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Re: Comments on SPARQL Query Language for RDF (21 July) (OWL inference and query)

From: Ora Lassila <ora.lassila@nokia.com>
Date: Tue, 06 Sep 2005 15:06:39 -0400
To: Dan Connolly <connolly@w3.org>
CC: <Art.Barstow@nokia.com>, <public-rdf-dawg-comments@w3.org>
Message-ID: <BF4360FF.EC91%ora.lassila@nokia.com>

Dan,

> From: ext Dan Connolly <connolly@w3.org>
> Date: Tue, 06 Sep 2005 13:44:10 -0500
> On Tue, 2005-09-06 at 11:00 -0400, Ora Lassila wrote:
>> Dan et al,
> [...]
>>> The link was not as clear as it could be in the 2005-07-21 version;
>>> In response to comments from other reviewers, we have clarified
>>> the references section. In the current editor's draft, you
>>> may want to look at...
>>>   http://www.w3.org/2001/sw/DataAccess/rq23/#RDF-MT
>> 
>> I will obviously have to read the most recent version first to see what you
>> have done. It does seem, though, that since queries are (potentially) run
>> against the deductive closure of a graph, the query language does not extend
>> easily to representations which do not have a single unique deductive
>> closure (which as far as I understand happens as soon as you have mutually
>> exclusive entailments as in OWL).
> 
> Our most recent version doesn't address the case of entailments
> that don't have a unique deductive closure.
> 
> That would be out of the scope of our charter. More below...

Understood, but if my memory serves me correctly, the charter called for
some kind of an "extension mechanism" to accommodate (some) out-of-scope
issues. Did the group decide that OWL Semantics was going to be one of those
out-of-scope items *not* covered by an extension mechanism, or is there
still a possibility to extend this for OWL? To me it is not clear that there
is such a possibility.

> 
> [skipping the transitive closure discussion for now...]
> 
>>>> 3) It does not seem possible to extend SPARQL to be
>>>> used with OWL (primarily, perhaps, because of comment
>>>> #1 above).
>>> 
>>> A number of WG members (UMD, Agfa) are succesfully using SPARQL
>>> with OWL.
>>> 
>>> By charter, OWL inference is orthogonal to query:
>>> 
>>> [[
>>> 2.1 Specification of RDF Schema/OWL semantics
>>> 
>>> The protocol will allow access to a notional RDF graph. This may in
>>> practice be the virtual graph which would follow from some form of
>>> inference from a stored graph. This does not affect the data access
>>> protocol, but may affect the description of the data access service. For
>>> example, if OWL DL semantics are supported by a service, that may be
>>> evident in the description of the service or the virtual graph which is
>>> queried, but it will not affect the protocol designed under this
>>> charter.
>>> ]]
>>> http://www.w3.org/2003/12/swa/dawg-charter#rdfs-owl-queries
>>> 
>> 
>> I guess I don't really understand how such an approach would work. I can see
>> that you could query a graph that *syntactically* represents your OWL but
>> not more. The beauty of RDF is that we have a uniquely defined deductive
>> closure, but this is not the case for OWL.
> 
> Indeed, OWL entailments that don't have a single deductive closure
> are not accommodated by the approach in the charter.

Hmm... in recent discussions with Ian Horrocks I got the impression that
basing the query language semantics on the model theory (rather than the
idea of an "expanded" graph/closure) would have allowed extensions to
languages like OWL. I am yet to fully understand the details of that myself,
but you might want to talk to him.

> 
>>  Of course, the "neats" do not
>> like this approach even with RDF, but myself, being a "scruffy", can live
>> with it. ;-)
> 
> Hmm... now it's even less clear to me what changes, if any, you're
> asking for. Help?

What I meant was that for *RDF* and the "inferential component" defined by
the RDF Semantics document, I am OK with querying against the deductive
closure, but I was guessing that there are people who would not be. As for
inventing some ad hoc inference rules to account for the simple lack of
transitive closure does, to me, seems like a cumbersome way to do things.

> 
>> I will have to take a look what exactly the UMD and Agfa folks have done.
> 
> I misspoke when I said that UMD was successfully using OWL with SPARQL.
> 
> Note clarification from Hendler:
> 
> [[
>   We have continued to support the design of SPARQL because we do a
> significant amount of work with respect to RDF triplestores and we
> very much want to do distributed triple store linking, but we do not
> consider SPARQL to be very useful for OWL as per Nokia's comments.
> ]]
>  -- 
> http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2005Sep/0009.html

OK, so it is just Agfa then?

    - Ora

> 
>>     - Ora
>> 
> -- 
> Dan Connolly, W3C http://www.w3.org/People/Connolly/
> D3C2 887B 0F92 6005 C541  0875 0F91 96DE 6E52 C29E
> 
Received on Tuesday, 6 September 2005 19:45:43 GMT

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