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

On Mon, Jul 24, 2017 at 3:22 PM, Dmitry Lazurkin <dilaz03(at)gmail(dot)com> wrote:

> 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)
> ...
>

​You haven't constrained the outer relation (i.e., :values_clause) to be
non-null which is what I believe is required for the semi-join algorithm to
be considered.​

David J.