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RE: Reqirement 3.5: subgraph results

From: Seaborne, Andy <andy.seaborne@hp.com>
Date: Fri, 7 May 2004 09:58:01 +0100
Message-ID: <E864E95CB35C1C46B72FEA0626A2E808031A9043@0-mail-br1.hpl.hp.com>
To: Howard Katz <howardk@fatdog.com>, Steve Harris <S.W.Harris@ecs.soton.ac.uk>, RDF Data Access Working Group <public-rdf-dawg@w3.org>



-------- Original Message --------
> From: public-rdf-dawg-request@w3.org <>
> Date: 6 May 2004 19:06
> 
> > -----Original Message-----
> > From: public-rdf-dawg-request@w3.org
> > [mailto:public-rdf-dawg-request@w3.org]On Behalf Of Steve Harris
> > Sent: Wednesday, May 05, 2004 8:00 AM
> > To: RDF Data Access Working Group
> > Subject: Re: Reqirement 3.5: subgraph results
> 
>     [ snip ... ]
> 
> > This is similar to my concern:
> > 
> > If a query includes some extension function (after 3.3), say a
> > function that takes a radius and the URIs for two geo-spatial
> > co-ordinate nodes and returns TRUE if one is in the radius of the
> > other. The complete graph used to answer that query is not
> > neccesarily known to the query engine - especailly if the function is
> > implemented at a lower level. Asking extension functions (for
> > example) to give the triples that it used to answer the question
> > seems unneccesarily onerous. 

If external functions are only filters, saying "accept" or "reject" (or
"undefined" probably), based on some bindings of the variables then there
does nto appear to be a problem.  External functions should not be able to
bind variables.  The current solution of the graph pattern does define a
part of the graph so it could be used to define a solution graph.

If the external function returns a solution as a graph it gets unecessarily
complicated.

> 
> That's really my main question to the group: are we designing
> a language in
> which specific and explicit knowledge of graph structure has
> to be embedded
> in the query?

Going off the subject topic:

On the wider point here, I think there are two issues here: locating parts
of the graph of interest and extracting information from the graph aroudn
those parts.

Explicit graph structure is used to locate exactly which nodes are of
interest.  Having located the nodes of interest, there is an extraction
process that gets data out of the graph - the results (there is than the
issue of presenting the results).

If the query can only specify the exact shape of the results as in variables
bound in graph patterns then the server can't return all it knows (the
motorcycle parts UC), only what the query has scoped.

> It seems to me that we are or should be, but
> it's not clear to
> me that's been decided one way or another by the group. Has
> this question
> been answered to other people's satisfaction, or are we still
> in a bit of
> murky territory here?
> 
> I don't see a contradiction in allowing external functions to
> do things
> under the hood and out of sight btw: they provide a needed
> escape mechanism
> for letting clever implementers get around the limits of having to be
> triple-wise explicit in the query. In me 'umble at any rate ...
> 
> Howard

	Andy
Received on Friday, 7 May 2004 04:58:40 GMT

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