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> On Tue, 26 Aug 2003, Michael Kay wrote:
> > I was never a fan of hierarchical databases myself (I worked extensively
> > with Codasyl databases) but the statement that "redundancy cannot be
> > avoided" is quite wrong. I've just been re-reading the relevant chapter
> > from Tsichritzis and Lochovsky's "Data Models" (1982) which has an
> > extensive discussion of the various techniques developed by vendors and
> > users to support m:n relationships without redundancy: the most
> > comprehensive solution being "spanning trees" which allowed multiple
> > hierarchic views over the same data records. And although "foreign keys"
> > were not part of the model, they were widely used in practice at the
> > application level (just as they are in XML). The solutions seem rather
> > ad-hoc (I said I'm not a fan), but it's quite wrong to say that they
> > don't exist.
Michael, I looked up the book, especially pages 155 - 157.. Note that it
also mentions that we should note that there are two kinds of
redundancies: logical and physical.. Logical redundancies cannot be
avoided in hierarchical model.
there are ways by which we can design so that there exist logical
redundancies, but not physical storage redundancies..
cheers and regards - murali.
> going off in another track, we may think in terms of logical model as
> well as physical model.. we are thinking of
> XML/relational/hierarchical as logical models.
> (a) If we have redundancies in logical model, then users might get
> "unexpected" results, they modify something, something else also changes
> and so on..
> (b) If we have redundancies in physical model, we burden the physical
> model with consistency etc, updates might take longer etc..
> But there are cases when redundancy is useful also..
> What people who work on research can provide is ways by which we can
> remove redundancy, but whether a redundancy should be removed or not will
> depend on the application requirements...