Re: On specialization constraints time of application

"Cimode" <cimode_at_hotmail.com> wrote in message news:bba2f111-0023-499a-8e0b-97c39d45336f_at_h11g2000yqb.googlegroups.com...
> Lately, I stumbled about the problem of how constraints are to be
> implemented in the case of relations runtime-only (I call them virtual
> - they can *roughly* be assimilated to functions/views)
> representations. I thought that it is necessity for a TRDBMS to allow
> the implementation of constraints over the domains of attributes that
> are part of these relations. As a consequence, the question came out
> about the modalities of constraint implementations in declarative
> mindset:
>

I'm not sure exactly what you're driving at here. Are you concerned with implementing constraints defined on relational expressions, such as a unique constraint defined on a view?

> > Since a TRDBMS should allow constraint specialization in a declarative
> > way, how
> could a TRDBMS allow constraint declaration of virtual relations in a way
> that does
> not require a representation of the virtual relation in order to validate
> that its
> constraints are verified.

Are you seeking a deterministic solution for mapping inserts, updates and deletes that target a relation derived from a set of base relations to those base relations? I don't think there is one.

> > Are virtual relations to be considered a part of the database relation.

I don't think so. Despite the good intentions of Date and Darwen, among others, in advocating for the Principle of Interchangability, it still has huge unresolved issues that relegate it to being just wishful thinking. For example, how do you map an assignment to a relvar that is defined as a union or a join to assignments to the base relvars in the union or the join?

> > So far, I have by default taken the approach of considering attributes
> > of virtual
> relations as being subjected to ANY constraint that carries on some
> attribute that
> allows these attributes to be built. I wander about the risks and
> opportunities of
> such approach and whether some work has been done on that subject.

I'm not sure what it is you're trying to say here. If you're saying that the contstraints defined on the base relation schemata impact the set of possible derived relations (values), then I would agree, since the predicate of the virtual relation schema includes in its definition the predicates of the base relations that appear in the definition of the virtual relation schema.

> I would to have the opinion of some people here and any material that
> dealt with the specific subject at hand.
>
> Thank you/
Received on Sat Jun 13 2009 - 17:03:32 CEST