Re: Statistics on array values

Thanks Tom for the clear explanation.
Unfortunately I don't get actual improvements. I use PG 11 and I run the
following commands:

ALTER TABLE subscriptions ALTER tags SET STATISTICS 1000;
ANALYZE subscriptions;

However the bias remains pretty much the same (slightly worse after). Any
idea?

On Sun, Feb 2, 2020 at 6:11 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Marco Colli <collimarco91(at)gmail(dot)com> writes:
> > Let's say that you have a simple query like the following on a large
> table
> > (for a multi-tenant application):
> > SELECT "subscribers".* FROM "subscribers" WHERE
> "subscribers"."project_id"
> > = 123 AND (tags @> ARRAY['de']::varchar[]);
>
> > If you run EXPLAIN ANALYZE you can see that stats are completely wrong.
> > For example I get an expected count of 3,500 rows whereas the actual
> > result is 20 rows. This also results in bad query plans...
>
> > In a previous discussion someone said that this wrong estimate is because
> > @> uses a fixed selectivity of 0.001, **regardless of actual data**!!
> > Is that true?
>
> Hasn't been true since 9.2.
>
> You might get some insight from looking into the most_common_elems,
> most_common_elem_freqs, and elem_count_histogram fields of the pg_stats
> view.
>
> It seems likely to me that increasing the statistics target for this array
> column would help. IIRC, estimates for values that don't show up in
> most_common_elems are going to depend on the lowest frequency that *does*
> show up there ... so if you want better resolution for non-common values,
> you need more entries.
>
> regards, tom lane
>