Re: rows selectivity overestimate for @> operator for arrays

On Fri, May 27, 2022 at 12:19 PM Alexey Ermakov <
alexey(dot)ermakov(at)dataegret(dot)com> wrote:

> Hello, please look into following example:
>
> postgres=# create table test_array_selectivity as select
> array[id]::int[] as a from generate_series(1, 10000000) gs(id);
> SELECT 10000000
> postgres=# explain analyze select * from test_array_selectivity where a
> @> array[1];
> QUERY PLAN
>
> -----------------------------------------------------------------------------------------------------------------------------
> Seq Scan on test_array_selectivity (cost=0.00..198531.00 rows=50000
> width=32) (actual time=0.023..2639.029 rows=1 loops=1)
> Filter: (a @> '{1}'::integer[])
> Rows Removed by Filter: 9999999
> Planning Time: 0.078 ms
> Execution Time: 2639.038 ms
> (5 rows)
>
>
> for row estimation rows=50000=10000000*0.005 we are using constant
> DEFAULT_CONTAIN_SEL if I'm not mistaken.
> and we're using it unless we have something in most_common_elems (MCE)
> in statistics which in this case is empty.
>
>
This was discussed before at
https://www.postgresql.org/message-id/flat/CAMkU%3D1x2W1gpEP3AQsrSA30uxQk1Sau5VDOLL4LkhWLwrOY8Lw%40mail.gmail.com

My solution was to always store at least one element in the MCE, even if
the sample size was too small to be reliable. It would still be more
reliable than the alternative fallback assumption. That patch still
applies and fixes your example, or improves it anyway and to an extent
directly related to the stats target size. (It also still has my bogus code
comments in which I confuse histogram with n_distinct).

Then some other people proposed more elaborate patches, and I never wrapped
my head around what they were doing differently or why the elaboration was
important.

Since you're willing to dig into the source code and since this is directly
applicable to you, maybe you would be willing to go over to pgsql-hackers
to revive, test, and review these proposals with an eye of getting them
applied in v16.

I'm not sure if there is a simple fix for this, maybe store and use
> something like n_distinct for elements for selectivity estimation ? or
> perhaps we should store something in MCE list anyway even if frequency
> is low (at least one element) ?
>

n_distinct might be the best solution, but I don't see how it could be
adapted to the general array case. If it could only work when the vast
majority or arrays had length 1, I think that would be too esoteric to be
accepted.

Cheers,

Jeff