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Re: can subqueries be executed first in SPARQL?

From: Peter F. Patel-Schneider <pfpschneider@gmail.com>
Date: Fri, 17 Jun 2016 04:16:50 -0700
To: james anderson <james@dydra.com>
Cc: public-sparql-dev@w3.org
Message-ID: <640163d5-18a7-cb98-d933-4680b4cde600@gmail.com>
> 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.


peter
Received on Friday, 17 June 2016 11:17:22 UTC

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