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https://issues.apache.org/jira/browse/MAPREDUCE-3310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13863627#comment-13863627
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Alejandro Abdelnur commented on MAPREDUCE-3310:
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just committed an addendum fixing javadoc warnings (apologies for the noise).
> 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
> Fix For: 1.3.0, 2.4.0
>
> 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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