SELECT DISTINCT too slow

Hello,

I have a table with cca 400,000 rows. The table contains column "key" of varchar(20) type containing 10 distinct values. I want to get out what distinct values are present in the column. I use this simple query, which is very slow:

SELECT DISTINCT Key FROM MRTPContactValue

Here is the query plan:

QUERY PLAN
Unique (cost=64882.26..66964.59 rows=9 width=9) (actual time=26139.972..29593.164 rows=10 loops=1)
-> Sort (cost=64882.26..65923.43 rows=416466 width=9) (actual time=26139.964..27975.944 rows=416466 loops=1)
Sort Key: "key"
-> Seq Scan on mrtpcontactvalue (cost=0.00..8669.66 rows=416466 width=9) (actual time=0.026..2460.535 rows=416466 loops=1)
Total runtime: 29603.159 ms

I've tried index on the "key" column but no improvement.

Is there a way to speed the SELECT up?

Thank you for any suggestions.

-- 
Miroslav Šulc