how to enforce index sub-select over filter+seqscan

Hello!

I have this table:

create table test (
    s1 varchar(255),
    s2 varchar(255),
    i1 integer,
    i2 integer,

... over 100 other fields

);

table contains over 8 million records

there's these indexes:

create index is1 on test (s1);
create index is2 on test (s2);
create index ii1 on test (i1);
create index ii2 on test (i2);
create index ii3 on test (i1, i2);

and then i run this query:

select
*
from (
    select  *
    from test
    where
        is1 = 'aa' or is2 = 'aa'
    )
where
    is1 = 1
    or (is1  = 1
        and is2 = 1)
    or (is1  = 2
        and is2 = 2)
    or (is1  = 3
        and is2 = 3)


where part of outer query can have different count of
    "or (is1  = N
        and is2 = M)"
expressions, lets name this number X.

When X is low planner chooses index scan using is1 and is2,
then BitmapAnd that with index scan using  ii1, ii2 or ii3.

But when X is big enough (> 15) planner chooses seqscan and filter on
i1, i2, s1, s2.
Seqscan is very slow and I want to avoid it. Subquery is very fast
and i don't know why postgres chooses that plan.

I know I can set enable_seqscan = off.
Is there other ways to enforce index usage?

postgres pg_class have right estimate of rowcount.

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