Here is a simple test case:

select * from a
where a.id in (select b.id from b where l ='a') ;

gives:
PLAN (B INDEX (B_ID)) PLAN (A NATURAL)

Would it be possible to access A with index A_ID ?
like in:

select * from a
where a.id in (1,2)

PLAN (A INDEX (A_ID, A_ID))

Regards
PAscal

here is the DDL:
create table A (id integer not null , l varchar(10));
create unique index a_id on a (id);
create table B (id integer not null , l varchar(10));
create unique index b_id on b (id);