- From: Axel Polleres <axel.polleres@deri.org>
- Date: Tue, 12 Jan 2010 14:57:10 +0000
- To: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
- Cc: "SPARQL Working Group" <public-rdf-dawg@w3.org>
On 12 Jan 2010, at 14:50, Birte Glimm wrote: > I think this might be used as another hack, but I can't believe that > this is intended and it looks like a hack. If we do that, then an > inconsistent AG implies ... My understanding is that an inconsistent AG implies nothing. It's simply up to the extension for a particular entailment regime to define what happens in case on an inconsistent AG. I don't really see what's the hack here. For this WD, I'd simply suggest to weaken the resp. Editor's note... but I have to dial in now and don't have a concrete proposal yet. ;-) Axel > ... that there is no scoping graph and, in the > absence of a scoping graph, condition 3 never applies and I can just > define whatever I want as query answer. I have to think how that can > be done properly though. The problem is that without a consistency > check, we don't know whether we are in this situation and if we are, > condition C1, which limits answers, is not usable as it is now because > it depends on SG. > This would mean that we either have to reformulate > C1 so that it is independent of SG or to have different conditions for > consistent and inconsistent graphs (without necessarily being able to > distinguish in which situation we are). > > I don't see the editorial note as wrong or too strict though because > at the moment, it is assumed that there is a scoping graph and the the > scoping graph can be inconsistent. The entailment regimes assumes that > SG is always equal to AG apart from bnode renaming. This means that if > AG is inconsistent, then SG is defined and SG is E-equivalent to AG, > but SG is not consistent as it would be required. Your suggestion is > to not have an SG at all. > > Birte > > > > > 2010/1/12 Axel Polleres <axel.polleres@deri.org>: > > > > On 12 Jan 2010, at 12:58, Birte Glimm wrote: > > > >> Axel, > >> we've spend some more thoughts on the inconsistency issue here. > >> > >> >>> > I am not sure, actually, condition 1. doesn't require consistency of SG, it only says: > >> >>> > "The scoping graph, SG, corresponding to any consistent active graph AG is uniquely specified and is E-equivalent to AG." > >> >>> > > >> >>> > So, hmmmm, *actually*, this wording actually doesn't limit at all what the scoping graph to an > >> >>> > inconsistent graph is: In fact, this even seems to let open that the SG for an inconsistent > >> >>> > graph is e.g. empty, implementation dependent, etc. > >> > >> I now discussed that with Boris (Ian is super busy at the moment) and > >> in our understanding the SG is at best undefined if AG is inconsistent > >> or, rather, there is no scoping graph in that case. > > > > Yes indeed, but that is just what I meant: there is no condition on the scoping graph if AG is inconsistent. > > To my understanding, this allows us "to do what we want in case of inconsistencies, we just have to specify it: > > This understanding doesn't follows from the condition itself, but from that sentence right before the conditions: > > > > "The effect of a query on an inconsistent graph is not covered by this specification, but must be specified by the particular > > SPARQL extension." > > > > In particular, this doesn't preclude that an entailment regime specifies an SG even > > for inconsistent AGs, or say that it is implementation-dependent: if consistency is checked by the > > implementation we raise an error, otherwise we can specify a consistent SG. This would seem to be perfectly > > inline with what we currently have and make the resp. editorial note unnecessary. > > > >> Thus, if AG is > >> inconsistent, then you could do something that does not use a scoping > >> graph, but if you do that, you violate condition 3 that basically says > >> that SG must entail the answers. > > > > Still, slight disagreement here, because I really read here that you CAN > > specify a consistent SG for such cases, as long as you say how. And even if not, again the condition is void, since it only applies to "for any scoping graph SG" (= forall), so if there is no scoping graph, again the condition doesn't apply... > > > >> Condition 3 cannot be satisfied if > > > > ... ex falso quod libet. > > > >> you have no SG or SG is undefined and you cannot have an SG because SG > >> would have to be consistent (E-equivalence). > >> > >> Birte > > > > best, > > Axel > > > > -- > Dr. Birte Glimm, Room 306 > Computing Laboratory > Parks Road > Oxford > OX1 3QD > United Kingdom > +44 (0)1865 283529 >
Received on Tuesday, 12 January 2010 14:57:44 UTC