Pavel Stehule: How to fix Hibernate bug by conditional index

Yesterday I found significant grow of seq read tuples. After some investigation I found query with strange predicate:

WHERE 1 = case when pb1_0.parent_id is not null then 0 end

It is really strange, and I had to ask, who wrote it.

The reply is - Hibernate. It is a transformation of predicate parent_id = (?) when the list of id is empty.

Unfortunately, PostgreSQL is not able to detect so this predicate is always false, and then the repeated execution ended in repeated full scans.

Fortunately, Postgres has simple workaround - conditional index

CREATE INDEX ON TABLE pb(id)
  WHERE 1 = case when pb1_0.parent_id is not null then 0 end

This index is always empty, and then index scan is fast.

This issue should be fixed in more recent versions of Hibernate where predicate 1=0 is generated instead.