[jira] [Created] (HIVE-24471) Add support for combiner in hash mode group aggregation

["mahesh kumar behera (Jira)" <ji...@apache.org>]

mahesh kumar behera created HIVE-24471:
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             Summary: Add support for combiner in hash mode group aggregation 
                 Key: HIVE-24471
                 URL: https://issues.apache.org/jira/browse/HIVE-24471
             Project: Hive
          Issue Type: Bug
          Components: Hive
            Reporter: mahesh kumar behera
            Assignee: mahesh kumar behera


In map side group aggregation, partial grouped aggregation is calculated to reduce the data written to disk by map task. In case of hash aggregation, where the input data is not sorted, hash table is used. If the hash table size increases beyond configurable limit, data is flushed to disk and new hash table is generated. If the reduction by hash table is less than min hash aggregation reduction calculated during compile time, the map side aggregation is converted to streaming mode. So if the first few batch of records does not result into significant reduction, then the mode is switched to streaming mode. This may have impact on performance, if the subsequent batch of records have less number of distinct values. To mitigate this situation, a combiner can be added to the map task after the keys are sorted. This will make sure that the aggregation is done if possible and reduce the data written to disk.



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