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Re: Reqirement 3.5: subgraph results

From: Steve Harris <S.W.Harris@ecs.soton.ac.uk>
Date: Wed, 5 May 2004 15:59:40 +0100
To: RDF Data Access Working Group <public-rdf-dawg@w3.org>
Message-ID: <20040505145940.GD1971@login.ecs.soton.ac.uk>

On Tue, May 04, 2004 at 01:45:33 -0700, Rob Shearer wrote:
> > "1.8 Derived Graphs
> > 
> > The working group must recognize that RDF graphs are often constructed
> > by aggregation from multiple sources and through logical 
> > inference, and
> > that sometimes the graphs are never materialized. Such graphs may be
> > arbitrarily large or infinite."
> >  -- http://www.w3.org/2003/12/swa/dawg-charter#derivedGraphs
> 
> "Logical inference" extends a *lot* farther than just appending triples
> to an RDF graph.
> There are a lot of ways to say "at least one of these two edges needs to
> exist". It can be a consequence of an OWL ontology; it can be a
> consequence of a rule encoded in a rules language; it can be a
> consequence of any semantic layer you want to put on top of the basic
> RDF data model. It does explicitly say something about what RDF graphs
> are possible and what are not. But such knowledge does not necessarily
> have any sensible encoding in RDF itself. (The best we've seen is
> changing the query to little more than "get me the answers" and then
> adding triples to the source RDF that say "this is an answer"; an
> approach which is both bizarre and quite impractical in the case of more
> than one variable which needs to be bound.)

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.

- Steve
Received on Wednesday, 5 May 2004 11:07:07 GMT

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