Document that we don't support ORDER BY with general expressions on

the output of UNION/INTERSECT/EXCEPT.

diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 9dbc4958121a267b2f73e21da08242f3191c5f2b..a34771571ecb33fac4fd6cba68c20d055f755f02 100644 (file)
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -1,5 +1,5 @@
 
 
@@ -616,7 +616,12 @@ SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;
     
 SELECT name FROM distributors ORDER BY code;
     
-
+    A limitation of this feature is that an ORDER BY clause applying to the
+    result of a UNION, INTERSECT, or EXCEPT query may only specify an output
+    column name or number, not an expression.
+   
+    
+   
     Note that if an ORDER BY item is a simple name that matches both
     a result column name and an input column name, ORDER BY will interpret
     it as the result column name.  This is the opposite of the choice that