Re: Possible problems with Date & McGoveran View Updating

"Bob Badour" <bbadour_at_golden.net> wrote in message news:Vd97b.717$zB.69216404_at_mantis.golden.net...
> "Mikito Harakiri" <mikharakiri_at_ywho.com> wrote in message
> news:Uj47b.22$741.237_at_news.oracle.com...
> >
> > In language of view equations that translates into an equation
> >
> > Ax=b
> >
> > having more than one solution.
>
> I am not certain what you mean. Are you saying that in the case of
ignoring
> symmetry and arbitrarily deciding to insert to only one base relvar there
is
> more than one solution?

No, your update policy allows one solution only.

Union view update has 3 solutions: insert into the first relation, insert into the second, and insert into the both. You just arbitrarily chosen one of them. Your choice formally is just [arbitrary] constraint that you add to the view.

> Or are you comparing the entire situation to an
> equation with multiple roots?

Yes. Suppose we have a system of equations

x1+x2 = y1
x1+x3 = y2
2*x1+x2+x3 = y3

and being asked to find x1, x2, and x3 given y1, y2, and y3. It is easy to see that if y1+y2!=y3, then there is no solutions, while if y1+y2=y3, then there is a continum of solutions. This continuum of solutions would be disambiguated if we add one more independent linear equation.

> > There is a clear method of making equation solution unique: add more
> > equations!
>
> I assume you are agreeing with my suggestion to declare the constraints.

Constraint, or additional view. For example, we certainly have not enough information to disambiguate union view alone, but can disambiguate union view if also know (symmetric) difference.

select x1, x2 from (
  select x as x1 from A
  union
  select x as x1 from B
), (
  select x as x2 from A
  minus
  select x as x2 from B
  union
  select x as x2 from B
  minus
  select x as x2 from A
)

If the value (1,1) is inserted, then we certainly aren't allowed to insert it into both A and B. Otherwise the second operand of the CP would be empty.

The ambiguity between inserting into A and inserting into B is still unresolved. We eliminated one possibility out of 3 only, but I hope that the idea is clear.

> > 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.
>
> Suppose we extend the syntax for view definition to allow systematic
default
> values:
>
> view A as
> select a from X
> default X set b = 0 on insert to A

Not general enough.

> > 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.
>
> The constraint method basically restates POOD. Or alternatively, POOD
states
> a good design has sufficient constraints declared to disambiguate the
> systems of equations.
>
> The default value method constrains specific operations without
constraining
> the base relvars.

Constraints are one possibility. Extending views (aka adding more equations) like in the "union" example above is more general approach. Received on Tue Sep 09 2003 - 02:01:08 CEST