Index not used on group by

Hello all,

I have table ma_data, that contain above 300000 rows.
This table has primary key id, and field alias_id.
I create index (btree)on this field.
Set statistic:

ALTER TABLE "public"."ma_data"
ALTER COLUMN "alias_id" SET STATISTICS 998;

So, when I do something like
SELECT alias_id FROM ma_data GROUP BY alias_id
and have (with seq_scan off):

Group (cost=0.00..1140280.63 rows=32 width=4) (actual time=0.159..2640.090 rows=32 loops=1)
-> Index Scan using reference_9_fk on ma_data (cost=0.00..1139526.57 rows=301624 width=4) (actual time=0.120..1471.128 rows=301624 loops=1)
Total runtime: 2640.407 ms
(3 rows)

As I understand there are some problems with visibility of records,
but some others DBMS used indexes without problems(for example
FireBird)? Or maybe some another information be helpful for me and
community.

--
С наилучшими пожеланиями,
Репко Андрей Владимирович mailto:repko(at)sart(dot)must-ipra(dot)com