- From: Andy Seaborne <andy.seaborne@talis.com>
- Date: Tue, 05 Jan 2010 21:45:07 +0000
- To: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
- CC: Axel Polleres <axel.polleres@deri.org>, SPARQL Working Group <public-rdf-dawg@w3.org>
On 05/01/2010 7:43 PM, Birte Glimm wrote: ... >> 4) "The term RDF-L denotes the set of all RDF Literals, RDF-B the set >> of all blank nodes in RDF graphs" >> >> Hmmm, why "in RDF graphs" and in which graphs? BTW, this question also >> applies to Query as well. > > That is more a query comment. I just repeat the query definitions as I > also state above, to remind readers. If Andy and Steve want to change > that, I happily use a changed definition. The reason is lost to me - RDF-B is only used to get to RDF-T anyway. (this used not to be true because at one time, you could have bnodes, as non-distinguished variables, in the predicate position). ... >> * Another remark which is not critical, but maybe we should re-discuss >> it at some point ... BGP extension says that entailment regimes must >> specify: >> - well-formed graphs >> - SG must be unitquely specified >> - entailment relation >> - finiteness condition for answers >> - handling of inconsistent graphs >> >> It doesn't *actually* say that it should define restrict "which qeries >> are legal", does it? I anyway don't think that the definition of BGP >> extension >> does preclude such restrictions, but it isn't actually required by the >> original definition. > > True. The closest to that is "An entailment regime specifies 1) ... 2) > an entailment relation between subsets of well-formed graphs and > well-formed graphs". and "2 -- For any basic graph pattern BGP and > pattern instance mapping P, P(BGP) is well-formed for E". I am not > sure whether I can interpret that as a possibility of defining what > legal/supported queries are. I think I once discussed that with Andy > and he suggested that all queries are legal, but some queries might > have empty answers. In particular for OWL Direct Semantics, I would > prefer to restrict not only the queried graphs but also the queries > themselves. If a query BGP cannot be parsed into ontology structures > then Direct Semantics entailment is just not defined. In that case I > would prefer to raise an error instead of giving an empty answer. > The other problem are update queries. Here we decided, I think, that > we put a note somewhere that the entailment regimes document does not > define the behaviour of systems for update queries. Once there is more > implementation experience one can then specify what implemented > systems do, which is most likely to use standard simple entailment for > update queries. I can add a note in this direction. > >> 6) This remark might be overshooting (at leat for this WD), but: >> >> "The scoping graph, SG, corresponding to any consistent active graph >> AG is uniquely specified and is E-equivalent to AG." >> [...] >> "All entailment regimes specified here use the same definition of a >> scoping graph as given in SPARQL 1.0. Thus, the required equivalence >> is immediate." >> >> I am a bit worried that *actually* the definition of the scoping graph >> as given in SPARQL 1.0 is *NOT* uniquely specified, since it obviously >> doesn't >> uniquely determine the blank nodes. Not sure whether this is really an >> issue, but it seems a bit awkward. >> >> Maybe the condition should be weakened to something like >> >> "The scoping graph, SG, corresponding to any consistent active graph >> AG is uniquely (except blank node identifiers) specified and is >> E-equivalent to AG." IIRC It's not supposed to uniquely specify it in the query spec but to give the framework - the entailment regime should specify the scoping graph in a compatible manner. >> Not really ideal either, but better than before? >> If we agree on that change, we can include that with a remark to ask >> for comments? > > That is again a comment for Query and I agree it is a valid comment. > Several of the given conditions/definitions are not ideal IMO, that > being one of them. I would also prefer to use a skolemized scoping > graph directly, but that is also not possible, so I define this kind > of work around to meet the Query conditions. We further violate > already against the condition that the scoping graph must be > consistent according to the conditions in the Query spec, which we > cannot guarantee with the current RDFS entailment regime definition. I > would prefer to be more consistent, i.e., either remove the > consistency requirement everywhere or have it throughout. I think we have more latitude with the defns here because they exist for the purposes of extension and so are less tested by the spec as published as the REC. I don't have the bandwidth to work on it before the upcoming publications. We need wider review than just the WG can currently manage - maybe we need to seek out explicit reviews. Andy
Received on Tuesday, 5 January 2010 21:45:36 UTC