Add a warning about possible strange behavior of volatile functions

in cursors.  This has always been the case, but given the lack of user
complaints about it, I'm not going to bother back-patching this.

Author: tglsfdc

doc/src/sgml/ref/declare.sgml

@@ -1,5 +1,5 @@
 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/declare.sgml,v 1.46 2009/04/10 17:56:21 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/declare.sgml,v 1.47 2009/06/10 19:21:37 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -228,6 +228,20 @@ DECLARE <replaceable class="parameter">name</replaceable> [ BINARY ] [ INSENSITI
     <literal>SCROLL</literal> may not be specified in this case.
    </para>
 
+   <caution>
+    <para>
+     Scrollable and <literal>WITH HOLD</literal> cursors may give unexpected
+     results if they invoke any volatile functions (see <xref
+     linkend="xfunc-volatility">).  When a previously fetched row is
+     re-fetched, the functions might be re-executed, perhaps leading to
+     results different from the first time.  One workaround for such cases
+     is to declare the cursor <literal>WITH HOLD</literal> and commit the
+     transaction before reading any rows from it.  This will force the
+     entire output of the cursor to be materialized in temporary storage,
+     so that volatile functions are executed exactly once for each row.
+    </para>
+   </caution>
+
    <para>
     If the cursor's query includes <literal>FOR UPDATE</> or <literal>FOR
     SHARE</>, then returned rows are locked at the time they are first