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 d295ec252108542e7b79dde15bd41163bdf0db52..bbd6605c9936333648c43dcfede89c1cc23a7fa8 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -9981,6 +9981,16 @@ 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 id="functions-coalesce-nvl-ifnull">