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..1471.128rows=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@sart.must-ipra.com