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From "Jonathan Eagles (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-3310) Custom grouping comparator cannot be set for Combiners
Date Thu, 16 Jan 2014 20:43:22 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-3310?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13873905#comment-13873905

Jonathan Eagles commented on MAPREDUCE-3310:

It looks like the behavior before is to default combiner group comparator to sort output key
comparator. If I route Tez's getCombinerKeyGroupingComparator() to getSortComparator(), would
this have the same functionality as pre-MAPREDUCE-3310. This would allow Tez to compile for
both hadoop-2.2.0 and hadoop-2.4.0.


> 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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