Re: SELECT MAX(c) FROM (SELECT ... FOR UPDATE) AS foo

On Tue, 17 Aug 2004, Markus Bertheau wrote:

> , 17.08.2004, 16:12, Bruno Wolff III :
> > > SELECT MAX(position) FROM (SELECT position FROM classes WHERE name =
> > > 'foo' FOR UPDATE OF classes) AS foo
> > >
> > > It's clear which rows should be locked here, I think.
> >
> > Even if it was allowed, it probably wouldn't be good enough because it won't
> > protect against newly inserted records.
>
> Can you detail an example where this wouldn't be good enough?

Another transaction can come along and insert a row with name='foo' into
classes with a higher position value after you've done the above but
before you commit.

T1: begin;
T2: begin;
T1: select max(position) from (select position from classes where
name='foo' for update of classes) as foo;
-- say this gets 5
T2: insert into classes (name, position) values ('foo', 10);
-- This wouldn't be blocked by the for update lock.
T2: commit;
-- now if you were to do the T1 select above, you'd get a different
-- answer in read committed.

If we had predicate locking, I think you could probably manage these cases
in serializable mode, but for now I'm not sure anything less than a table
lock would do.