Allow ORDER BY/GROUP BY/etc items to match targetlist items regardless of

any implicit casting previously applied to the targetlist item.  This is
reasonable because the implicit cast, by definition, wasn't written by the
user; so we are preserving the expected behavior that ORDER BY items match
textually equivalent tlist items.  The case never arose before because there
couldn't be any implicit casting of a top-level SELECT item before we process
ORDER BY etc.  But now it can arise in the context of aggregates containing
ORDER BY clauses, since the "targetlist" is the already-casted list of
arguments for the aggregate.  The net effect is that the datatype used for
ORDER BY/DISTINCT purposes is the aggregate's declared input type, not that
of the original input column; which is a bit debatable but not horrendous,
and to do otherwise would require major rework that doesn't seem justified.

Per bug #5564 from Daniel Grace.  Back-patch to 9.0 where aggregate ORDER BY
was implemented.

src/backend/parser/parse_clause.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/parser/parse_clause.c,v 1.198 2010/02/26 02:00:50 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/parser/parse_clause.c,v 1.199 2010/07/18 19:37:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -21,6 +21,7 @@
 #include "commands/defrem.h"
 #include "nodes/makefuncs.h"
 #include "nodes/nodeFuncs.h"
+#include "optimizer/clauses.h"
 #include "optimizer/tlist.h"
 #include "optimizer/var.h"
 #include "parser/analyze.h"
@@ -1430,8 +1431,20 @@ findTargetlistEntrySQL99(ParseState *pstate, Node *node, List **tlist)
 	foreach(tl, *tlist)
 	{
 		TargetEntry *tle = (TargetEntry *) lfirst(tl);
+		Node	   *texpr;
 
-		if (equal(expr, tle->expr))
+		/*
+		 * Ignore any implicit cast on the existing tlist expression.
+		 *
+		 * This essentially allows the ORDER/GROUP/etc item to adopt the same
+		 * datatype previously selected for a textually-equivalent tlist item.
+		 * There can't be any implicit cast at top level in an ordinary SELECT
+		 * tlist at this stage, but the case does arise with ORDER BY in an
+		 * aggregate function.
+		 */
+		texpr = strip_implicit_coercions((Node *) tle->expr);
+
+		if (equal(expr, texpr))
 			return tle;
 	}
 

src/test/regress/expected/aggregates.out

@@ -830,3 +830,24 @@ select string_agg(a,',') from (values(null),(null)) g(a);
  
 (1 row)
 
+-- check some implicit casting cases, as per bug #5564
+select string_agg(distinct f1 order by f1) from varchar_tbl;  -- ok
+ string_agg 
+------------
+ aababcd
+(1 row)
+
+select string_agg(distinct f1::text order by f1) from varchar_tbl;  -- not ok
+ERROR:  in an aggregate with DISTINCT, ORDER BY expressions must appear in argument list
+LINE 1: select string_agg(distinct f1::text order by f1) from varcha...
+                                                     ^
+select string_agg(distinct f1 order by f1::text) from varchar_tbl;  -- not ok
+ERROR:  in an aggregate with DISTINCT, ORDER BY expressions must appear in argument list
+LINE 1: select string_agg(distinct f1 order by f1::text) from varcha...
+                                               ^
+select string_agg(distinct f1::text order by f1::text) from varchar_tbl;  -- ok
+ string_agg 
+------------
+ aababcd
+(1 row)
+

src/test/regress/sql/aggregates.sql

@@ -362,3 +362,9 @@ select string_agg(a,',') from (values('aaaa'),('bbbb'),('cccc')) g(a);
 select string_agg(a,',') from (values('aaaa'),(null),('bbbb'),('cccc')) g(a);
 select string_agg(a,',') from (values(null),(null),('bbbb'),('cccc')) g(a);
 select string_agg(a,',') from (values(null),(null)) g(a);
+
+-- check some implicit casting cases, as per bug #5564
+select string_agg(distinct f1 order by f1) from varchar_tbl;  -- ok
+select string_agg(distinct f1::text order by f1) from varchar_tbl;  -- not ok
+select string_agg(distinct f1 order by f1::text) from varchar_tbl;  -- not ok
+select string_agg(distinct f1::text order by f1::text) from varchar_tbl;  -- ok