Re: ISSUE-68: Updated definition

On 03/09/2016 12:46 AM, Holger Knublauch wrote:
> On 9/03/2016 18:17, Peter F. Patel-Schneider wrote:
>> I'm pretty sure that this fails in a number of places.
>>
>> It can break the shared variable connection for MINUS.   (I think that FILTER
>> is OK, but I'm not sure.)
> 
> Do you have an example for this?

SELECT ?this
WHERE { ?this ex:a ex:b MINUS { ?this ex:a ex:b } }

>>
>> The substitution can modify variables from different scopes, which will change
>> results.
> 
> Do you have an example for this?

SELECT ?this ?that
WHERE { ?this ex:a ex:b
   SELECT ?that WHERE { ?this ex:a ?that } }

>> Skolemization in the SPARQL code means that the blank node will not match back
>> to itself in the graph it came from.
> 
> Conceptually, the bnodes will also need to be skolemized in the data graph.
> 
> All this is an entirely conceptual definition. Actual implementations are
> unlikely to ever use this mechanism, but instead operate on Algebra and API
> level.

You can't provide something that may not be possible in practice unless there
is something that is possible in practice, and you haven't shown that there
there is that something.  If the only way to do this is to skolemize the data
graph as well, which wasn't even in the description, then you haven't shown
that there is an acceptable way to handle bnodes.

> 
> Holger

peter

Received on Wednesday, 9 March 2016 15:17:44 UTC