select distinct too slow

[postgresql 7.1.3, Sun ultrasparc, Solaris 2.5.1]
create table myjunk(run_name text,wafer_num int2,pass int2,test_type text,yloc int2,xloc int2,test_num int,test_data float8,comments text,primary key(run_name,wafer_num,yloc,xloc,test_type,test_num))
[insert 240765 rows]
vacuum analyze myjunk;

There are only 3 distinct values of run_name right now.

select distinct run_name from myjunk;
[takes ~ 14 seconds]

explain select distinct run_name from myjunk;

Unique (cost=34896.75..35498.66 rows=24076 width=12)
-> Sort (cost=34896.75..34896.75 rows=240765 width=12)
-> Seq Scan on myjunk (cost=0.00..6785.65 rows=240765 width=12)

How can I make this query be fast? I tried adding an index on run_name alone, that did not help.

I guess I can keep a separate table with unique run_name's... sounds kind of ugly.
If I did this, what sort of trigger or rule could maintain it automatically -- and how could it
be done without grossly slowing down large data loads into that table?

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