Re: Possible problems with Date & McGoveran View Updating

Mikito Harakiri wrote:
>
> In math they spent much more time researching what the conditions for
> equation roots existence & uniqueness are, as opposed to developing
> methods for solving them. Date's view update theory doesn't bother with
> analysis, and jumps to the methods right away. That is a little bit
> suspicious.

An excellent point. So let us set the stage to do some analysis. Lets assume that we have a view V that is defined by a query Q that maps a database to a relation.

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.

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.

Good, now we have all that in place we can start to investigate which views in which schemas by which sets of updates are updatable and which are reversibly updatable.

Mikito, the stage is yours. :-)

• Jan Hidders