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Re: entailment review - part 1

From: Andy Seaborne <andy.seaborne@talis.com>
Date: Tue, 05 Jan 2010 21:45:07 +0000
Message-ID: <4B43B2E3.6050500@talis.com>
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 GMT

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