XML and the Relational Model (was Re: A standard approach to g
--- lbradshaw@d... wrote: > I am saying I have not seen a proof that XML > supports, or can be engineered > to support, the Relational Model. I'm not sure what you mean by that. Clearly any XML schema can be normalized into relations (didn't Codd prove something to that effect?). I'm less sure about this, but my reading of C. J. Date's various discussions of the relationship between the RM and the OO suggest that one could condider a particular XML schema a "type" and -- so long as a useful set of operators were defined on the type -- there's no difference between the RM's support (or lack of support) for types such as dates/times and types such as "invoices" or whatever. I believe there's a fair amount of work underway to reconcile the underpinnings of the relational model and XML's tree model. See for example http://citeseer.nj.nec.com/jagadish01tax.html and http://www.eecs.umich.edu/db/timber/files/physical.pdf Also, FWIW, I made the assertion in a townhall on XML and databases at XML 2002 that within 5-10 years the XML model and the RM would be theoretically reconciled, and some notable luminaries in the field were nodding their heads :-) > 2) Data based applications developed using the > Relational Model, which are > well engineered and designed, will feature lower > cost over time with > greater flexibility. > 4) Rigorous, scientific proofs exist, and are easily > found, for adherence > to the Relational Model (RM). > 5) I have seen nothing better than RM for improving > software application > reliability, flexibility, maintainability and > lowering software system > costs overall. If something better exists, as a > methodology with scientific > proofs, I would dearly like to see it. Well, I'll just ask the question that Simon and I have posed to Fabian Pascal a few times, resulting in a storm of invective but no real answer: If the RM is the answer to all data management problems, why do people keep reinventing and effectively using variants of the hierarchical model (such as OODBMS and XML DBMS and the numerous hybrid solutions that the dominant SQL DBMS vendors offer)? I don't buy the "the industry is too stupid and fad-obsessed to do the Right Thing" argument. There are an awful lot of smart people and very deep R&D pockets at Microsoft, Oracle, and IBM; if implementing and exploiting the RM "properly" could give one a competitive advantage over the others, they would have done this already. Instead, they've all turned to XML or hybrid SQL-XML-OO approaches. It would appear that all the "scientific proofs" in the world don't help one if the techniques they espouse can't be implemented and used by real humans. We don't write real software using Turing's mathematics, although it's important to have that as a theoretical foundation. I suspect that we'll eventually conclude something similar about Codd's -- it's wonderful theoretical foundation for databases, but not a practical tool for ordinary mortals ... and probably never will be for a large subset of real-world problems, especially those where the "types" are complex, poorly understood, rapidly evolving, etc.
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