Re: Foreign key reference counting strategy?

On Sat, Oct 14, 2006 at 07:58:06PM -0400, Tom Lane wrote:
> Michael Fuhr <mike(at)fuhr(dot)org> writes:
> > On Sat, Oct 14, 2006 at 03:52:04PM +0200, Markus Schaber wrote:
> >> Create an "after delete" trigger on the referencing table that checks
> >> whether there still are records with the same key (IF EXISTS()), and
> >> deletes the referenced row otherwise.
>
> > In a concurrent environment that delete can fail with a foreign key
> > constraint violation because IF EXISTS won't see uncommitted changes
> > in other transactions.
>
> No, I don't think so, because the DELETE will already be holding
> exclusive lock on the doomed PK row, which any would-be inserters of
> matching FK rows will be blocked on. AFAICS the DELETE should go
> through and then the inserters will fail.

Unless the inserters got there first. I just tested both ways; if
the insert acquires the lock first then the delete fails, but if the
delete acquires the lock first then the insert fails.

--
Michael Fuhr