Slow SELECT with distinct, in a TIMESTAMP type column

Hi,

I've a table "send_total" like this:

Table "send_total"
Column | Type |
Modifiers
-----------+--------------------------+-------------------------------------------------------
id | integer | not null default
nextval('"send_total_id_seq"'::text)
smsdate | timestamp with time zone |
idclient | integer | not null
recv | integer | default 0
send | integer | default 0
recv_conf | integer | default 0
send_conf | integer | default 0
conf0 | integer | default 0
conf1 | integer | default 0
conf2 | integer | default 0
conf3 | integer | default 0
conf4 | integer | default 0
conf5 | integer | default 0
conf6 | integer | default 0
conf7 | integer | default 0
Indexes: send_total_idclient_idx,
send_total_smsdate_idx
Primary key: send_total_pkey
Unique keys: send_total_idclismsdate
(idclient and smsdate must be unique)

The problem is that my query is too slow when i use distinct:

pgsql> explain analyze SELECT distinct(smsdate) FROM send_total;

NOTICE: QUERY PLAN:

Unique (cost=15840.31..15870.81 rows=1220 width=8) (actual
time=56358.93..56452.78 rows=6670 loops=1)
-> Sort (cost=15840.31..15840.31 rows=12203 width=8) (actual
time=56358.91..56436.95 rows=12062 loops=1)
-> Seq Scan on send_total (cost=0.00..15012.03 rows=12203
width=8) (actual time=0.55..56205.25 rows=12062 loops=1)
Total runtime: 56518.26 msec

This query is becoming slower and slower, day by day.
What i have to do to optimize my query or database??? I've this database
working since January 2002.

I've tried to make the following command, but without success:

$ /usr/bin/vacuumdb -z -d dbname -t send_total

Thanks in advance,

M.P.Garcia

--
M.P.Garcia
PT Inovação, SA
Serviços e Redes Móveis
Rua José Ferreira Pinto Basto - 3810 Aveiro
Tel: 234 403 253 - Fax: 234 424 160
E-mail: marcos-p-garcia(at)ptinovacao(dot)pt