Re: Misestimate when applying condition like id = id

Michael Lewis <mlewis(at)entrata(dot)com> writes:
> Simplified example query-
> explain
> select * from organizations o where id = id;

> On a PG10 instance I get-
> Seq Scan on organizations o (cost=0.00..4.21 ***rows=1*** width=37)
> Filter: (id = id)

> On a PG11 instance with the same or very very similar data, I get-
> Seq Scan on organizations o (cost=0.00..5.70 ***rows=270*** width=37)
> Filter: (id IS NOT NULL)

> Given this is a not null field, it translates to "is true" basically. The
> real query that this example was extracted from apparently had a typo in
> the join resulting in this bogus id = id condition, but I still curious
> about where this estimate change comes from.

It's an intentional v11 change, cf

https://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=8ec5429e2

regards, tom lane