Re: can subqueries be executed first in SPARQL?

good morning;

> On 2016-06-17, at 14:18, Peter F. Patel-Schneider <pfpschneider@gmail.com> wrote:
> 
> On 06/17/2016 05:01 AM, james anderson wrote:
>> good afternoon;
>> 
>>> On 2016-06-17, at 13:16, Peter F. Patel-Schneider <pfpschneider@gmail.com
>>> <mailto:pfpschneider@gmail.com>> wrote:
>>> 
>>>> On 17 Jun 2016, at 03:58, james anderson <james@dydra.com
>>>> <mailto:james@dydra.com>> wrote:
>>>> 
>>>> good morning;
>>>> 
>>>>> On 2016-06-17, at 01:11, Peter F. Patel-Schneider <pfpschneider@gmail.com
>>>>> <mailto: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 <mailto: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?
> 
> Aah.  I used "bound" where SPARQL uses "maps".  That's showing my background
> in logic.

i have no argument there. my implementation basis is environments.
my concern is the license taken  with scoping rules and the permission given to perform lexical operations in the midst of the discussion of the definition for an algebra operation.
is there any advantage to going down that pat, to set up such evidently straw men?

best regards, from berlin,

> 
>> ---
>> james anderson | james@dydra.com <mailto:james@dydra.com> | http://dydra.com> 
> 
> Evaluate the above against the graph (with the same base)
>  :k a :k .
>  :j a :k .
> The bgp evaluation results in a multiset with two solution mappings
> { { (x,k), (y,k) }, { (x,j), (y,k) } }
> The first produces a non-empty value for the project whereas the second
> produces an empty value.
> 
> peter

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