performance for MIN,MAX aggregates

Hello all,

Just a little simple example:

        isbs=# \d radauth
                       Table "public.radauth"
          Column  |            Type             | Modifiers
        ----------+-----------------------------+-----------
         dttm     | timestamp(0) with time zone |
         username | text                        |
         realm    | text                        |
         logline  | text                        |
        Indexes: radauth_dttm_username btree (dttm, username)

        isbs=# explain select min(dttm) from radauth;
                                     QUERY PLAN
        --------------------------------------------------------------------
         Aggregate  (cost=2591.75..2591.75 rows=1 width=8)
           ->  Seq Scan on radauth  (cost=0.00..2363.00 rows=91500 width=8)
        (2 rows)

        isbs=# \timing
        Timing is on.
        isbs=# select min(dttm) from radauth;
                      min
        -------------------------------
         Wed 15 Jan 00:10:35 2003 KRAT
        (1 row)

        Time: 1455,40 ms


As you can see, PostgreSQL use _sequential_ scans for determination of MINimal
datetime in the raduauth table, but index on dttm exists for this table.
Why not using index in the query?
btree indexes - is binary tree internally and questions like "fetch min/max
element table" can be done very quickly, without seqscans. Furhermore, even
touching table is unuseful in such cases - all required information can be
located in the index.  Is it possible to force PostgreSQL use indexes
for MIN/MAX aggregate functions?   Thanks a lot for any comments.


--
  best regards,
Ruslan A Dautkhanov  rusland@scn.ru