Type casting and indexes

I hope this hasn't been answered before, I've looked at the docs here:
  http://www.postgresql.org/docs/view.php?version=7.3&idoc=1&file=index.html

Anyway, I've found a (bug|feature|standard?) with type casting and index usage.

I've got a table with a column that's a timestamp with time zone. This column is indexed. If I issue the "normal" query
of: 

    SELECT count(*) FROM foo WHERE bar > '2003-05-05':;timestamp

I get the following EXPLAIN ANALYZE output:

urldb=> explain select count(*) from foo where bar > '2003-05-05'::timestamp;
                             QUERY PLAN
------------------------------------------------------------------------
 Aggregate  (cost=89960.75..89960.75 rows=1 width=0) (actual time=
             56706.58..56706.58 rows=1 loops=1)
   ->  Seq Scan on urlinfo  (cost=0.00..87229.45 rows=1092521 width=0) (actual
        time=25.37..56537.86 rows=27490 loops=1)
         Filter: (ratedon > ('2003-05-05 00:00:00'::timestamp without time
                  zone)::timestamp with time zone)
 Total runtime: 56706.67 msec

So it seems that the type conversion is killing the use of the index, even though the type conversion has to happen for
thecondition to be tested. 

If I change this query slightly, by casting to timestamptz, I get the following EXPLAIN ANALYZE output:

                              QUERY PLAN
-------------------------------------------------------------------------
 Aggregate  (cost=38609.70..38609.70 rows=1 width=0) (actual time=547.58..547.58
             rows=1 loops=1)
   ->  Index Scan using urlinfo_on on urlinfo  (cost=0.00..38578.97 rows=12295
        width=0) (actual time=0.18..381.95 rows=27490 loops=1)
         Index Cond: (ratedon > '2003-05-05 00:00:00-07'::timestamp with time
                      zone)
 Total runtime: 548.17 msec

That's much better! Is this the way it's supposed to work?

--------------------------
David Olbersen
iGuard Engineer
11415 West Bernardo Court
San Diego, CA 92127
1-858-676-2277 x2152