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;
updateorders;    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