A long time ago, Peter pointed out that ruleutils.c didn't dump simple

constant ORDER/GROUP BY entries properly:
http://archives.postgresql.org/pgsql-hackers/2001-04/msg00457.php
The original solution to that was in fact no good, as demonstrated by
today's report from Martin Pitt:
http://archives.postgresql.org/pgsql-bugs/2008-01/msg00027.php
We can't use the column-number-reference format for a constant that is
a resjunk targetlist entry, a case that was unfortunately not thought of
in the original discussion.  What we can do instead (which did not work
at the time, but does work in 7.3 and up) is to emit the constant with
explicit ::typename decoration, even if it otherwise wouldn't need it.
This is sufficient to keep the parser from thinking it's a column number
reference, and indeed is probably what the user must have done to get
such a thing into the querytree in the first place.

src/backend/utils/adt/ruleutils.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/ruleutils.c,v 1.268 2008/01/01 19:45:52 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/ruleutils.c,v 1.269 2008/01/06 01:03:16 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -175,7 +175,7 @@ static void get_coercion_expr(Node *arg, deparse_context *context,
 				  Oid resulttype, int32 resulttypmod,
 				  Node *parentNode);
 static void get_const_expr(Const *constval, deparse_context *context,
-			   bool showtype);
+			   int showtype);
 static void get_sublink_expr(SubLink *sublink, deparse_context *context);
 static void get_from_clause(Query *query, const char *prefix,
 				deparse_context *context);
@@ -2258,15 +2258,20 @@ get_rule_sortgroupclause(SortClause *srt, List *tlist, bool force_colno,
 	expr = (Node *) tle->expr;
 
 	/*
-	 * Use column-number form if requested by caller or if expression is a
+	 * Use column-number form if requested by caller.  Otherwise, if
-	 * constant --- a constant is ambiguous (and will be misinterpreted by
+	 * expression is a constant, force it to be dumped with an explicit
-	 * findTargetlistEntry()) if we dump it explicitly.
+	 * cast as decoration --- this is because a simple integer constant
+	 * is ambiguous (and will be misinterpreted by findTargetlistEntry())
+	 * if we dump it without any decoration.  Otherwise, just dump the
+	 * expression normally.
 	 */
-	if (force_colno || (expr && IsA(expr, Const)))
+	if (force_colno)
 	{
 		Assert(!tle->resjunk);
 		appendStringInfo(buf, "%d", tle->resno);
 	}
+	else if (expr && IsA(expr, Const))
+		get_const_expr((Const *) expr, context, 1);
 	else
 		get_rule_expr(expr, context, true);
 
@@ -3409,7 +3414,7 @@ get_rule_expr(Node *node, deparse_context *context,
 			break;
 
 		case T_Const:
-			get_const_expr((Const *) node, context, true);
+			get_const_expr((Const *) node, context, 0);
 			break;
 
 		case T_Param:
@@ -4388,7 +4393,7 @@ get_coercion_expr(Node *arg, deparse_context *context,
 		((Const *) arg)->consttypmod == -1)
 	{
 		/* Show the constant without normal ::typename decoration */
-		get_const_expr((Const *) arg, context, false);
+		get_const_expr((Const *) arg, context, -1);
 	}
 	else
 	{
@@ -4407,13 +4412,13 @@ get_coercion_expr(Node *arg, deparse_context *context,
  *
  *	Make a string representation of a Const
  *
- * Note: if showtype is false, the Const is the direct argument of a coercion
+ * showtype can be -1 to never show "::typename" decoration, or +1 to always
- * operation with the same target type, and so we should suppress "::typename"
+ * show it, or 0 to show it only if the constant wouldn't be assumed to be
- * to avoid redundant output.
+ * the right type by default.
  * ----------
  */
 static void
-get_const_expr(Const *constval, deparse_context *context, bool showtype)
+get_const_expr(Const *constval, deparse_context *context, int showtype)
 {
 	StringInfo	buf = context->buf;
 	Oid			typoutput;
@@ -4430,7 +4435,7 @@ get_const_expr(Const *constval, deparse_context *context, bool showtype)
 		 * about type when reparsing.
 		 */
 		appendStringInfo(buf, "NULL");
-		if (showtype)
+		if (showtype >= 0)
 			appendStringInfo(buf, "::%s",
 							 format_type_with_typemod(constval->consttype,
 													  constval->consttypmod));
@@ -4506,13 +4511,15 @@ get_const_expr(Const *constval, deparse_context *context, bool showtype)
 
 	pfree(extval);
 
-	if (!showtype)
+	if (showtype < 0)
 		return;
 
 	/*
-	 * Append ::typename unless the constant will be implicitly typed as the
+	 * For showtype == 0, append ::typename unless the constant will be
-	 * right type when it is read in.  XXX this code has to be kept in sync
+	 * implicitly typed as the right type when it is read in.
-	 * with the behavior of the parser, especially make_const.
+	 *
+	 * XXX this code has to be kept in sync with the behavior of the parser,
+	 * especially make_const.
 	 */
 	switch (constval->consttype)
 	{
@@ -4534,7 +4541,7 @@ get_const_expr(Const *constval, deparse_context *context, bool showtype)
 			needlabel = true;
 			break;
 	}
-	if (needlabel)
+	if (needlabel || showtype > 0)
 		appendStringInfo(buf, "::%s",
 						 format_type_with_typemod(constval->consttype,
 												  constval->consttypmod));