[jira] [Commented] (MAPREDUCE-3310) Custom grouping comparator cannot be set for Combiners

["Tom White (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/MAPREDUCE-3310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13863042#comment-13863042 ] 

Tom White commented on MAPREDUCE-3310:
--------------------------------------

+1

> Custom grouping comparator cannot be set for Combiners
> ------------------------------------------------------
>
>                 Key: MAPREDUCE-3310
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-3310
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>          Components: client
>    Affects Versions: 0.20.1
>         Environment: All
>            Reporter: Mathias Herberts
>            Assignee: Alejandro Abdelnur
>         Attachments: MAPREDUCE-3310-branch-1.patch, MAPREDUCE-3310-branch-1.patch, MAPREDUCE-3310-trunk.patch, MAPREDUCE-3310-trunk.patch, MAPREDUCE-3310-trunk.patch, MAPREDUCE-3310-trunk.patch
>
>
> Combiners are often described as 'Reducers running on the Map side'.
> As Reducers, Combiners are fed <K,{V}>, where {V} is built by grouping values associated with the 'same' key.
> For Reducers, the comparator used for grouping values can be set independently of that used to sort the keys (using Job.setGroupingComparatorClass).
> Such a configuration is not possible for Combiners, meaning some things done in Reducers cannot be done in Combiners (such as secondary sort).
> It would be handy to have a Job.setCombinerGroupingComparatorClass method that would allow the setting of the grouping comparator used when applying a Combiner.



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