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https://issues.apache.org/jira/browse/HIVE-3496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13460132#comment-13460132
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Kevin Wilfong commented on HIVE-3496:
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https://reviews.facebook.net/D5577
> Query plan for multi-join where the third table joined is a subquery
> containing a map-only union with hive.auto.convert.join=true is wrong
> ------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: HIVE-3496
> URL: https://issues.apache.org/jira/browse/HIVE-3496
> Project: Hive
> Issue Type: Bug
> Components: Query Processor
> Affects Versions: 0.10.0
> Reporter: Kevin Wilfong
> Assignee: Kevin Wilfong
>
> Take the following query as an example:
> EXPLAIN SELECT * FROM
> src11 a JOIN
> src12 b ON (a.key = b.key) JOIN
> (SELECT * FROM (SELECT * FROM src13 UNION ALL SELECT * FROM src14)a )c ON
> c.value = b.value;
> When hive.auto.convert.join=true, the two joins are implemented separately as
> conditional tasks with two mapjoins and a backup common join. In the second
> join, the conditional task will be a backup task, contained in the
> ConditionalTask, and a root task. This is clearly wrong, and leads to query
> failures.
> I've traced this to the joinUnionPlan method of GenMapRedUtils. If the union
> operator was performed in its own map reduce task and it could be a root
> task, when it is added to the mapper of the existing task which performs the
> join in the reducer, this task will get made a root task without first
> checking if the existing (non-union) task has any dependencies.
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