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

From: Ivan Herman <ivan@w3.org>
Date: Fri, 9 Oct 2009 05:12:07 -0000 (UTC)
Message-ID: <4e0424de7bbc475cffceb9b523bb96af.squirrel@webmail.sophia.w3.org>
To: "Birte Glimm" <birte.glimm@comlab.ox.ac.uk>
Cc: "Ivan Herman" <ivan@w3.org>, "Axel Polleres" <axel.polleres@deri.org>, "Seaborne, Andy" <andy.seaborne@hp.com>, "SPARQL Working Group" <public-rdf-dawg@w3.org>
Hey Birte,

On Thu, October 8, 2009 1:35 pm, Birte Glimm wrote:
> [snip]
>>>> 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.

Hm. And again hm:-)

We should probably leave this open and concentrate on the RDFS case now. I must admit
that I just thought of owl:import without variables, ie, where all these issues do not
really arise. But you are right, let us leave this open.

>> 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...

I must say I am  a little bit mixed up here, maybe you can help... We discussed the
issues of restricting entailements specific graphs when those graphs are defined through
the named graph mechanism of sparql. But I am now messed up on how the FROM NAMED and
the GRAPH statements would exactly influence entailement, ie when is anything
restricted. Could you try to summarize this for a better understanding? Maybe this is
where my confusion comes from... but I am lost a bit:-(

>> 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.

Hm (again:-). Yes, you are actually right, I am not sure the spec says anything. My
impression is that the spec is silent at that point and a URI to a graph amy refer to
any format that the processor understands. If that is so, we may not have a problem with
OWL if the processor understands non RDF/XML formats. Maybe it is worth to add this to a
possible service descriptions, though.

But it is certainly a problem with RIF. Indeed, turtle may not be a standard format but
it is an RDF serialization syntax. In this sense, both the OWL 2 functional syntax and
the M'ter syntax can be considered as an RDF serialization syntax, because they can be
converted, in a standard way, to RDF. But an RIF rule set _cannot_:-(



>> 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.
> Birte
>> 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
> Oxford
> OX1 3QD
> United Kingdom
> +44 (0)1865 283529

Ivan Herman, W3C Semantic Web Activity Lead
URL: http://www.w3.org/People/Ivan/
FOAF: http://www.ivan-herman.net/foaf.rdf
Received on Friday, 9 October 2009 05:12:37 UTC

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