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Re: Querying Named Graph using SPARQL

From: David Booth <david@dbooth.org>
Date: Wed, 14 Aug 2013 09:34:09 -0400
Message-ID: <520B8751.3050307@dbooth.org>
To: Axel Polleres <axel@polleres.net>
CC: Quentin Reul <Quentin.H.Reul@gmail.com>, semantic-web@w3.org, public-rdf-wg@w3.org, public-sparql-dev@w3.org, Andy Seaborne <andy@apache.org>, Steve Harris <steve.harris@garlik.com>
[Copying Andy Seaborne and Steve Harris for their input.]

Hi Axel,

The part of the spec that I was trying to bring to your attention was 
where it says that the empty group graph pattern "does not bind *any* 
variables" (my emphasis).  If ?G is not bound then that short form of 
listing existing graphs would not work.  But I am not sure from reading 
the spec whether ?G is supposed to be bound or not.  The SPARQL algebra 
says:

http://www.w3.org/TR/sparql11-query/#defn_evalGraph
[[
eval(D(G), Graph(var,P)) =
      Let R be the empty multiset
      foreach IRI i in D
         R := Union(R, Join( eval(D(D[i]), P) , Ω(?var->i) )
      the result is R
]]
Hmm, there's right parenthesis missing, which I guess I'll report 
separately.

It looks like Ω(?var->i) is binding the graph variable, but then it's 
doing a "Join", which is defined as:
http://www.w3.org/TR/sparql11-query/#defn_algJoin
[[
Join(Ω1, Ω2) = { merge(μ1, μ2) | μ1 in Ω1and μ2 in Ω2, and μ1 and μ2 are 
compatible }
]]

I couldn't readily find the definition of the merge function, but I 
think I found the definition of "compatible":
http://www.w3.org/TR/sparql11-query/#defn_algCompatibleMapping
[[
Two solution mappings μ1 and μ2 are compatible if, for every variable v 
in dom(μ1) and in dom(μ2), μ1(v) = μ2(v).
]]
which seems to be saying that the variables being joined must be bound 
to the same values.  But since the empty basic graph pattern does not 
bind ?G, I *think* this means that Ω(?var->i) would *not* be compatible 
and ?G would therefore not be bound in the result.

So, if I have properly followed the SPARQL algebra, I *think* this means 
that the short form that you suggested for listing graphs will not work, 
and it is not possible to get a list of graphs that includes empty 
graphs.  (Hence the Sesame 2.7.1 behavior is correct.)

Andy or Steve, have I got this right?

Thanks,
David

On 08/13/2013 03:10 PM, Axel Polleres wrote:
>> [[ The group pattern: { } matches any graph (including the empty
>> graph) with one solution that does not bind any variables. ]]
>
> This only means that upon
>
> SELECT ?G WHERE { GRAPH ?G {} }
>
> also empty named graphs should be returned, which would not be the
> case for
>
> SELECT ?G WHERE { GRAPH ?G { ?S ?P ?O } }
>
> Obviously, this makes a difference for all graph stores that support
> empty named graphs. So, to my understanding at least, this is not a
> bug in the spec.
>
> HTH, Axel
>
> On 13 Aug 2013, at 20:21, David Booth <david@dbooth.org> wrote:
>
>> Hi Axel,
>>
>> That doesn't work in Sesame 2.7.1 at least, apparently because ?G
>> is not bound, even though there is one solution.  The SPARQL 1.1
>> spec says: http://www.w3.org/TR/sparql11-query/#emptyGroupPattern
>> [[ The group pattern: { } matches any graph (including the empty
>> graph) with one solution that does not bind any variables. ]]
>>
>> Is this a bug in the spec?
>>
>> David
>>
>> On 08/13/2013 11:48 AM, Axel Polleres wrote:
>>> Hi Quentin,
>>>
>>> how about just
>>>
>>> SELECT ?G WHERE { GRAPH ?G {} }
>>>
>>> (no need to dump all triples, if the only concern is which ?G
>>> exist)
>>>
>>> BTW, public-sparql-dev@w3.org may be the list you wanted to use.
>>>
>>> best, Axel
>>>
>
>
>
>
Received on Wednesday, 14 August 2013 13:34:38 UTC

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