XML as a Hierarchical DBMS (HDBMS) and Relational Theory

Hello all,

I was wondering if anyone was working on a
hierarchical database?  Or (better yet)
extensions to PostgreSQL that allowed for
hierarchical expressions.  Following is a
post I made to the XML list.

Best,

Clark Evans


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Subject: HDBMS vs RDBMS
   Date: Fri, 05 Feb 1999 03:23:41 +0000
   From: Clark Evans <clark.evans@manhattanproject.com>
     To: XML Developers' List <xml-dev@ic.ac.uk>
     CC: murata@apsdc.ksp.fujixerox.co.jp
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> > Comparing apples to apples, the DOM has elements and attributes, and a
> > Recordset has rows and columns. Most bank accounts in the world today are
> > well represented as rows and columns. There are times when the slightly more
> > complex concept of elements and attributes has a better impedance match to
> > the data being modelled than rows and columns.
>
> This is, in essence, the debate of the 70's between the hierarchical
> model (HDBMS) and the relational model (RDBMS).  The relational
> people "won", in part, beacuse they had a mathematical theory
> which formally defined how their database works.
>
> I see MURATA Makoto's work as being the mathematical formalism
> required to explain how a hierarchical database would work.  This
> to me is exciting.
>
> If anything, I would say that any *reasonable* database in the
> future must handle both and what would be wonderful to see
> is a mathematical formalism that allowed both perspectives to
> work in a complementary fashon.
>
> Already, relational databases are adding hierarchical features,
> witness the "CONNECT BY" clause in Oracle.  And, the hierarchical
> people are busily adding relational features (XML Link).
>
> I think the problem is that the data needs to be both viewed
> as a set of relations _and_ as a hierarchy.   I feel that it
> will be tempting to "toss out" relational theory in favor
> of hierarchial databases.  I think the true solution will
> involve some sort of "DUAL" which allows for a gateway
> between the "world of sets" and the "world of trees".
>
> Perhaps objects provide the language necessary to unify these
> two different pictures of information.
>
> >         Perhaps not yet, but if I want to automate transforms, XSL or the
> > transformation language subset 'XTL' is a leap in the right direction. A
> > large part of computer programming consists of interfacing one API to
> > another. I'm not saying that XSL helps with this at all but pointing out
> > that transformations and impedance matching is an important task. If we have
> > the ability to express transforms directly this greatly reduces the need to
> > do traditional coding and bit twiddling.
>
> The XTL is, in effect, the equivalent of SQL for a relational database.
>
> An SQL statement takes one or more relations and produces another
> relation.  So true that XTL will do a similar thing to trees.
>
> This is indeed very exciting.  After XTL is worked out, then
> we only have two more transformations left, RDBMS->HDBMS and
> HDBMS->RDBMS.  And neither of these is trivial.
>
> Thoughts?
>
> Clark Evans