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https://issues.apache.org/jira/browse/HIVE-8625?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14187498#comment-14187498
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Jesús Camacho Rodríguez commented on HIVE-8625:
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The results for the tests are correct, the plans are different.
> Some union queries result in plans with many unions with CBO on
> ---------------------------------------------------------------
>
> Key: HIVE-8625
> URL: https://issues.apache.org/jira/browse/HIVE-8625
> Project: Hive
> Issue Type: Bug
> Components: CBO
> Affects Versions: 0.14.0, 0.15.0
> Reporter: Jesús Camacho Rodríguez
> Priority: Minor
> Attachments: HIVE-8625.patch
>
>
> For some queries e.g. union9, union16, we are getting plans with many binary
> unions when CBO is on. The reason is that an identity select operator is
> introduced after each union by the CBO; thus, the method that was merging the
> unions when the translation of the AST into a logical plan was being done is
> not executed, as it does not detect there is another union operator.
> With the fix in the patch attached, the method will merge the two unions if
> (1) one is the child of the other one, or (2) if one is a descendant of the
> other one and there is an identity select operator between them.
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