diff --git a/doc/src/sgml/plpgsql.sgml b/doc/src/sgml/plpgsql.sgml
new file mode 100644
index 08c3658..a2482de
*** a/doc/src/sgml/plpgsql.sgml
--- b/doc/src/sgml/plpgsql.sgml
*************** PERFORM <replaceable>query</replaceable>
*** 940,945 ****
--- 940,948 ----
       result.  Write the <replaceable>query</replaceable> the same
       way you would write an SQL <command>SELECT</> command, but replace the
       initial keyword <command>SELECT</> with <command>PERFORM</command>.
+      For <keyword>WITH</> queries, use <keyword>PERFORM</> and then
+      place the query in parentheses.  (In this case, the query can only
+      return one row.)
       <application>PL/pgSQL</application> variables will be
       substituted into the query just as for commands that return no result,
       and the plan is cached in the same way.  Also, the special variable