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https://issues.apache.org/jira/browse/HIVE-3276?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13427510#comment-13427510
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Nadeem Moidu commented on HIVE-3276:
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The patch is partially complete. https://reviews.facebook.net/D4431
> optimize union sub-queries
> --------------------------
>
> Key: HIVE-3276
> URL: https://issues.apache.org/jira/browse/HIVE-3276
> Project: Hive
> Issue Type: Bug
> Reporter: Namit Jain
> Assignee: Nadeem Moidu
>
> It might be a good idea to optimize simple union queries containing
> map-reduce jobs in at least one of the sub-qeuries.
> For eg:
> a query like:
> insert overwrite table T1 partition P1
> select * from
> (
> subq1
> union all
> subq2
> ) u;
> today creates 3 map-reduce jobs, one for subq1, another for subq2 and
> the final one for the union.
> It might be a good idea to optimize this. Instead of creating the union
> task, it might be simpler to create a move task (or something like a move
> task), where the outputs of the two sub-queries will be moved to the final
> directory. This can easily extend to more than 2 sub-queries in the union.
> This is only useful if there is a select * followed by filesink after the
> union. This can be independently useful, and also be used to optimize the
> skewed joins https://cwiki.apache.org/Hive/skewed-join-optimization.html
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