Re: [GENERAL] Perfomance of IN-clause with many elements and possiblesolutions

On 25.07.2017 01:15, David G. Johnston wrote:

On Mon, Jul 24, 2017 at 3:12 PM, Dmitry Lazurkin <dilaz03@gmail.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 = 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.