doc: Some additional information about when to use referential actions

doc/src/sgml/ddl.sgml

@@ -1062,6 +1062,27 @@ CREATE TABLE order_items (
     operation will fail.
    </para>
 
+   <para>
+    The appropriate choice of <literal>ON DELETE</literal> action depends on
+    what kinds of objects the related tables represent.  When the referencing
+    table represents something that is a component of what is represented by
+    the referenced table and cannot exist independently, then
+    <literal>CASCADE</literal> could be appropriate.  If the two tables
+    represent independent objects, then <literal>RESTRICT</literal> or
+    <literal>NO ACTION</literal> is more appropriate; an application that
+    actually wants to delete both objects would then have to be explicit about
+    this and run two delete options.  In the above example, order items are
+    part of an order, and it is convenient if they are deleted automatically
+    if an order is deleted.  But products and orders are different things, and
+    so making a deletion of a product automatically cause the deletion of some
+    order items could be considered problematic.  The actions <literal>SET
+    NULL</literal> or <literal>SET DEFAULT</literal> can be appropriate if a
+    foreign-key relationship represents optional information.  For example, if
+    the products table contained a reference to a product manager, and the
+    product manager entry gets deleted, then setting the product's product
+    manager to null or a default might be useful.
+   </para>
+
    <para>
     Analogous to <literal>ON DELETE</literal> there is also
     <literal>ON UPDATE</literal> which is invoked when a referenced