Re: NOT IN subquery optimization
| От | Richard Guo |
|---|---|
| Тема | Re: NOT IN subquery optimization |
| Дата | |
| Msg-id | CAN_9JTwZUsWLymsGA+BAJiGyqL5-gpEhu5zRF3gG9grCgmoCWA@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: NOT IN subquery optimization (David Rowley <david.rowley@2ndquadrant.com>) |
| Список | pgsql-hackers |
Greenplum Database does this optimization. The idea is to use a new join
type, let's call it JOIN_LASJ_NOTIN, and its semantic regarding NULL is
defined as below:
1. If there is a NULL in the outer side, and the inner side is empty, the
NULL should be part of the outputs.
2. If there is a NULL in the outer side, and the inner side is not empty,
the NULL should not be part of the outputs.
3. If there is a NULL in the inner side, no outputs should be produced.
An example plan looks like:
gpadmin=# explain (costs off) select * from t1 where a not in(select a from t2);
QUERY PLAN
-----------------------------------
Hash Left Anti Semi (Not-In) Join
Hash Cond: (t1.a = t2.a)
-> Seq Scan on t1
-> Hash
-> Seq Scan on t2
(5 rows)
Thanks
Richard
On Tue, Feb 26, 2019 at 7:20 AM David Rowley <david.rowley@2ndquadrant.com> wrote:
On Tue, 26 Feb 2019 at 11:51, Li, Zheng <zhelli@amazon.com> wrote:
> Resend the patch with a whitespace removed so that "git apply patch" works directly.
I had a quick look at this and it seems to be broken for the empty
table case I mentioned up thread.
Quick example:
Setup:
create table t1 (a int);
create table t2 (a int not null);
insert into t1 values(NULL),(1),(2);
select * from t1 where a not in(select a from t2);
Patched:
a
---
1
2
(2 rows)
Master:
a
---
1
2
(3 rows)
This will be due to the fact you're adding an a IS NOT NULL qual to
the scan of a:
postgres=# explain select * from t1 where a not in(select a from t2);
QUERY PLAN
------------------------------------------------------------------
Hash Anti Join (cost=67.38..152.18 rows=1268 width=4)
Hash Cond: (t1.a = t2.a)
-> Seq Scan on t1 (cost=0.00..35.50 rows=2537 width=4)
Filter: (a IS NOT NULL)
-> Hash (cost=35.50..35.50 rows=2550 width=4)
-> Seq Scan on t2 (cost=0.00..35.50 rows=2550 width=4)
(6 rows)
but as I mentioned, you can't do that as t2 might be empty and there's
no way to know that during planning.
--
David Rowley https://urldefense.proofpoint.com/v2/url?u=http-3A__www.2ndQuadrant.com_&d=DwIBaQ&c=lnl9vOaLMzsy2niBC8-h_K-7QJuNJEsFrzdndhuJ3Sw&r=5r3cnfZPUDOHrMiXq8Mq2g&m=dE1nglE17x3nD-oH_BrF0r4SLaFnQKzwwJBJGpDoaaA&s=dshupMomMvkDAd92918cU21AJ1E1s7QwbrxIGSRxZA8&e=
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