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Date: Wed, 25 Jul 2012 19:24:35 +0000 (UTC)
From: "Nadeem Moidu (JIRA)" <jira@apache.org>
To: hive-dev@hadoop.apache.org
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Subject: [jira] [Commented] (HIVE-3276) optimize union sub-queries
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    [ https://issues.apache.org/jira/browse/HIVE-3276?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13422533#comment-13422533 ] 

Nadeem Moidu commented on HIVE-3276:
------------------------------------

A wiki page has been added for the same here https://cwiki.apache.org/confluence/display/Hive/Union+Optimization . I have started work on this and will be uploading the patch soon. Feel free to give any feedback on the same. Thanks.
                
> 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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