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https://issues.apache.org/jira/browse/HIVE-3915?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13556994#comment-13556994
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Kevin Wilfong commented on HIVE-3915:
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https://reviews.facebook.net/D8019
> Union with map-only query on one side and two MR job query on the other
> produces wrong results
> ----------------------------------------------------------------------------------------------
>
> Key: HIVE-3915
> URL: https://issues.apache.org/jira/browse/HIVE-3915
> Project: Hive
> Issue Type: Bug
> Components: Query Processor
> Affects Versions: 0.11.0
> Reporter: Kevin Wilfong
> Assignee: Kevin Wilfong
>
> When a query contains a union with a map only subquery on one side and a
> subquery involving two sequential map reduce jobs on the other, it can
> produce wrong results. It appears that if the map only queries table scan
> operator is processed first the task involving a union is made a root task.
> Then when the other subquery is processed, the second map reduce job gains
> the task involving the union as a child and it is made a root task. This
> means that both the first and second map reduce jobs are root tasks, so the
> dependency between the two is ignored. If they are run in parallel (i.e. the
> cluster has more than one node) no results will be produced for the side of
> the union with the two map reduce jobs and only the results of the other side
> of the union will be returned.
> The order TableScan operators are processed is crucial to reproducing this
> bug, and it is determined by the order values are retrieved from a map, and
> hence hard to predict, so it doesn't always reproduce.
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