I am seeing some odd locking behaviour when deleting a parent record (Postgres 9.4.4).
Setup: create table pk_rel (id int primary key); create table fk_rel (pk_id int references pk_rel (id), x text unique); insert into pk_rel values (1), (2); insert into fk_rel values (1, 'a');
This example works as I expected.
Session 1: =>begin; =>update fk_rel set pk_id = 2;
Session 2: =>delete from pk_rel where id = 1;
[Fails with FK violation]
But the following case, I do not understand.
Session 1: =>begin; =>update fk_rel set x = 'b';
Session 2: =>delete from pk_rel where id = 1;
[Blocks waiting for Session 1]
Session 1: =>insert into fk_rel values (1, 'a');
[Blocks waiting for Session 2]
At this point, Session 1 fails with a deadlock, and Session fails with a FK violation.
So, why is this happening? Why doesn't Session 2 fail the FK check immediately, like in the first case? And why is it that updating fk_rel.x introduces a lock conflict, but updating fk_rel.pk_id does not?