Re: Perfomance of IN-clause with many elements and possible solutions

On 25.07.2017 01:15, David G. Johnston wrote:
> On Mon, Jul 24, 2017 at 3:12 PM, Dmitry Lazurkin <dilaz03(at)gmail(dot)com
> <mailto:dilaz03(at)gmail(dot)com>>wrote:
>
> And I have one question. I don't understand why IN-VALUES doesn't
> use Semi-Join? PostgreSQL has Hash Semi-Join... For which task
> the database has node of this type?
>
>
> ​Semi-Join is canonically written as:
>
> SELECT *
> FROM tbl
> WHERE EXISTS (SELECT 1 FROM tbl2 WHERE tbl.id <http://tbl.id> =
> tbl2.id <http://tbl2.id>)
>
> The main difference between IN and EXISTS is NULL semantics.
>
> David J.
>

ALTER TABLE ids ALTER COLUMN id SET NOT NULL;
EXPLAIN (ANALYZE, BUFFERS) SELECT count(*) FROM ids WHERE id IN
:values_clause;

Aggregate (cost=245006.46..245006.47 rows=1 width=8) (actual
time=3824.095..3824.095 rows=1 loops=1)
Buffers: shared hit=44248
-> Hash Join (cost=7.50..235006.42 rows=4000019 width=0) (actual
time=1.108..3327.112 rows=3998646 loops=1)
...

Hmmm. No Semi-Join.

PostgreSQL can use Semi-Join for IN too.