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https://issues.apache.org/jira/browse/JENA-389?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Andy Seaborne updated JENA-389:
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Summary: Subquery containing a FILTER NOT EXISTS can cause wrong results
(was: Subquery containing a FILTER NOT EXISTS can cause unexpected results)
> 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
>
> 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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