Re: Possible problems with Date & McGoveran View Updating

"Jan Hidders" <jan.hidders_at_pandora.be> wrote in message news:5SJ7b.12871$Nh6.347237_at_phobos.telenet-ops.be...
> Lets assume
> that we have a view V that is defined by a query Q that maps a database to
> a relation.

Minor comment. There could be 2 (or more) queries Q1 and Q2 mapping into relations R1 and R2, correspondingly. You can still accomodate this case in your defintion if you assume Q = CartesianProduct(Q1, Q2) and R=R1 x R2.

> Given a database D an addition of a set of tupels A to the view V is
called
> *well-defined* if there is a unique smallest superinstance D' of D such
> that D' satisifies the schema and Q(D') = Q(D) + A. Analogously we define
a
> deletion of a set of tupels as *well-defined* but with "superinstance"
> replaced with "subinstance" and set union replaced with set difference.
> Relational assignments can be simulated by a deletion followed by an
> addition.

Terminolody and defintion amended:

Q( D + deltaD ) = V + deltaV

where "+" is union (or difference;-)

The Q is called invertible if for any deltaV there exists unique deltaD satisfying the formula above.

Question. Is the definition equivalent to

exists Q^(-1) such that Q(D) == Q * D = V <=> D = Q^(-1) * V

and therefore switching term "well-defined" to "invertible" is justifiable?

> Finally, we define a view V as *updatable* by a certain set of updates U
if
> for all databases D that satisfy the schema all updates in U are
> well-defined. Additionally we say that V is *commutatively updatable* by U
> if for all databases D that satisfy the schema it holds that if two series
> of updates from U have the same result when applied to Q(V) and perform
> only well-defined updates then they result in the same database when
> applied to D.

I don't understand why introducing a series of updates is necessary. If the database instance is not fixed and the view is invertible, then it must follow that the effect of composition of 2 updates in any order is the same. Examples and some motivation? Received on Wed Sep 10 2003 - 21:05:47 CEST