From: | Sophia Wright <sjw9010(at)gmail(dot)com> |
---|---|
To: | pgsql-general(at)postgresql(dot)org |
Subject: | Strange deadlock in foreign key check |
Date: | 2015-08-06 14:19:07 |
Message-ID: | CAJTwwh1LtEPVhSzzLr3c78hhGN4_e7p5owPCUQHc0emPTU1Hmw@mail.gmail.com |
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Lists: | pgsql-general |
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?
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