diff --git a/doc/src/sgml/installation.sgml b/doc/src/sgml/installation.sgml
index 673ec243eaa5b93f13aafc6fa713709418cd93ff..1dee8f4ccba1470a0a1c2e77c0ae64a9bbe76a4b 100644
--- a/doc/src/sgml/installation.sgml
+++ b/doc/src/sgml/installation.sgml
@@ -203,8 +203,12 @@ su - postgres
       language, you need a <productname>Python</productname>
       installation with the header files and
       the <application>distutils</application> module.  The minimum
-      required version is <productname>Python</productname>
-      2.3.  <productname>Python 3</productname> is supported if it's
+      required version is <productname>Python</productname> 2.3.
+      For full functionality, a 2.3.x installation must include the
+      separately-available <filename>cdecimal</> module; in particular
+      the <application>PL/Python</> regression tests will not pass if that
+      is missing.
+      <productname>Python 3</productname> is supported if it's
       version 3.1 or later; but see
       <![%standalone-include[the <application>PL/Python</> documentation]]>
       <![%standalone-ignore[<xref linkend="plpython-python23">]]>