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 the index.

I thought this might have been from using a partial index, but the same results happen with a full index. I just can't seem to 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