Re: views of binary operations

Bob Badour wrote:
> Marshall wrote:
>
> > Consider named views of binary operations on relations.
> >
> > Given a relational operator "op" and relation variables A and B,
> > and a declaration of:
> >
> > r = A op B
> >
> > the language evaluates the expression "A op B" and assigns the
> > result to r.
> >
> > However, if we declare this as a view, we do not evaluate A op B
> > at the time of the declaration, but instead (re)evaluate A op B each
> > time we make reference to r in later expressions.
> >
> > r = view(A op B)
> >
> > Is it the case that we want exactly the above, or might we
> > want a more fine-grained control? Might we ever want
> >
> > r = A op view(B)
> >
> > In other words, the value of A at the time of the declaration and
> > value of B at the time of evaluations of r? In which case, we
> > then have four possibilities:
> >
> > r = A op B
> > r = view(A) op B
> > r = A op view(B)
> > r = view(A) op view(B)
> >
> > (Actual evaluation of the expression is deferred if either of
> > the operands is a view.)
> >
> > In other words, when we have a view of a binary relation operation,
> > are we necessarily creating a view of the entire expression, or are
> > we making views of the operands?
> >
> > (I am asking in the theoretical sense, and not about SQL per se,
> > although if current practice in SQL sheds light on the question,
> > I am interested in that as well.)
>
> I cannot make sense of what you are asking.
That's because Marshall refers to a very complex question that requires a level of abstraction impossible to be dealt with SQL perspective. I doubt even, he, realizes the complexity of such question.

> Are you confusing views and snapshots by any chance?
No, he's not confusing anything at SQL perspective level. He however confuses relvars and projection of relvars as views. Received on Sun Jul 16 2006 - 12:54:43 CEST