Date: Wed, 28 Oct 2020 20:13:43 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Michael Lewis <mlewis@entrata.com>
Cc: Philip Semanchuk <philip@americanefficient.com>,
	pgsql-performance@postgresql.org
Subject: Re: Understanding bad estimate (related to FKs?)
Message-ID: <20201029011343.GO5380@telsasoft.com>
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On Mon, Oct 26, 2020 at 11:20:01AM -0600, Michael Lewis wrote:
> On Mon, Oct 26, 2020 at 11:14 AM Philip Semanchuk <philip@americanefficient.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?

I guess it shouldn't matter, since the FKs should remove all but one of the
conditions.

Maybe you saw this other thread, which I tentatively think also affects your
case (equijoin with nonjoin condition)
https://www.postgresql.org/message-id/AM6PR02MB5287A0ADD936C1FA80973E72AB190%40AM6PR02MB5287.eurprd02.prod.outlook.com