Re: NOT IN subquery optimization

On Tue, 26 Feb 2019 at 11:51, Li, Zheng <zhelli(at)amazon(dot)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 http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services