| From: | Michael Lewis <mlewis(at)entrata(dot)com> |
|---|---|
| To: | Philip Semanchuk <philip(at)americanefficient(dot)com> |
| Cc: | Justin Pryzby <pryzby(at)telsasoft(dot)com>, postgres performance list <pgsql-performance(at)postgresql(dot)org> |
| Subject: | Re: Understanding bad estimate (related to FKs?) |
| Date: | 2020-10-26 17:20:01 |
| Message-ID: | CAHOFxGpcN+=Vo10=2roxOOm8UXu8zstdaggRq0vvu00wcC6YjQ@mail.gmail.com |
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| Lists: | pgsql-performance |
On Mon, Oct 26, 2020 at 11:14 AM Philip Semanchuk <
philip(at)americanefficient(dot)com> wrote:
> >> The item I'm focused on is node 23. The estimate is for 7 rows, actual
> is 896 (multiplied by 1062 loops). I'm confused about two things in this
> node.
> >>
> >> The first is Postgres' estimate. The condition for this index scan
> contains three expressions --
> >>
> >> (five_uniform = zulu_five.five_uniform) AND
> >> (whiskey_mike = juliet_india.whiskey_mike) AND
> >> (bravo = 'mike'::text)
>
Are the columns correlated? Have you tried to create extended statistics
and see if the estimate changes? I believe that extended stats will not
directly help with joins though, only group bys and perhaps choosing an
index scan vs table scan when comparing the correlated columns to static
values rather than joining up tables. Wouldn't be much effort to try it
though.
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