Document incompatibility of comparison expressions with VARIADIC array arguments

COALESCE, GREATEST and LEAST all look like functions taking variable
numbers of arguments, but in fact they are not functions, and so
VARIADIC array arguments don't work with them. Add a note to the docs
explaining this fact.

The consensus is not to try to make this work, but just to document the
limitation.

Discussion: https://postgr.es/m/CAFj8pRCaAtuXuRtvXf5GmPbAVriUQrNMo7-=TXUFN025S31R_w@mail.gmail.com

diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 03859a78eaa35c71efb2cfd532173d651d4479fe..7fbcdfeae56ba3b70229005281372b43575846f7 100644 (file)
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -12481,6 +12481,15 @@ SELECT setval('foo', 42, false);    <lineannotation>Next <function>nextval</func
    </para>
   </tip>
 
+   <note>
+    <para>
+     Although <token>COALESCE</token>, <token>GREATEST</token>, and
+     <token>LEAST</token> are syntactically similar to functions, they are
+     not strictly functions, and thus cannot be used with explicit
+     <token>VARIADIC</token> array arguments.
+    </para>
+   </note>
+
   <sect2 id="functions-case">
    <title><literal>CASE</literal></title>