query not using index

Hi,

I have created the following test table:
create table history (event_id int8 not null, event_type int, event_name
char(10), event_ts timestamp, user_data_ref int8);

I've populated the table with 20,000,000 rows.

I have created an index table:
create index history_idx on history (event_id);

When I issue the following query, it takes about 6 minutes:
select event_id from history where event_id = 1;

I have run vacuum, vacuum analyze but it had no effect on the speed.
Explain sais the followings:

explain select event_id from history where event_type = 120;
NOTICE: QUERY PLAN:

Seq Scan on history (cost=0.00..446079.00 rows=200000 width=8)

even if I do a drop index history_idx, explains sais the same.

Exactly the same thing happens if I create this table with event_id int8
primary key.

Since I don't think, that sequential scan is better then index scan in this
case,
could someone tell me why my index is not being used (- and still I'm
getting only 200000 rows scanned - )?

Thanks,
Norbert