index usage ... strange !?

He's a small example and I'm trying to speed up this query and I can manage it :-(

The table:

P3AT:

AO - char(15), with index
AT - Integer, with index
AV - VARCHAR(80), with index valind
CI - Integer, with index

We've about 23000 rows in this table. I would like to execute:

SELECT AO,AT,AV FROM P3AT
WHERE EXISTS( SELECT AO FROM P3AT WHERE AV='12' AND AT=12 AND CI=17)

Explain gives me:

Result(946.19,0,0)
InitPlan
-> INdex Scan using valind on p3at (222,265,12) (fine !)
->Seq Scan on p3at (946,22235,26) (urgghhh ?)

This statement takes about 2s to return the results.

The select statement within exists just needs 23ms to find the AO value !

SELECT AO FROM P3AT WHERE AO='12'

EXPLAIN GIVES ME:

INDEX SCAN USING ATROWIND ON P3AT (2,2,12)

In general I expect for each unique AO about 10-12 result rows. I've done
several vacuum analyze (the first one crashed my database by the way :-(, this
tool is really making me crazy - claiming that it can't remove the lock :-().

The reason seems to be the seq scan ... therefore how can I get rid of it !

I use PSQL 6.5.3 under SuSE 6.1.

Marten