Custom grouping comparator cannot be set for Combiners
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Key: MAPREDUCE-3310
URL: https://issues.apache.org/jira/browse/MAPREDUCE-3310
Project: Hadoop Map/Reduce
Issue Type: Improvement
Components: client
Affects Versions: 0.21.0, 0.20.205.0, 0.20.203.0, 0.20.2, 0.20.1
Environment: All
Reporter: Mathias Herberts
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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