RE: Why does not subquery pruning conditions inherit to parent query?

Friday, May 24, 2019 5:10 PM, David Rowley wrote:
> The planner can only push quals down into a subquery, it cannot pull quals
> from a subquery into the outer query.
>
> If you write the query like:
>
> explain select * from jta, (select a, max(b) from jtb group by a ) c1
> where jta.a = c1.a and c1.a = 1;
>
> you should get the plan that you want.

Thank you for your replay.

You are right. I should do that.
However, following query looks like the subquery qual is pushed down into the outer query.

postgres=# explain select * from jta, (select a from jtb where a = 1) c1 where jta.a = c1.a;
QUERY PLAN
------------------------------------------------------------------
Nested Loop (cost=0.00..81.94 rows=143 width=8)
-> Seq Scan on jta0 (cost=0.00..41.88 rows=13 width=4)
Filter: (a = 1)
-> Materialize (cost=0.00..38.30 rows=11 width=4)
-> Seq Scan on jtb0 (cost=0.00..38.25 rows=11 width=4)
Filter: (a = 1)
(6 rows)

So, I think I could improve this behavior.
Why such a difference occur?

regards,

Sho Kato
> -----Original Message-----
> From: David Rowley [mailto:david(dot)rowley(at)2ndquadrant(dot)com]
> Sent: Friday, May 24, 2019 5:10 PM
> To: Kato, Sho/加藤 翔 <kato-sho(at)jp(dot)fujitsu(dot)com>
> Cc: pgsql-hackers(at)postgresql(dot)org
> Subject: Re: Why does not subquery pruning conditions inherit to parent
> query?
>
> On Fri, 24 May 2019 at 19:44, Kato, Sho <kato-sho(at)jp(dot)fujitsu(dot)com> wrote:
> > I execute following query to the partitioned table, but the plan is
> different from my assumption, so please tell me the reason.
> >
> > postgres=# explain select * from jta, (select a, max(b) from jtb where
> a = 1 group by a ) c1 where jta.a = c1.a;
> > QUERY PLAN
> >
> --------------------------------------------------------------------
> --
> > -- Hash Join (cost=38.66..589.52 rows=1402 width=12)
> > Hash Cond: (jta0.a = jtb0.a)
> > -> Append (cost=0.00..482.50 rows=25500 width=4)
> > -> Seq Scan on jta0 (cost=0.00..35.50 rows=2550 width=4)
> > -> Seq Scan on jta1 (cost=0.00..35.50 rows=2550 width=4)
> > -> Seq Scan on jta2 (cost=0.00..35.50 rows=2550 width=4)
> > -> Seq Scan on jta3 (cost=0.00..35.50 rows=2550 width=4)
> > -> Seq Scan on jta4 (cost=0.00..35.50 rows=2550 width=4)
> > -> Seq Scan on jta5 (cost=0.00..35.50 rows=2550 width=4)
> > -> Seq Scan on jta6 (cost=0.00..35.50 rows=2550 width=4)
> > -> Seq Scan on jta7 (cost=0.00..35.50 rows=2550 width=4)
> > -> Seq Scan on jta8 (cost=0.00..35.50 rows=2550 width=4)
> > -> Seq Scan on jta9 (cost=0.00..35.50 rows=2550 width=4)
> > -> Hash (cost=38.53..38.53 rows=11 width=8)
> > -> GroupAggregate (cost=0.00..38.42 rows=11 width=8)
> > Group Key: jtb0.a
> > -> Seq Scan on jtb0 (cost=0.00..38.25 rows=11
> width=8)
> > Filter: (a = 1)
> > (18 rows)
> >
> > I assume that subquery aggregate only pruned table and parent query
> joins pruned table and subquery results.
> > However, parent query scan all partitions and join.
> > In my investigation, because is_simple_query() returns false if
> subquery contains GROUP BY, parent query does not prune.
> > Is it possible to improve this?
>
> The planner can only push quals down into a subquery, it cannot pull quals
> from a subquery into the outer query.
>
> If you write the query like:
>
> explain select * from jta, (select a, max(b) from jtb group by a ) c1
> where jta.a = c1.a and c1.a = 1;
>
> you should get the plan that you want.
>
> --
> David Rowley http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services
>
>