Re: The Practical Benefits of the Relational Model

"Peter Koch Larsen" <pkl_at_mailme.dk> wrote in message news:<3d94aae2$0$18114$edfadb0f_at_dspool01.news.tele.dk>...
> > > Another vague and very central subject is that of view
> > > updateability: hardly discussed at all.
> >
> > View updatability is clearly spelled out by Date in several places.
> > The most recent of these is the 7th edition of An Introduction to
> > Database Systems. There, update rules are spelled out for each
> > relational operator. The Dataphor DAE fully supports view
> > updateability.
> [This part was partly written before that i noticed that An Introduction to
> Database Systems was actually referenced in TTM section on view updates]
> I have read the Introduction to DBMS, but probably not the seventh edition
> (I do not have my edition present, but it was from 1990'es). There I do
> remember reading about view-updateability, but it definitely did not propose
> that ALL views were in theory updateable. The brief mention of the subject
> in TTM claims that (with a back door wrt constraint violations (pg 151)) and
> follows up with an example of an update of a union, where both underlying
> relations are updated. While this does work wrt the view, there is no
> argumentation that this should be so - you could e.g. just update one of the
> relations. I am sceptical of such an approach, especially when the choice
> seems so arbitrary and justification is not present. Anyway it is a
> disappointment on such a central point not to see references to books that
> are not by the present authors.

I agree: those are just a set of ad-hock rules hardly illuminating anything about the problem. If we allow domain operators, however, (and D4 allows them, right?) then the problem becomes more challenging. Telling the rules for relational operators is not enough anymore, we have to be able to solve equations. For example, is a view

select x+y, x-y from point

updateable? If it is, then how about

select arccos(x/(x*x+y*y)), sqrt(x*x+y*y) from point

?

Even if we don't allow domain operators, what can we infer about

select id, 'VOICE' type, voice phone

      from contact
      union
select id, 'FAX' type, fax phone
      from contact

?

Therefore, claim like "The Dataphor DAE fully supports view updateability" is just naive marketing. Received on Sat Sep 28 2002 - 01:21:01 CEST