Обсуждение: grouping pushdown
Hi hackers,
I came across a problem on how to improve the performance of queries with GROUP BY clause when the grouping columns have much duplicate data. For example:
create table t1(i1) as select 1 from generate_series(1,10000);
create table t2(i2) as select 2 from generate_series(1,10000);
select i1,i2 from t1, t2 group by i1,i2;
i1 | i2
----+----
1 | 2
QUERY PLAN
-----------------------------------
HashAggregate
Group Key: t1.i1, t2.i2
Batches: 1 Memory Usage: 24kB
-> Nested Loop
-> Seq Scan on t1
-> Materialize
-> Seq Scan on t2
Planning Time: 0.067 ms
Execution Time: 15864.585 ms
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. For example, the plan may looks like:
expected QUERY PLAN
----------------------------------------
Group
Group Key: t1.i1, t2.i2
-> Sort
Sort Key: t1.i1, t2.i2
-> Nested Loop
-> HashAggregate
Group Key: t1.i1
-> Seq Scan on t1
-> HashAggregate
Group Key: t2.i2
-> Seq Scan on t2
Moreover, queries with expressions as GROUP BY columns may also take advantage of this feature, e.g.
select i1+i2 from t1, t2 group by i1+i2;
?column?
----------
3
expected QUERY PLAN
----------------------------------------
Group
Group Key: ((t1.i1 + t2.i2))
-> Sort
Sort Key: ((t1.i1 + t2.i2))
-> Nested Loop
-> HashAggregate
Group Key: t1.i1
-> Seq Scan on t1
-> HashAggregate
Group Key: t2.i2
-> Seq Scan on t2 Is someone has suggestions on this?
On Wed, 4 Jan 2023 at 23:21, Spring Zhong <spring.zhong@openpie.com> wrote: > The plan is apparently inefficient, since the hash aggregate goes after the Cartesian product. We could expect the query'sperformance 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]. David [1] https://commitfest.postgresql.org/41/3764/
David Rowley <dgrowleyml@gmail.com> wrote:
> On Wed, 4 Jan 2023 at 23:21, Spring Zhong <spring.zhong@openpie.com> wrote:
> > The plan is apparently inefficient, since the hash aggregate goes after the Cartesian product. We could expect the
query'sperformance 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