Re: Deadlock bug

> It *is* allowed to, and in fact has already done so. The problem is
> that it now needs a sharelock on the referenced row in order to ensure
> that the FK constraint remains satisfied, ie, nobody deletes the
> referenced row before we commit the update. In the general case where
> the referencing row is new (or has a new FK value) in the current
> transaction, such a lock is necessary for correctness. Your case would
> work if we could optimize away the FK check, but with only a limited
> view of what's happened in the current transaction, it's not always
> possible to optimize away the check.

Hmmm. It seems to me that we'd need a sharelock on the referenced row
both times. Is the below sequence missing something?

process 1 process 1 locks process 2 process 2 locks

update session; exclusive lock
session row;
update orders; exclusive lock
orders row;
share lock
session row;
update orders; exclusive lock
requested orders row
(blocks);
share lock session row;
update orders; exclusive lock
orders row;
share lock
session row;

(in this example, there is an fk orders.sessionid --> session.id )

It certainly seems that process 2 is acquiring exactly the same locks
twice, since the referenced value is never being changed. So why does
it need a share lock the 2nd time and not the first? Or is the
sharelock in the first cycle being optimized away improperly?

--
-- Josh Berkus
PostgreSQL Experts Inc.
http://www.pgexperts.com