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Re: finite approximation of the minimal Herbrand model for a RIF Core/BLD ruleset.

From: Axel Polleres <axel.polleres@deri.org>
Date: Wed, 24 Feb 2010 13:23:04 +0000
Cc: "Birte Glimm" <birte.glimm@comlab.ox.ac.uk>, "Sandro Hawke" <sandro@w3.org>, "SPARQL Working Group" <public-rdf-dawg@w3.org>
Message-Id: <09DBC9C1-FCE1-42BA-8248-A2F79EB25F5F@deri.org>
To: "Jos de Bruijn" <debruijn@inf.unibz.it>
> If you have a yes/no query:
> 
> _:x rdf:type rdfs:ContainerMembershipProperty


Hmmm, tricky

First, I suppose 

ASK WHERE { ?X rdf:type rdfs:ContainerMembershipProperty}

shouldn't behave different from 

ASK WHERE { _:x rdf:type rdfs:ContainerMembershipProperty}

(it doesn't in the current spec, AFAIR, right Birte?)



So, as far as we are concerned, Jos' example boils down to:

SELECT * WHERE { ?X rdf:type rdfs:ContainerMembershipProperty}

By definition of my understanding of an "entailment regime" in SPARQL, 
an ASK query can give a positive answer if and only if the corresponding SELECT *
query would return a non-empty answer. However, there is no bnode identifier 
in the data which would warrant an answer. Answers to ASK queries are just 
like SELECT queries depending solely on BGP matching (i.e. existence of a solution for the BGP)
in SPARQL... to my understanding,  we have to deal with this "limitation", so, if you want, 
the kind of Boolean queries that Jos is looking for are not necessarily possible in SPARQL.

Hope it is clear what I meant to explain here...
I think we have a thin line between pragmatic/intuitive for what 
behaviour we can choose for those corner cases... but it should be consistent.

Axel


On 24 Feb 2010, at 12:39, Jos de Bruijn wrote:

> 
> 
> On 2010-02-24 13:34, Birte Glimm wrote:
> > [snip]
> >>> A common way to deal with this in a finite approximation way is
> >>> a) ignoring (specifically the infinite) axiomatic triples alltogether
> >>> b) take only those from the infinite axiomatic triples (those about container membership properties)
> >>> that appear in the graph... I believe the latter is what we do in the current RDF(S) entailment regime, yes Birte?
> >>
> >> b) seems to be the most reasonable way to go; but make sure to include
> >> at least one representative (for queries with blank nodes).
> >
> > What do you mean with one representative. Can you give an example? I
> > don't see what problems queries with blank nodes cause here. Maybe I
> > am overlooking something?
> 
> If you have a yes/no query:
> 
> _:x rdf:type rdfs:ContainerMembershipProperty
> 
> on the empty graph I am assuming one wants the answer "yes". If you do
> not consider any of the axiomatic triples concerning the container
> membership properties, the answer would be "no".
> 
> If you were to always consider the axiomatic triples pertaining to at
> least one container membership (or perhaps just a blank node), you are
> guaranteed to get the answer "yes" in this case.
> 
> 
> Jos
> 
> >
> > Birte
> >
> >> Unnecessarily ignoring parts of the semantics (as in a) seems rather a
> >> bad idea.
> >
> >
> >
> >>
> >> Cheers, Jos
> >>
> >>>
> >>> Axel
> >>>
> >>>>
> >>>>
> >>>> Jos
> >>>>
> >>>>>
> >>>>> Axel
> >>>>>
> >>>>>
> >>>>>> ============================================================================
> >>>>>> On 2010-02-24 12:07, Axel Polleres wrote:
> >>>>>>>
> >>>>>>> On 24 Feb 2010, at 11:04, Jos de Bruijn wrote:
> >>>>>>>> On 2010-02-24 11:28, Axel Polleres wrote:
> >>>>>>>>> Hi Jos,
> >>>>>>>>>
> >>>>>>>>> Can you check this briefly and tell me whether I don't oversimplify
> >>>>>>>>> things here?
> >>>>>>>>
> >>>>>>>> I will have a more detailed look at it later on, but a few first comments:
> >>>>>>>> - you do not consider equality between data values, e.g.
> >>>>>>>> "1"^^int="1"^^decimal
> >>>>>>>
> >>>>>>> hmmm, I am at the moment, not sure how far this is a problem, but I definitly should include this in the issues!
> >>>>>>>
> >>>>>>>
> >>>>>>>> - I did not see how a minimal model for RIF-RDF combinations is defined,
> >>>>>>>> in particular I see no blank nodes or RDF(S) semantics
> >>>>>>>
> >>>>>>> ? Can't we just treat them as skolem constants? We are just interested in query answering...
> >>>>>>
> >>>>>> 1- if you treat blank nodes as skolem constants you need to say so.
> >>>>>> 2- the RDF(S) semantics gives you more than just blank nodes.
> >>>>>>
> >>>>>>> if you agree, I forward your comments to SPARQL, ok?
> >>>>>>
> >>>>>> Sure.
> >>>>>>
> >>>>>>
> >>>>>> Jos
> >>>>>
> >>>>>
> >>>>
> >>>> --
> >>>> Jos de Bruijn
> >>>>   Web:   http://www.debruijn.net/
> >>>>   Phone: +39 0471 016224
> >>>>   Fax:   +39 0471 016009
> >>>>
> >>>
> >>
> >> --
> >> Jos de Bruijn
> >>  Web:   http://www.debruijn.net/
> >>  Phone: +39 0471 016224
> >>  Fax:   +39 0471 016009
> >>
> >
> >
> >
> 
> --
> Jos de Bruijn
>   Web:   http://www.debruijn.net/
>   Phone: +39 0471 016224
>   Fax:   +39 0471 016009
> 
Received on Wednesday, 24 February 2010 13:23:39 GMT

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