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Re: [xml-dev] SQL instead of XQuery [offtopic]

Len, Michael,

Your objections seem to be about collections of homogeneous documents 
being the unit of storage - but an XML database doesn't have to work 
that way. In the hotel example I could quite easily model hotel chain or 
resort information as a different type of document in a collection of 
heterogeneous documents.

To relate things to the relational model, I find it's sometimes useful 
to think of each document in a collection as a row in a table. However 
it can also be useful to think of each document as a table in a 
database. Since a single document is also a valid collection, you can 
also consider an LDAP model of using a single tree to store everything.

In short I don't think it's possible to write off collections of 
documents as a storage model so easily - they clearly offer a superset 
of the functionality in the relational model, and are therefore capable 
of naturally expressing far more complex structures.


Len Bullard wrote:
> No argument here, Michael.  A DBA has fits but they have the same fits about
> de-normalizing, and the truth is, storage is cheap now but speed is still at
> a premium.
> My company's owner made a case to me about simplicity of design.  He was
> building a utilities power system application.  He implemented algorithms
> for line length, power loss, ideal transformer loads, etc.  When he showed
> this to his engineers at the power company, they told him this was great
> stuff but that all of the calculations he was doing were standard, and what
> they really needed was a pick list to record what they do on a job.  In
> other words, a not inconsiderable number of applications are just data
> logging applications.  The revelation was that the computer is a very good
> typewriter.
> That said:  the document model works when there is a need to create or
> recreate the context instance of the data log (including the temporal
> context).  Otherwise, messages are messages.   I work with a CAP-capable
> application and will be including some new message types soon, but given all
> of the various contexts the data we log has to fit into, I wouldn't consider
> CAP a good way to organize for storage and reuse nor do I think meta-meta
> schemas such as GJXDM are a good template for table designs.  That is the
> next layer of the problem: schemas for schemas.  Here though, XQuery gets a
> compelling application: contexts wrapping contexts... ad insanitum.
> len
> From: Michael Kay [mailto:mike@saxonica.com] 
>> I think it a huge 
>> conceptual mistake to make document computing the centerpiece 
>> of database design although it is a big win for the GUI.
> It certainly can be a big mistake. I think there are two cases where it
> works well:
> (a) where the documents map well to the business objects that are the
> primary information content of the database. For example a database of
> hotels can work well when implemented as a database holding one document per
> hotel, similarly a database of medicinal drugs. You get issues about where
> to put information that doesn't belong directly with a hotel (e.g.
> information about a hotel chain or about a resort), but if you're clever you
> can present this to viewers (not updaters) as if it's just part of the hotel
> information. (One thing that seems to be lacking from most of today's XML
> databases is this concept of a persistent virtual document or view.)
> (b) where the purpose of the database is to record events and the events are
> captured by documents - for example safety inspection reports or insurance
> claims. In this situation a database of documents is exactly what you need.
> When these conditions don't hold, for example with an HR database, despite
> the fact that XML is very good at handling the flexibility of the data,
> document-centred modelling certainly has its limitations. But then so do
> other modelling techniques. I once asked a data modelling class to model a
> railway timetable - big mistake.
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John Snelson, Oracle Corporation            http://snelson.org.uk/john
Berkeley DB XML:        http://www.oracle.com/database/berkeley-db/xml
XQilla:                                  http://xqilla.sourceforge.net

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