How to optimize this simple query :-(

I've the follwoing two tables:

Table P3AT

AO CHAR(15)
AT Integer
AV VARCHAR(80)
AB LargeInteger

Table P3RL

RELS CHAR(15)
RELT CHAR(15)
SRCT Integer
and some other columns

Indices are on: P3AT.AO, P3AT.AT, P3RL.RELS, P3RL.RELT, P3RL.SRCT

Now I do the query:

a) SELECT AO,AT,AV FROM P3AT WHERE AO IN
(SELECT RELT FROM P3RL WHERE RELS='9#####3#####RW#' AND (SRCT=1004025))

or via

b) SELECT AO,AT,AV FROM P3AT WHERE EXISTS
(SELECT RELT FROM P3RL WHERE (RELT=AO) AND (RELS= ..) AND ...)

Both statements have the same explain result:

Seq Scan on p3at (cost=14458.84 rows=327480 width=28)
SubPlan
-> Index Scan using reltrgind om p3rl (cost=2.05 rows=1 width=12)

Both statements need about 3300 milliseconds to do the job ... and return the
three result rows ...

I gave it another sql-query and this works much better:

c) SELECT AO,AT,AV FROM P3AT,P3RL WHERE
(P3AT.AO=P3RL.RELT) AND (RELS= ...) AND (SRCT= ...)

This gives the following explain result (and the one I would have expected):

NestedLoop (cost=4.1 rows=5 width=40)
-> Index Scan using relsrcind on p3rl (cost=2.05 rows=1 width=12)
-> Index Scan using atrownind on p3at (cost=2.05 rows=327480 width=28)

and needs only 5 ms !

Marten