Re: Understanding bad estimate (related to FKs?)

> On Oct 26, 2020, at 1:20 PM, Michael Lewis <mlewis(at)entrata(dot)com> wrote:
>
> 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.

There’s not a lot of correlation between whiskey_mike and bravo --
stxkind stxndistinct stxdependencies
['d', 'f'] {"7, 12": 42} {"12 => 7": 0.000274}

Those stats didn’t help the planner.

I should have mentioned that five_uniform has ~63k unique values, whereas whiskey_mike has only 3, and bravo only 19.

Cheers
Philip