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Re: Actions 211 and 212: proposed changes to the extensions of basic graph pattern matching

From: Birte Glimm <birte.glimm@comlab.ox.ac.uk>
Date: Mon, 12 Apr 2010 17:31:48 +0100
Message-ID: <l2h492f2b0b1004120931yeda20819u764b84b926fbefbb@mail.gmail.com>
To: Andy Seaborne <andy.seaborne@talis.com>
Cc: SPARQL Working Group <public-rdf-dawg@w3.org>
>> Propsed text:
>> 4 -- Each SPARQL extension MUST provide conditions, which guarantee
>> that the answer set for every BGP and AG is uniquely specified up to
>> RDF graph equivalence. The conditions SHOULD prevent trivial infinite
>> answers such as those from axiomatic triples and infinite answers that
>> just differ in the identity of blank nodes.


> The first is fine but "trivial" has both technical and non-technical senses.
>  Can we ask the entailment regime to define what "trivial" means for each
> regime?
> Maybe:
> """
> , and should provide further conditions to prevent trivial infinite answers
> as appropriate to the regime.
> """

That could be an alternative. What I understand as "trivial" and what
the current regimes filter out are those answers that you can
reconstruct from the returned answers without knowing the queried
graph, so maybe I can try and make the more precise. E.g., for any of
the entailment regimes you can "complete" the finite answer set into
the "real" and possibly infinite answer set by adding all answers with
rdf:_n from the axiomatic triples and by replacing each answer with a
blank node with infinitely many copies that just differ in the blank
node names. This is obviously not what anybody would want to do and it
is also not too trivial because BGPs do not just consist of one triple
and interdependencies have to be considered. Thus, if we go in this
direction and make "trivial" that precise, then for each regime I
guess I would have to prove that we actually can construct the real
answer from any filtered one and I'd rather avoid that. Other than
that this is probably what we really want to filter out, answers that
can be reconstructed independently of the queried graph or rule set in
case of RIF.


>        Andy

Dr. Birte Glimm, Room 306
Computing Laboratory
Parks Road
United Kingdom
+44 (0)1865 283529
Received on Monday, 12 April 2010 16:32:25 UTC

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