Gopal V created HIVE-5093: ----------------------------- 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. -- This message is automatically generated by JIRA. If you think it was sent incorrectly, please contact your JIRA administrators For more information on JIRA, see: http://www.atlassian.com/software/jira