Re: The Practical Benefits of the Relational Model

Paul Vernon <paul.vernon_at_ukk.ibmm.comm> wrote in message news:<ank09n$1606$1_at_sp15at20.hursley.ibm.com>...
> Does setting the view V to it's current value fail?
>
> I.e. does
>
> SET V := V
>
> fail?
>
> And if so, do you think that that is justifiable, and/or desirable?
>
> Also do all possible DELETES and UPDATES also fail on the XOR view in
> question? If not, would you think that such incongruity is justifiable?
>

Yes, it would be mapped to a delete V, followed by an insert of the previous contents of V into V, which would fail. All possible updates on this particular view would also fail, but deletes in general would succeed.

I believe this behavior is not only justifiable, but desirable. As for justifiable, it is easy to manufacture a view using other operators which has the same incongruity with regard to insert and delete. For example, inserts into a view defined with projection will in general fail, but the deletes, and even some updates, will succeed. As for desirable, we make the case that the view updatability mechanism must present a uniform view :) of table variables, base or derived in order to preserve logical data independence. If we lose this, then the client applications must make up for it. The view updatability mechanism provided by Date and others has given us this independence. As soon as we start talking about views which are updatable, versus views which are not, the client applications have to get involved and we're back to writing update logic outside the DBMS.

Regards,
Bryn Rhodes
Alphora Received on Mon Oct 07 2002 - 21:51:41 CEST