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Re: Use of p* in test cases

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Sat, 13 Oct 2012 09:46:48 +0100
Message-ID: <50792A78.8000403@epimorphics.com>
To: public-rdf-dawg-comments@w3.org

(explanatory answer - not a formal WG response)

The spec deals with this by defining the node set of a graph as all 
subjects and objects (not properties).  This is closest to a 
mathematical graph, which is a set of vertexes and set of edges; an RDF 
graph is defined only as a set of edges.


ZeroOrMorePath is then defined using nodes(G) for when the path is 
unbounded at each end.  The node set in question is the "active graph" 
hence it's GRAPH <ng-01.ttl>.

When you start working with graph algorithms, the need for the set of 
vertexes arises.  The Node set of a graph is the minimal set to have all 
the vertexes taking the triples as labelled edges of a graph.  The 
properties could be included, but property paths connect vertexes to 
vertexes, treating the properties as edge labels, when viewed as a 
mathematical graph.


On 11/10/12 16:29, Jan Wielemaker wrote:
> Hi,
> It is not really clear to me whether comments on test cases should be
> sent to this list.  Maybe it is also a comment on the document though.
> I'm updating the ClioPatria SPARQL endpoint to 1.1.
> One issue that causes me trouble are test cases such as '(pp34) Named
> Graph 1'.  The query is
> prefix :  <http://www.example.org/>
> select ?t
> where {
>    GRAPH <ng-01.ttl> {
>      ?s :p1* ?t }
> }
> Given that the * operator matches zero steps and both end-points
> are variables, all resources are an answer.  But, what are all
> resources?  It seems the test cases assume all resources that
> appear as subject or object in the destination graph.  But,
> why not all predicates too?   ClioPatria can generate resources,
> but it has no clue to what graph these belong :-(
> Although I can write an implementation that will answer this
> question as the test-cases suggest it should be answered, I
> wonder whether it makes much sense to define the behaviour of
> this query exactly.  ?x :p* ?y queries can never be answered on
> any sensible sized RDF graph anyway.
> I'd propose to either use :p1+ for these queries or specify either
> the subject or the object.  That avoids these ambiguities.
>      Cheers --- Jan
> P.s.    Is there a better way to get the test cases than mirroring
>      http://www.w3.org/2009/sparql/docs/tests/?  I found a mirror
>      git repository on GitHUB, but it was rather outdated when
>      I checked.
Received on Saturday, 13 October 2012 08:47:18 UTC

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