Re: Jan's well-defined view updates definition

"Jan Hidders" <jan.hidders_at_pandora.be> wrote in message news:Dtqab.27254$QG5.1552501_at_phobos.telenet-ops.be...
> >> Lets say I have base relations R(a,b) and S(b,c) with a foreign key R.b
> >> -> S.b and a view V that is defined by the natural join of R and S. The
> >> additions and deletions are both well-defined but if I add the tuple
> >> (a1,b1,c1) and then remove it then the end result is an additional
tuple
> >> (b1,c1) in S, which is not the same as the end result of adding 0
tuples.
> >> So for the class of updates that consists of inserts and deletes it is
> >> not commutatively updatable.
> >
> > Non commutativity is an obvious consequence of admitting approximate
> > solutions.
>
> Sure, but so what? What is so special about this property that makes you
> want to restrict severely which views users can update? I certainly can
see
> some benefits (otherwise I would not have proposed the definition) but you
> seem not even willing to contemplate the possibility of losing it.

The problem of inverting views and solving transformation equations seems to be more general than just view updates. For example, given

Q * D = V

one can be asked to find base relations D that look like V after being transfomed by Q. Here V is not a single view, but a set of views, of course. Once again, the way we treat view updates is highly unsymmetrical: we consider a single view, but many base relations. Adding symmetry and generalizing the problem is a typical mathematical approach. (Unlike computer science that jumps to definitions without trying to understand the problem;-)

There seems to be a way to consolidate our positions, though. Given a well-defined view update, if we can specify an additional view constraint equation that gives the unique solution, we both would be happy, right? In your Emp example there is such an equation (found by trial and error), but what general case is like?

Anyway, could you please outline next step(s), assuming that we accepted well-definess and commutativity? Received on Fri Sep 19 2003 - 01:24:04 CEST