Re: more closed-world chatter

From: Marshall <marshall.spight_at_gmail.com>
Date: 6 May 2007 10:11:15 -0700
Message-ID: <1178471475.364868.156500@y5g2000hsa.googlegroups.com>

On May 6, 9:49 am, paul c <toledobythe..._at_oohay.ac> wrote:

> Not to distract my betters on this topic, but I guess I should have
> originally mentioned that I think the D&D stipulation "It is required
> that if <A,T1> is in Hr1 and <A,T2> is in Hr2, then T1 = T2" could be
> gotten around simply by ensuring that no two relvars have such an
> attribute. This would be easy for a "catalog" to enforce.

Sure, that works. But it doesn't thrill me; it means that now you might be trying to create a relvar and fail because of some other relvar that you're barely aware of.

My latest thinking is just to take T1 and T2 out of the header and put them on the constraints. That means you don't need any second-order constraints like the one you propose, and it furthermore means you can join any two relations and be sure of getting a relation. (I.e. closure.)

Marshall Received on Sun May 06 2007 - 12:11:15 CDT