Re: more closed-world chatter
From: paul c <toledobythesea_at_oohay.ac>
Date: Sun, 06 May 2007 17:21:11 GMT
Message-ID: <b8o%h.162591$DE1.122575_at_pd7urf2no>
>
>
> Sure, that works. But it doesn't thrill me; it means that now you
> might be trying to create a relvar and fail because of some
> other relvar that you're barely aware of.
>
> My latest thinking is just to take T1 and T2 out of the header
> and put them on the constraints. That means you don't need
> any second-order constraints like the one you propose, and
> it furthermore means you can join any two relations and be
> sure of getting a relation. (I.e. closure.)
Date: Sun, 06 May 2007 17:21:11 GMT
Message-ID: <b8o%h.162591$DE1.122575_at_pd7urf2no>
Marshall wrote:
> 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.)
Also, earlier, I think Jon H said something like "constraints are associated with relvars". I would rather say constraints mention relvars. Because they may mention more than one, I would rather think of them as being associated with a db.
p Received on Sun May 06 2007 - 19:21:11 CEST