Re: NOT IN subquery optimization

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                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services