diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 954c3a9..07723b8 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -432,11 +432,14 @@ CREATE TABLE products (
     A not-null constraint is always written as a column constraint.  A
     not-null constraint is functionally equivalent to creating a check
     constraint <literal>CHECK (<replaceable>column_name</replaceable>
-    IS NOT NULL)</literal>, but in
+    IS DISTINCT FROM NULL)</literal>, but in
     <productname>PostgreSQL</productname> creating an explicit
     not-null constraint is more efficient.  The drawback is that you
     cannot give explicit names to not-null constraints created this
-    way.
+    way.  Note that according to the SQL standard, <literal>NOT NULL</literal>
+    is equivalent to <literal>CHECK (<replaceable>columnn_name</replaceable>
+    IS NOT NULL)</literal>, which has different semantics for composite
+    types.
    </para>
 
    <para>