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Re: summary on options for JP-4 Comment about the semantics of property paths

From: Axel Polleres <axel.polleres@deri.org>
Date: Sat, 11 Feb 2012 20:02:08 +0100
Cc: "Lee Feigenbaum" <lee@thefigtrees.net>, "SPARQL Working Group" <public-rdf-dawg@w3.org>
Message-Id: <3A9CE8A4-2B0D-4BC5-9AB6-5301DBDAAE1E@deri.org>
To: "Gregory Williams" <greg@evilfunhouse.com>

On 11 Feb 2012, at 19:21, Gregory Williams wrote:

> On Feb 11, 2012, at 1:14 PM, Axel Polleres wrote:
> > To make this more tangible, what I was thinking is the following:
> >
> > Take
> >
> > Q:  SELECT ?S WHERE { ?s :p/:q [] }
> >
> > and the following graph
> >
> >  :s1 :p [ :q :o1 ].
> >  :s1 :p [ :q :o1 ].
> >  :s1 :p [ :q :o2 ].
> I think this is a particularly strange query to use to base this discussion, because:
> a) (as Andy noted) things get particularly strange when you don't have a term on one end of the path
> b) it's a fixed length path, which isn't the underlying problem as I understand it. I certainly hope we're not discussing changing the cardinality semantics of fixed-length paths…

Sure, the query was kept simple just to show the problem why I think that it's not trivial to find an equivalent way of expressing Jorge's semantics,
So the path expression wasn't my point, but the blank node... I simply don't know a way how to write 
an equivalent DISTINCT subquery where these things wouldn't intermingle. 

So, I am unsure how exactly we "implement" Option 1:

> Option 1... keep everything as it is in the grammar, and explain which DISTINCT path subqueries can be optimized

What I am missing is an understanding/definition of which DISTINCT path subquerise could be optimized and how. 
Jorge's paper outlines how this can be done for their semantics, if  we don't have a way to express their 
semantics, what are the queries that can be optimized and how? 


> I'm having a hard time coming up with a real query like this that used arbitrary-length paths, that didn't involve counting, and that couldn't be done with an ASK query.
> .greg
Received on Saturday, 11 February 2012 19:02:39 UTC

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