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From "Lefty Leverenz (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-3276) optimize union sub-queries
Date Tue, 13 May 2014 10:19:16 GMT

    [ https://issues.apache.org/jira/browse/HIVE-3276?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13996244#comment-13996244
] 

Lefty Leverenz commented on HIVE-3276:
--------------------------------------

The configuration parameters are now documented in the wiki:

* [hive.optimize.union.remove |https://cwiki.apache.org/confluence/display/Hive/Configuration+Properties#ConfigurationProperties-hive.optimize.union.remove]
* [hive.mapred.supports.subdirectories |https://cwiki.apache.org/confluence/display/Hive/Configuration+Properties#ConfigurationProperties-hive.mapred.supports.subdirectories]

> optimize union sub-queries
> --------------------------
>
>                 Key: HIVE-3276
>                 URL: https://issues.apache.org/jira/browse/HIVE-3276
>             Project: Hive
>          Issue Type: Bug
>    Affects Versions: 0.10.0
>            Reporter: Namit Jain
>            Assignee: Namit Jain
>             Fix For: 0.10.0
>
>         Attachments: HIVE-3276.1.patch, hive.3276.10.patch, hive.3276.11.patch, hive.3276.12.patch,
hive.3276.13.patch, hive.3276.14.patch, hive.3276.2.patch, hive.3276.3.patch, hive.3276.4.patch,
hive.3276.5.patch, hive.3276.6.patch, hive.3276.7.patch, hive.3276.8.patch, hive.3276.9.patch
>
>
> 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 very 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/confluence/display/Hive/Skewed+Join+Optimization.
> If there is a select, filter between the union and the filesink, the select
> and the filter can be moved before the union, and the follow-up job can
> still be removed.



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