- From: Axel Polleres <axel.polleres@deri.org>
- Date: Wed, 6 Jan 2010 14:18:45 +0000
- To: "Birte Glimm" <birte.glimm@comlab.ox.ac.uk>
- Cc: "Andy Seaborne" <andy.seaborne@talis.com>, "SPARQL Working Group" <public-rdf-dawg@w3.org>
On 6 Jan 2010, at 13:59, Birte Glimm wrote: > 2010/1/5 Andy Seaborne <andy.seaborne@talis.com>: > > > > 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). > > To me it seems to be unnecessary, but not really hurting on the other > hand. Maybe it has historical reasons as Andy suggested. > > > ... > > > >>> * 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. > > I agree that the Query spec says an extension to BGP matching has to > uniquely specify the SG. That seems to me a bit over the top or maybe > I am interpreting that too strictly? You have to pick one of a number > of equivalent scoping graphs. The ones you can choose from are the > ones that differ in bnode names and the only requirement is that > bnodes are suitably named apart, i.e., the query and the SG should not > share any bnodes. I would just leave it as that unless there are > complaints. Well, this actually *is* sort of a complaint. I admittedly would feel safer, if the condition was changed to something like the suggested: "The scoping graph, SG, corresponding to any consistent active graph AG is uniquely (except blank node identifiers) specified and is E-equivalent to AG." or "The scoping graph, SG, corresponding to any consistent active graph AG is uniquely (modulo equivalence) specified and is E-equivalent to AG." Would that be acceptable? > It would be hard to specify it so that it is more unique, > not knowing the given inputs. The result would not differ either way. yes, but leaving it as it stands now requires uniqueness, which is in my opinion too strict. > > >>> 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. > > What I could do is to put in an editorial note into the entailment > regimes doc that mentions that the SG is only unique up to bnode names > and that the SG is not necessarily consistent (well that's already in > an ed note) explicitly asking for feedback on whether that is seen as > problematic given the requirements from the Query spec. Is that a good > idea? Sure, an ed note would make me happy for now, if we can't reach agreement on that suggested change. best, Axel > Birte > > > Andy > > > > > > -- > Dr. Birte Glimm, Room 306 > Computing Laboratory > Parks Road > Oxford > OX1 3QD > United Kingdom > +44 (0)1865 283529 >
Received on Wednesday, 6 January 2010 14:19:19 UTC