Subject: Re: multi column foreign key for implicitly unique columns
From: Markus Bertheau <twanger@bluetwanger.de>
To: Stephan Szabo <sszabo@megazone.bigpanda.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Richard Huxton <dev@archonet.com>,
	pgsql-sql@postgresql.org
In-Reply-To: <20040817080425.U61699@megazone.bigpanda.com>
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Date: Tue, 17 Aug 2004 17:12:11 +0200
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=D0=92 =D0=92=D1=82=D1=80, 17.08.2004, =D0=B2 17:06, Stephan Szabo =D0=BF=
=D0=B8=D1=88=D0=B5=D1=82:
> On Tue, 17 Aug 2004, Markus Bertheau wrote:
>=20
> > =D0=92 =D0=92=D1=82=D1=80, 17.08.2004, =D0=B2 16:46, Tom Lane =D0=BF=D0=
=B8=D1=88=D0=B5=D1=82:
> >
> > > 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?
>=20
> 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

--=20
Markus Bertheau <twanger@bluetwanger.de>