W3C home > Mailing lists > Public > public-rdf-dawg-comments@w3.org > October 2012

Re: Use of p* in test cases

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Sat, 13 Oct 2012 16:36:00 +0100
Message-ID: <50798A60.9090007@epimorphics.com>
To: public-rdf-dawg-comments@w3.org

On 13/10/12 09:54, Jan Wielemaker wrote:
> Hi Andy,
> I see.  If enumerating all nodes is what the test it about, it is fine.
> If the test is about whether property paths combine with GRAPH, I'd
> prefer if the node-test is a single test and this test uses :p1+.
>      Cheers --- Jan

(not having written the test ... and not a formal WG response)

The test, which is "approved", looks to me to be in part that the
property path is with a specific graph - specifically the scope of node 
set being the active graph not all whole dataset of the query.

ARQ throws its hands up and executes it "directly" -- it does calculate 
the node set if there isn't anything else to start on.


> On 10/13/2012 10:46 AM, Andy Seaborne wrote:
>> Jan,
>> (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.
>> http://www.w3.org/2009/sparql/docs/query-1.1/rq25.xml#defn_nodeSet
>> 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.
>>      Andy
>> 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 15:36:29 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 6 January 2015 20:52:13 UTC