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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