[jira] [Resolved] (JENA-389) Subquery containing a FILTER NOT EXISTS can cause wrong results

["Andy Seaborne (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/JENA-389?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Andy Seaborne resolved JENA-389.
--------------------------------

       Resolution: Fixed
    Fix Version/s: Jena 2.10.1
    
> Subquery containing a FILTER NOT EXISTS can cause wrong results
> ---------------------------------------------------------------
>
>                 Key: JENA-389
>                 URL: https://issues.apache.org/jira/browse/JENA-389
>             Project: Apache Jena
>          Issue Type: Bug
>          Components: ARQ
>    Affects Versions: Jena 2.10.0
>            Reporter: Rob Vesse
>            Assignee: Andy Seaborne
>             Fix For: Jena 2.10.1
>
>
> Highlighted by Tim Harsch on Answers.SemanticWeb.com - http://answers.semanticweb.com/questions/20737/why-doesnt-subquery-variable-get-projected
> He found a couple of queries (run against the trivial books database on sparql.org which should give the same results yet yield entirely different results).
> Query 1:
> SELECT *
> {
>   SELECT (COUNT(?x1) as ?openTriplets)
>   WHERE {
>     ?x1 ?a1 ?y1 .
>     ?y1 ?b1 ?z1 .
>     FILTER NOT EXISTS {?z1 ?c1 ?x1}
>   }
> }
> Result 1 - ?openTriplets has count of 6
> Query 2:
> SELECT ?openTriplets
> {
>   SELECT (COUNT(?x1) as ?openTriplets)
>   WHERE {
>     ?x1 ?a1 ?y1 .
>     ?y1 ?b1 ?z1 .
>     FILTER NOT EXISTS {?z1 ?c1 ?x1}
>   }
> }
> Result 2 - ?openTriplets = 0
> This seems to be because the explicit mention of the variable name in Query 2 results in a different algebra being generated because of how ARQ renames variables to give correct scoping.
> Algebra for Query 1:
> (base <http://example/base/>
>    (project (?openTriplets)
>      (extend ((?openTriplets ?.0))
>       (group () ((?.0 (count ?x1)))
>         (filter (notexists (bgp (triple ?z1 ?c1 ?x1)))
>           (bgp
>             (triple ?x1 ?a1 ?y1)
>            (triple ?y1 ?b1 ?z1)
>          ))))))
> Algebra for Query 2:
> (base <http://example/base/>
>    (project (?openTriplets)
>      (project (?openTriplets)
>        (extend ((?openTriplets ?/.0))
>          (group () ((?/.0 (count ?/x1)))
>            (filter (notexists (bgp (triple ?//z1 ?//c1 ?//x1)))
>              (bgp
>                (triple ?/x1 ?/a1 ?/y1)
>                (triple ?/y1 ?/b1 ?/z1)
>              )))))))
> As can be seen with the second algebra the extra project appears to cause ARQ to rename variables differently.  This results in the NOT EXISTS being treated as distinct from the BGP it encloses meaning it matches everything resulting in all values being eliminated hence the count of zero as the result of the second query.

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