Re: grouping pushdown

David Rowley <dgrowleyml(at)gmail(dot)com> wrote:

> On Wed, 4 Jan 2023 at 23:21, Spring Zhong <spring(dot)zhong(at)openpie(dot)com> wrote:
> > The plan is apparently inefficient, since the hash aggregate goes after the Cartesian product. We could expect the query's performance get much improved if the HashAggregate node can be pushed down to the SCAN node.
>
> > Is someone has suggestions on this?
>
> I think this is being worked on. See [1].

Well, the current version of that patch requires the query to contain at least
one aggregate. It shouldn't be a big deal to modify it. However note that this
feature pushes the aggregate/grouping only to one side of the join ("fake"
aggregate count(*) added to the query):

SET enable_agg_pushdown TO on;

EXPLAIN select i1,i2, count(*) from t1, t2 group by i1,i2;
QUERY PLAN
--------------------------------------------------------------------------------
Finalize GroupAggregate (cost=440.02..440.04 rows=1 width=16)
Group Key: t1.i1, t2.i2
-> Sort (cost=440.02..440.02 rows=1 width=16)
Sort Key: t1.i1, t2.i2
-> Nested Loop (cost=195.00..440.01 rows=1 width=16)
-> Partial HashAggregate (cost=195.00..195.01 rows=1 width=12)
Group Key: t1.i1
-> Seq Scan on t1 (cost=0.00..145.00 rows=10000 width=4)
-> Seq Scan on t2 (cost=0.00..145.00 rows=10000 width=4)

If both sides should be grouped, finalization of the partial aggregates would
be more difficult, and I'm not sure it'd be worth the effort.

> [1] https://commitfest.postgresql.org/41/3764/

--
Antonin Houska
Web: https://www.cybertec-postgresql.com