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Re: BIND tests

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Mon, 24 Jan 2011 16:05:20 +0000
Message-ID: <4D3DA340.7080905@epimorphics.com>
To: Lee Feigenbaum <lee@thefigtrees.net>
CC: SPARQL Working Group <public-rdf-dawg@w3.org>


On 24/01/11 15:20, Lee Feigenbaum wrote:
> On 1/24/2011 4:33 AM, Andy Seaborne wrote:
>>
>>
>> On 24/01/11 03:22, Lee Feigenbaum wrote:
>>> Hi everyone,
>>>
>>> This mail discharged my ACTION-371
>>> (http://www.w3.org/2009/sparql/track/actions/371), to look at the bind
>>> tests.
>>>
>>> I looked over the 7 tests in
>>> http://www.w3.org/2009/sparql/docs/tests/data-sparql11/bind/ and
>>> believed them to be accurate and reasonable coverage of the BIND
>>> keyword.
>>>
>>> I added one new test, bind08, which I believe tests correctly the fact
>>> that BIND occurs _after_ a group is ended, and hence a FILTER appearing
>>> before BIND cannot act on the BINDed variables. This is the test:
>>>
>>> == query ==
>>>
>>> PREFIX : <http://example.org/>
>>>
>>> SELECT ?s ?p ?o ?z
>>> {
>>> ?s ?p ?o .
>>> FILTER(?z = 3 )
>>> BIND(?o+1 AS ?z)
>>> }
>>>
>>> == results ==
>>>
>>> (no solutions)
>>>
>>> Can someone please check the truth of this test? I don't have a BIND
>>> implementation to test this with at the moment. Also, if you feel that
>>> this should not be the semantics of BIND as the WG agreed on a couple of
>>> months ago, please let us know.
>>>
>>> Once this test has been looked at, I'd like to propose that we approve
>>> these 8 bind tests either this Tuesday or (more likely) one week from
>>> Tuesday.
>>>
>>> Lee
>>
>> Unfortunately, we missed that the WG agreement is at odds with SPARQL
>> 1.0 [1]
>>
>> In SPARQL 1.0, a filter applies to the whole group, where "group" means
>> syntax units between the {}.
>>
>> Andy
>>
>> [1] http://www.w3.org/TR/rdf-sparql-query/#scopeFilters
>>
>> [[
>> A constraint, expressed by the keyword FILTER, is a restriction on
>> solutions over the whole group in which the filter appears.
>> ]]
>
> Well, I think that it's less that it changes that part of SPARQL 1.0 and
> more that it provides a new way to mark the end of the group. (BIND is
> effectively the equivalent of "} {".) Still, it clearly makes for some
> weirdness, which is what bind08 is showing.
>
> We discussed exactly bind08 at
> http://www.w3.org/2009/sparql/meeting/2010-11-02#BIND_and_FILTER_order_execution,
> and there was consensus that this is the behavior that we want, so while
> it is a strange new way to mark the end of the group (and start of a new
> group), I don't think it's a decision we ought to revisit now...

(meta: I'm not advocating a change because I think it's better - I'm 
bring this out because in working through the detailed design I came 
across this interaction with SPARQL 1.0.)

Group here is not a basic graph pattern.  It is (from SPARQL 1.0), the 
elements between {}.  [2]  We did discuss whether it inside or outside a 
BGP, but I found another design constraint from SPARQL 1.0.

The discussion I recall was all about BGPs, and the record seems to 
agree and show we were a bit mixed between BGP and syntax groups. We 
missed the further implication from SPARQL 1.0.

BIND can end a BGP, and that makes entailment work when it's just 
"outside the BGP".  It can't end a group.

Test 5 and test 8 must give the same results - as must:

SELECT ?s ?p ?o ?z
{
   FILTER(?z = 3 )
   ?s ?p ?o .
   BIND(?o+1 AS ?z)
}

	Andy

[2] http://www.w3.org/TR/rdf-sparql-query/#GroupPatterns


>
> Lee
>
>
Received on Monday, 24 January 2011 16:06:07 GMT

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