Re: can subqueries be executed first in SPARQL?

good afternoon;

> On 2016-06-17, at 13:16, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
> 
>> On 17 Jun 2016, at 03:58, james anderson <james@dydra.com> wrote:
>> 
>> good morning;
>> 
>>> On 2016-06-17, at 01:11, Peter F. Patel-Schneider <pfpschneider@gmail.com>
> wrote:
>>> 
> [...]
>> given this form:
>> 
>> Input:
>> 
>> 1 SELECT ?x WHERE {
>> 2    ?x a ?y .
>> 3    FILTER EXISTS {
>> 4      SELECT ?z WHERE { ?z ?w ?v . FILTER sameTerm(?x,?y) }
>> 5    }
>> 6  }
>> 
>> Algebra structure:
>> 
>> 1 (base <http://example/base/>
>> 2   (project (?x)
>> 3     (filter (exists
>> 4                (project (?z)
>> 5                  (filter (sameTerm ?x ?y)
>> 6                    (bgp (triple ?z ?w ?v)))))
>> 7       (bgp (triple ?x <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
> ?y)))))
>> 
>> why would the result change were the project evaluated logically later?
>> 
>> best regards, from berlin,
>> ---
>> james anderson | james@dydra.com | http://dydra.com
> 
> Evaluating the project first results in an empty solution sequence because x
> and y are unbound.  Evaluating the project after an exists substitution will
> result in a solution sequence with a solution mapping binding z for each
> triple in the active graph if x and y were bound to the same term.


“bound to the same term”, how?

---
james anderson | james@dydra.com | http://dydra.com