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https://issues.apache.org/jira/browse/JENA-633?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13892649#comment-13892649
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Andy Seaborne commented on JENA-633:
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Interestingly, reversing the join LHS and RHS makes the optimization correct.
So {{JoinClassifier}} needs to check the RHS differently from the LHS and
possibly swap them.
> optimizer sequences instead of joins / subqueries with limits project more
> results than they should
> ---------------------------------------------------------------------------------------------------
>
> Key: JENA-633
> URL: https://issues.apache.org/jira/browse/JENA-633
> Project: Apache Jena
> Issue Type: Bug
> Components: ARQ, Jena
> Affects Versions: Jena 2.11.0
> Reporter: Joshua Taylor
> Assignee: Andy Seaborne
> Labels: optimization, sparql
> Attachments: SubqueryLimitBug.java
>
>
> A subquery with `limit 1` specified can `leak` more values to the surrounding
> query. Rob Vesse tracked this down to a difference between the basic algebra
> and the optimized algebra [1] . One query has the following basic and
> optimized Ops; the "join" in the former shouldn't get turned into a
> "sequence" in the latter.
> {noformat}
> Basic:
> (join
> (bgp (triple ?resource <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
> <urn:ex:Thing>))
> (slice _ 1
> (project (?resource ?p)
> (order (?p)
> (bgp (triple ?resource <urn:ex:p> ?p))))))
> Optimized:
> (sequence
> (bgp (triple ?resource <http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
> <urn:ex:Thing>))
> (project (?resource ?p)
> (top (1 ?p)
> (bgp (triple ?resource <urn:ex:p> ?p)))))
> {noformat}
> [1]
> http://answers.semanticweb.com/questions/26343/outer-query-results-include-more-values-than-subquery-with-limit-n-produces/26344
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