Re: optimizing a query

On 06/21/2016 03:33 PM, Jonathan Vanasco wrote:
> 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?
>

The below works without including t_a in the FROM?

>
> 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
>
>
>

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com