"Group by" and "index".

Hi,

PostgreSQL 6.5.3.

I've created the following index:

>create index test_index on test (filed1);
>vacuum;

And then:

>explain select field1 from test group by field1;
NOTICE:  QUERY PLAN:

Group  (cost=2510.40 rows=58709 width=12) ->  Sort  (cost=2510.40 rows=58709 width=12)       ->  Seq Scan on test
(cost=2510.40rows=58709 width=12)
 

It don't use this index :-(. I've tried to create two types of index:
b-tree and hash - both doesn't work until "group" is present. If I type

> explain select field1 from test where field1='tra-ta-ta';

NOTICE:  QUERY PLAN:

Index Scan using test_index on test (cost=2.05 rows=2 width=12)


It work this way.


Any ideas?

_____________
Vlad Marchenko