Re: Possible problems with Date & McGoveran View Updating

"Bob Badour" <bbadour_at_golden.net> wrote in message news:Ia07b.686$wf7.65737782_at_mantis.golden.net...
> "Alfredo Novoa" <alfredo_at_ncs.es> wrote in message
> news:e4330f45.0309080357.3a627b52_at_posting.google.com...
> > Of course, but the DBMS does not have enough information in order to
> > decide which is the true solution.
>
> If it inserts the tuple into only one relvar, it must decide which. If it
> inserts the tuple into both relvars, it eliminates an arbitrary decision
> while preserving important mathematical properties.

In language of view equations that translates into an equation

Ax=b

having more than one solution.

> > Yes, it is. If a problem is undecidable it is better to say "the
> > problem is undecidable" rather than inventing a possible false result
> > without any warning.
>
> It is not undecidable. The only way it can create a false result without
> warning is if the designer omitted an important integrity constraint. If
the
> designer completes his job, the problem goes away.
>
> See principle #4 at: http://www.dbdebunk.com/page/page/622150.htm

There is a clear method of making equation solution unique: add more equations!

> > It is even clearer with project views;
> >
> > var x real relvar { a Integer, b Integer} key { a, b };
> > var vx virtual x { b };
> >
> > insert into vx relation { tuple { b 1 } };
> >
> > We can deduce infinite possible solutions. Which solution should we
> > choose? All?

Given

table X (

    a integer,
    b integer
)

the view

view A as
select a from X

is not updateable, since the equation

Ax=y

has nonunique solution no matter what value relation y has. If we throw in more equations, for example

view A1 as
select b from X

then, the view that combines A and A1 together is updateable.

Constraints are views that evaluate to constants, they could be added to disambiguate the original view the same way as the "normal" views.

All these ideas are parallel to equations solving in algebra. Given 2 variables x1 and x2, the equation

x1 = y1

doesn't have unique solution since x2 can be chosen arbitrarily. However, if we add

x2 = y2

then our system of equation has unique solution (no matter what values variables y1 and y2 have). Alternatively, we can add a constraint

x2 = 0

which disambiguates our system of equations as well. Received on Mon Sep 08 2003 - 20:50:02 CEST