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Re: entailment review - part 2

From: Axel Polleres <axel.polleres@deri.org>
Date: Tue, 12 Jan 2010 13:45:44 +0000
Cc: "SPARQL Working Group" <public-rdf-dawg@w3.org>
Message-Id: <DB4A116D-3E55-404B-BCD3-5A612C787B47@deri.org>
To: "Birte Glimm" <birte.glimm@comlab.ox.ac.uk>

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
Received on Tuesday, 12 January 2010 13:46:18 GMT

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