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Re: [TF-ENT] Querying datasets with default plus named graphs

From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
Date: Thu, 8 Oct 2009 14:35:57 +0100
Message-ID: <492f2b0b0910080635p6b8e5a3aj460f4051fbeff1b6@mail.gmail.com>
To: Ivan Herman <ivan@w3.org>
Cc: Axel Polleres <axel.polleres@deri.org>, "Seaborne, Andy" <andy.seaborne@hp.com>, SPARQL Working Group <public-rdf-dawg@w3.org>
>>> In OWL, I can of course use owl:import in my WHERE clause (Birte, this is
>>> all right, isn't it?) which is not that bad, the user has to make things
>>> explicit. But this does not help the RDFS case.
>> In OWL you can use imports, but I suppose you mean FROM and not WHERE
>> clause. If the ontology you are querying (as given in the FROM (NAMED)
>> clause) contains imports, then all imports will be loaded and the
>> axioms from the imported ontologies will be taken into account for
>> finding the query answers.
> No, I actually meant putting an owl:import into the WHERE clause. Would that
> be possible? If I simply look at it as an RDF statement than that would be
> part of the overall graph, just as I can add an RDF triple in the WHERE
> clause...

Well, but in the WHERE clause it maily "restricts" solutions, i.e., I
will only return solutions that make the BGP true (possibly under some
entailment regime). Now, if you write
SELECT * WHERE { ?x owl:imports ?y. }
That does not force the query processor to import anything under any
entailment regime, even if I were to replace the variables with IRIs
of ontologies. Under OWL DL (if I remember correctly), your ontology
would not even entail _:x owl_imports IRI_of_some_imported_ontology
(_:x being the  blank node that represents this ontology) because as
annotations, imports do not cause such an entailment. They are rather
an imperative instruction that tell the reasoner that it should also
consider the axioms from the ontology that is imported. The axioms
will be taken into account for entailments, but neither annotations
nor imports can be queried if we require solutions to be entailed. We
have a note for the OWL entailment regime that this is the case (for
annotations) and that we might want to think about applying different
semantics for them, but so far RDF(S) kept us busy and we didn't have
the time to think about any solutions for that problem. It is on the
list though.

> I am not worried about the OWL case. More of the RDFS case: how does FROM
> NAMED and RDFS cooperate (there is no import statement...)

As I understand it, from named can be used to access graphs in the
data set of the query processor. You can do merges into a fresh
default graph. Even though this might not be nicest thing in
particular for some entailment regimes, this is something that needs
to be addressed in the SPARQL query document. The requirement might
come from entailment regimes, but entailment regimes are based on
SPARQL and if SPARQL does not define it, then we cannot use it. I
personally do not want to raise an issue and a request for that, but
if others feel like doing it...

>>> It also raises an issue on the RIF side. RIF rules cannot be expressed in
>>> RDF. How would one add RIF rules to an entailement regime if we wanted to
>>> cover RIF? It might be a showstopper for that case:-(


> And what you say is perfectly o.k. in view of the RIF specification.
> However: in SPARQL, FROM and FROM NAMED are defined  to specify RDF
> datasets. OWL and RDFS are (or can be expressed in) RDF. RIF rules cannot.
> That actually may create problems for OWL, too. There is no problem if the
> OWL ontology in the FROM clause is in RDF. But would the spec allow to refer
> too OWL ontologies in functional and/or Manchester syntax via the FROM or
> FROM NAMED clauses?

Question to the SPARQL implementors/experts. Can I specify my RDF data
in turtle and query that in accordance with the spec? If not in
accordance with the spec, do systems support turtle input?
If yes, then I cannot see, why not functional or manchester syntax.
This is obviously not normative. Any system might reject non-RDF-XML
input, but many systems might happily take it.
If not even turtle is allowed, are there any plans for doing that as
an optional syntax? If not, I guess we have to live with RDF XML. That
would probably be the end for RIF though, for OWL RDF ML is normative
and any conformant system must support it anyway, so it is not as bad
for OWL.

> I would expect we should be able to do that, but that might affect the query
> language specification.

Again, that is up to the general SPARQL/Query spec and however want to
raise an issue for that can do so.


> I remember Axel and I had some corridor chat at some point that would allow
> adding a media type to the FROM (NAMED) clause...
> Ivan
>> Birte
>>> Ivan
>>> --
>>> Ivan Herman, W3C Semantic Web Activity Lead
>>> Home: http://www.w3.org/People/Ivan/
>>> mobile: +31-641044153
>>> PGP Key: http://www.ivan-herman.net/pgpkey.html
>>> FOAF: http://www.ivan-herman.net/foaf.rdf
> --
> Ivan Herman, W3C Semantic Web Activity Lead
> Home: http://www.w3.org/People/Ivan/
> mobile: +31-641044153
> PGP Key: http://www.ivan-herman.net/pgpkey.html
> FOAF: http://www.ivan-herman.net/foaf.rdf

Dr. Birte Glimm, Room 306
Computing Laboratory
Parks Road
United Kingdom
+44 (0)1865 283529
Received on Thursday, 8 October 2009 13:36:38 UTC

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