Misestimate when applying condition like id = id

[Michael Lewis <mlewis@xxxxxxxxxxx>]

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.

Michael Lewis  |  Database Engineer

Entrata