Re: multi column foreign key for implicitly unique columns

В Втр, 17.08.2004, в 17:06, Stephan Szabo пишет:
> On Tue, 17 Aug 2004, Markus Bertheau wrote:
>
> > В Втр, 17.08.2004, в 16:46, Tom Lane пишет:
> >
> > > I think one reason for this is that otherwise it's not clear which
> > > unique constraint the FK constraint depends on.  Consider
> > >
> > >     create table a (f1 int unique, f2 int unique);
> > >
> > >     create table b (f1 int, f2 int,
> > >             foreign key (f1,f2) references a(f1,f2));
> > >
> > > How would you decide which constraint to make the FK depend on?
> >
> > Either way, the semantics are the same, right?
>
> Unfortunately, not in the case of dropping the chosen constraint.

Can't you choose at fk check time rather than fk creation time?

> Theoretically in that case, you'd probably have to extend the spec there
> as well to say that you check any dependent objects again to see if they
> would still be valid rather than dropping them (on cascade) or erroring
> (on restrict).

That also makes sense and is more efficient as I see it.

Thanks

--
Markus Bertheau <twanger@bluetwanger.de>