- From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
- Date: Mon, 8 Feb 2010 13:50:01 +0000
- To: Axel Polleres <axel.polleres@deri.org>, Andy Seaborne <andy.seaborne@talis.com>
- Cc: SPARQL Working Group <public-rdf-dawg@w3.org>
> 1) consistency requirement for entailment regimes > > Issue text: For efficient implementations, it might be undesirable to enforce > consistency checking, e.g. for RDFS. So, in order to follow: > "The effect of a query on an inconsistent graph is not covered by this specification, > but must be specified by the particular SPARQL extension." > My (Axel- chairthatoff) interpretation is that I don't see that this implies that an > extension has to uniquely define the behavior on > inconsistent graphs, actually it could leave several options open (e.g. for implementations that > do or don't perform consistency checking.) The problem with the current definition is that it does not guarantee finite answers in the case of inconsistencies. This is because SG is defined in any case as equivalent to AG, which results in SG being inconsistent. Now answers are those that are entailed by SG and satisfy the extra conditions, which limit infinite answers from blank nodes and axiomatic triples. An inconsistent graph, however, entails everything and things such as: ex:a ex:b "<a1>"^^rdf:XMLLiteral . ex:a ex:b "<a2>"^^rdf:XMLLiteral . ... ex:a ex:b "<an>"^^rdf:XMLLiteral are all valid answers not filtered out by the current conditions. The conditions were defined with the handling of inconsistent graphs by current systems in mind. Current system behaviour can be captured by saying that in case of an inconsistent AG, SG is defined as a "repaired" AG where malformed XML literals are replaced by arbitrary wellformed ones. With that definition of SG finite answers are guaranteed and systems can keep doing what they do. Another way, which I am not sure whether that satisfies the conditions on entailment regimes is to say that in case AG is inconsistent the system behavior is not defined precisely by the spec and systems that don't check consistency are required to make sure that they always return finite answers in a system specific way. This might be the better option since repairing RDF graphs will no longer work for OWL Full since there are many reasons for inconsistencies and many ways of repairing (not just replacing malformed literals). A third way is to tighten conditions on answers in general and require that not only subject bindings occur in the input vocabulary, but also the bindings for predicates and objects. > 2) uniqueness of scoping graph > > The current spec of extending BGP matching requires uniqueness of the scoping graph, whereas actually the definition > of a scoping graph for simple entailment is only unique up to homomorphic (i.e. simple) equivalence: > > "1 -- The scoping graph, SG, corresponding to any consistent active graph AG is uniquely specified and is E-equivalent to AG." > > It is there fore discussed to clarify this condition, e.g.: > > "The scoping graph, SG, corresponding to any consistent active graph > AG is uniquely (modulo simple equivalence) specified and is E-equivalent to AG." The query spec claims that simple entailment satisfies these conditions, which is also only true up to bnode renaming: "It is straightforward to show that SPARQL satisfies these conditions for the case where E is simple entailment, given that the SPARQL condition on SG is that it is graph-equivalent to AG but shares no blank nodes with AG or BGP (which satisfies the first condition). " and "... pattern solutions are therefore understood to be not from the active graph of DS itself, but from an RDF graph, called the scoping graph, which is graph-equivalent to the active graph of DS but shares no blank nodes with DS or with BGP" Thus also simple entailment just takes any graph-equivalent scoping graph, which does not give a *unique* definition for the SG. Changing the spec in this regard seems also necessary for simple entailment. > 3) Definition of RDF-B > > ""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? It might be clearer/easier to simply drop "in RDF graphs", > or to specify which graphs are talked about here. Agreed. > 4) Definition of Pattern Instance Mapping > > Birte's suggested clarafication, cf. http://lists.w3.org/Archives/Public/public-rdf-dawg/2010JanMar/0029.html Andy, what is a mapping pattern? Birte -- Dr. Birte Glimm, Room 306 Computing Laboratory Parks Road Oxford OX1 3QD United Kingdom +44 (0)1865 283529
Received on Monday, 8 February 2010 13:50:34 UTC