diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
index 0ef55a0f79c..b5b16937d90 100644
--- a/doc/src/sgml/ddl.sgml
+++ b/doc/src/sgml/ddl.sgml
@@ -4056,8 +4056,8 @@ ALTER INDEX measurement_city_id_logdate_key
 
      <listitem>
       <para>
-       Unique constraints on partitioned tables must include all the
-       partition key columns.  This limitation exists because
+       Unique constraints (and hence primary keys) on partitioned tables must
+       include all the partition key columns.  This limitation exists because
        <productname>PostgreSQL</productname> can only enforce
        uniqueness in each partition individually.
       </para>