Re: View updates (was "is something a RDBMS")

Mikito,

good that you bring up the problems in determining the updateability of a view. That is exactly why in the thread "does a table always need a PK?" I asked Bob Badour and Lee Fesperman to show where that rule 6 of Codd is formally defined. They seem to be ignorant of these problems.

We can guess that generally determining if a view is updateable is an undecidable problem. That would make it impossible ever to make a database which satisfies Codd's rule 6.

Best regards,

Heikki

"Mikito Harakiri" <mikharakiri_at_ywho.com> kirjoitti viestissä news:Wv53b.19$84.119_at_news.oracle.com...
> "Mikito Harakiri" <mikharakiri_at_ywho.com> wrote in message
> news:dZ43b.18$84.104_at_news.oracle.com...
> >
> > D&D ad-hock updateability rules.
>
> Let's clarify why I call those "ad-hock".
>
> Given a view A transforming set of relational vars x into another set of
> relational vars y, we can write it like this:
>
> Ax=y
>
> Solving view updates involves finding another view A^(-1) ("inverting"
> operator A) so that
>
> x=A^(-1) y
>
> This notation is perfectly justified when we write classic linear algebra
> example problem in terms of relations
>
> select x1+x2 as y1, 2*x1+3*x2 as y2 from X
>
> You may notice that vector (x1,x2) is relational var x, and (y1,y2) is
> relational var y, while matrix [[1,1],[2,3]] is operator A in terms of
> notation introduced earlier.
>
> Linear Algebra has a method of inverting the above view to
>
> select 3*y1-y2 as x1, -2*y1+1 as x2 from Y
>
> It is this math method that I'm contrasting to D&D ad-hock approach.
>
> Of course, I don't have a slightest idea what the method is when we write
> equations involving relational operators. The major problem is that in
math
> we can freely apply laws to transform expressions like this
>
> u+v=w
>
> into
>
> u=w-v
>
> while we have major difficulties manipulating relational expressions. For
> example,
>
> u union v = w
>
> and we don't seem be able to apply any tranformation to it which would
move
> v to the other side of the equation.
>
> In short, given the context of the problem above it seems unlikely that
> solving equations in the relational algebra would be reduced to applying a
> set of simple rules soon.
>
>
>
Received on Wed Aug 27 2003 - 19:25:31 CEST