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RE: Reqirement 3.5: subgraph results

From: Howard Katz <howardk@fatdog.com>
Date: Fri, 7 May 2004 06:21:18 -0700
To: "Seaborne, Andy" <andy.seaborne@hp.com>, "Steve Harris" <S.W.Harris@ecs.soton.ac.uk>, "RDF Data Access Working Group" <public-rdf-dawg@w3.org>
Message-ID: <IKEOLCDFPBBPPAHGNKKOEEEEELAA.howardk@fatdog.com>

> From: public-rdf-dawg-request@w3.org On Behalf Of Seaborne, Andy
> Sent: Friday, May 07, 2004 1:58 AM

     [ snip ... ]

> > > 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.

Why not? XQuery (among other languages; SQL comes to mind) allows this, and
it seems to be a useful thing. The environment for example can pass in
useful, named system constants. Here's a way of introducing the current
date-time into the query. Here's a way of introducing the name of the
current user. Since a variable can be bound to a sequence of arbitrary,
heterogeneous items in XQuery, it can even be used in that environment to
pass in a system-bound sequence of nodes and atomics of particular
significance to seed the data-model instance. The RDF equivalent would be to
pass in a pre-bound graph of triples. I'm sure smarter minds than me can
find numerous interesting uses for this type of capability.

>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.

In what way? Can you explain why you say this, Andy?

> > 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 09:19:54 GMT

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