Re: Possible problems with Date & McGoveran View Updating

"Jan Hidders" <jan.hidders_at_pandora.be> wrote in message news:s7n9b.21899$%C3.1400873_at_phobos.telenet-ops.be...
> > I would rather start with "invertible"
> >
> > #1 Transformation Q is locally invertible for database instance X and
set
> > #of
> > views Y if there exists Q^(-1) such that:
> >
> > Q * X = Y <=> X = Y * Q^(-1)
>
> I assume you meant "Q * X = Y <=> X = Q^(-1) * Y" ?
>
> That's a trivial property: every transformation is locally invertible in
> that sense. Note that because of the way you formulated it I only have to
> show that given an X an Y there is a Q^-1. An example would the function
> that maps every relation to X.

How would you specify transformation that maps every relation to X? The transformation formula must be instance independent.

When we write view equations, transformation formula contains relation variables only. It can't contain tuples specific to database instance, or tuples in a view(s).

Moreover, Q is allowed to contain base relation variables only. Likewise, Q^(-1) is allowed to contain view variables only.

Please disregard the nested variables nonsence comment in my previous message. Received on Mon Sep 15 2003 - 21:44:05 CEST