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   Re: [xml-dev] XML: logical and/or physical model?

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Michael Champion wrote:
> [snip]
> [1] Ken North mentioned D.L. Childs STDS work that influenced the  
> relational model.  Childs is a neighbor of mine, and has a rather  
> interesting metaphor for this:  The logical model is up in the world of  
> sunshine and light where the Eloi dwell with little concern for ugly  
> realities; the physical model is the one that lives down in the land of  
> the Morlocks who do the dirty work.  It's nice to be oblivious to  
> physical reality, but this sometimes leads to a really unpleasant  
> realization when you find out where you really sit in the food chain  :-)
> BTW I have a bunch of Childs recent stuff archived at  
> http://xsp.xegesis.org if anyone is interested in seeing where the STDS  
> thinking has gone in the 35-or so years since Codd cited it; I am  
> intrigued because he proposes a way to formally unite the relational  
> model and XML's implicit data model using an extended set theory that  
> makes order and hierarchy first class citizens.  In STDS (which I used  
> via an early RDBMS called Micro at the University of Michigan back when  
> dinosaurs roamed the earth), there is a formal relationship between the  
> logical and physical models, which allows query optimization to be  
> driven down almost to the hardware level.

Childs' paper "XST Technology: Theory & Practice" 
(http://xsp.xegesis.org/Xsp-tap.pdf) says that "When XML tags are 
equated to XST scopes, XML-structures become well-defined extended 
sets", but it doesn't seem to be so, or at least such an extended set 
does not preserve the semantics of XML. Order is significant in XML, 
independent of tag names. For example,


Expressed as an XST set scoped by tag name would be:

S = { { Hurrah[b], Hurrah[b] }[p] }

where [] correspond to superscripts in the paper's notation. But, as a 
set, this is equivalent to:

S = { { Hurrah[b] }[p] }

To correct this, one would have to use numbered scopes, where tag names 
could, in some cases (and not in others), serve as an alias to the 
scope. The rub is that there is no way to know in which cases tag names 
can be used as scopes, outside the XML-writer's head. The widely-used 
XML modeling languages (DTDs or schemas) do not describe "semantic" 
(application-level) order-significance, nor can it be inferred from what 
they do describe.

Bob Foster


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