optimizing a query

I have a handful of queries in the following general form that I can't seem to optimize any further (same results on
9.3,9.4, 9.5) 

I'm wondering if anyone might have a suggestion, or if they're done.

The relevant table structure:

    t_a2b
        a_id INT references t_a(id)
        b_id INT references t_b(id)
        col_a

    t_a
        id INT
        col_1 INT
        col_2 BOOL

The selects query the association table (t_a2b) and join in a related table (t_a) for some filtering.

In effort of simplifying the work, I've created indexes on t_a that have all the related columns.

    CREATE INDEX test_idx ON t_a(col_1, id) WHERE col_2 IS NOT FALSE;
        CREATE INDEX test_idx__a ON t_a(col_1, id) WHERE col_2 IS NOT FALSE;

postgres will query test_idx__a first (yay!) but then does a bitmap heap scan on t_a, and uses the raw t_a for the hash
join.  

I don't actually need any information from t_a - it's just there for the filtering, and ideally postgres would just use
theindex. 

I thought this might have been from using a partial index, but the same results happen with a full index.  I just can't
seemto avoid this hash join against the full table. 

anyone have a suggestion?


example query

    SELECT t_a2b.b_id AS b_id,
           count(t_a2b.b_id) AS counted
    FROM t_a2b
    WHERE
          t_a2b.col_a = 1
          AND
          t_a.col_1 = 730
          AND
          t_a.col_2 IS NOT False
    GROUP BY t_a2b.b_id
    ORDER BY     counted DESC,
                 t_a2b.b_id ASC