMUTABLE can be evaluated when
+ the query is planned rather than when it is executed. This means
+ that constant parts of a subexpression that is not evaluated during
+ query execution might still be evaluated during query planning.
+
+