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From "Edward Capriolo (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-5093) Use a combiner for LIMIT with GROUP BY and ORDER BY operators
Date Mon, 19 Aug 2013 18:18:47 GMT

    [ https://issues.apache.org/jira/browse/HIVE-5093?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13744083#comment-13744083
] 

Edward Capriolo commented on HIVE-5093:
---------------------------------------

I have to think this over a little bit. If my understanding is correct hive does not use a
combiner anywhere. I do not understand the win of using a combiner in limit query situations
then. How much optimization should we do for limit queries?
                
> Use a combiner for LIMIT with GROUP BY and ORDER BY operators
> -------------------------------------------------------------
>
>                 Key: HIVE-5093
>                 URL: https://issues.apache.org/jira/browse/HIVE-5093
>             Project: Hive
>          Issue Type: Bug
>          Components: Query Processor
>    Affects Versions: 0.12.0
>            Reporter: Gopal V
>            Assignee: Gopal V
>         Attachments: HIVE-5093-WIP-01.patch
>
>
> Operator trees of the following structure can have a memory friendly combiner put in
place after the sort-phase 
> "GBY-LIM" and "OBY-LIM"
> This will cut down on I/O when spilling to disk and particularly during the merge phase
of the reducer.
> There are two possible combiners - LimitNKeysCombiner and LimitNValuesCombiner.
> The first one would be ideal for the GROUP-BY case, while the latter would more useful
for the ORDER-BY case.
> The combiners are still relevant even if there are 1:1 forward operators on the reducer
side and for small data items, the MR base layer does not run the combiners at all.

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