Document effect of constant folding on CASE.

Back-patch to all supported versions.

Laurenz Albe

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 29576d7ca3210fc4e615fcaf9cda678a6f53a94f..36ec0691b5d2b5ff99e61e1d6a02080f10676260 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -9290,6 +9290,16 @@ SELECT a,
 SELECT ... WHERE CASE WHEN x <> 0 THEN y/x > 1.5 ELSE false END;
 </programlisting>
    </para>
+
+   <note>
+    <para>
+     As described in <xref linkend="xfunc-volatility">, functions and
+     operators marked <literal>IMMUTABLE</literal> can be evaluated when
+     the query is planned rather than when it is executed.  This means
+     that constant parts of a subexpression that is not evaluated during
+     query execution might still be evaluated during query planning.
+    </para>
+   </note>
   </sect2>
 
   <sect2>