On 25.07.2017 01:15, David G. Johnston wrote:
On Mon, Jul 24, 2017 at 3:12 PM, Dmitry Lazurkin <dila...@gmail.com
<mailto:dila...@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 <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.
