Re: a SELECT FOR UPDATE question

On Tue, Feb 08, 2005 at 12:58:34AM -0500, Tom Lane wrote:
> Michael Fuhr <mike(at)fuhr(dot)org> writes:
> >
> > I think you can infer that a process is working with a row in some
> > manner (UPDATE, SELECT FOR UPDATE) by looking at the row's xmax
> > column and checking pg_locks to see if any process is holding a
> > lock on the indicated transaction ID.
>
> I'm too tired to work out an example, but I think this probably doesn't
> work in general: the xmax on the version of the row you can see might
> not correspond to a live transaction, but that doesn't mean someone
> else doesn't hold a lock on the latest committed version of the row.

Okay...I had done some tests with UPDATE and SELECT FOR UPDATE and
observed what I wrote, but admittedly that covers only a few cases.
If you could point me in the right direction I'll try to work out
an example where my suggestion fails.

--
Michael Fuhr
http://www.fuhr.org/~mfuhr/