[jira] [Created] (HIVE-5093) Use a combiner for LIMIT with GROUP BY and ORDER BY operators

["Gopal V (JIRA)" <ji...@apache.org>]

Gopal V created HIVE-5093:
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             Summary: Use a combiner for LIMIT with GROUP BY and ORDER BY operators
                 Key: HIVE-5093
                 URL: https://issues.apache.org/jira/browse/HIVE-5093
             Project: Hive
          Issue Type: Bug
          Components: Query Processor
    Affects Versions: 0.12.0
            Reporter: Gopal V
            Assignee: Gopal V


Operator trees of the following structure can have a memory friendly combiner put in place after the sort-phase 

"GBY-LIM" and "OBY-LIM"

This will cut down on I/O when spilling to disk and particularly during the merge phase of the reducer.

There are two possible combiners - LimitNKeysCombiner and LimitNValuesCombiner.

The first one would be ideal for the GROUP-BY case, while the latter would more useful for the ORDER-BY case.

The combiners are still relevant even if there are 1:1 forward operators on the reducer side and for small data items, the MR base layer does not run the combiners at all.

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