[jira] [Created] (MAPREDUCE-5461) Let users be able to get latest Key in reduce()

["Lijie Xu (JIRA)" <ji...@apache.org>]

Lijie Xu created MAPREDUCE-5461:
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             Summary: Let users be able to get latest Key in reduce()
                 Key: MAPREDUCE-5461
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-5461
             Project: Hadoop Map/Reduce
          Issue Type: Improvement
          Components: task
    Affects Versions: 1.2.1
         Environment: Any environment
            Reporter: Lijie Xu


Reducer generates <K, List(V)> for reduce(). In some cases such as SecondarySort, although current V and next V share the same K, their actual corresponding Ks are different. For example, in SecondarySort, map() outputs
Key         Value
<1, 3>        3
<1, 1>        1
<2, 5>        5
<1, 8>        8

After partition by Key.getFirst(), sort and group by Key.getFirst(),
reducer gets:
Key         Value
------Group 1------
<1, 1>        1
<1, 3>        3
<1, 8>        8
------Group 2------
<2, 5>        5

reduce() receives:

Key      List<Value>
<1, 1>   List<1, 3, 8>
<2, 5>   List<5>

When invoking V.next(), we can get next V (e.g, 3). But we do not have API to get its corresponding Key (e.g, <1, 3>). We can only get the first Key (e.g., <1,1>).

If we let user be able to get latest key, SecondarySort does not need to emit value in map(). So that the network traffic is better.

Another example is Join. If we can get latest Key, we do need to put table label in both key and value.



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