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https://issues.apache.org/jira/browse/HIVE-24471?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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mahesh kumar behera reassigned HIVE-24471:
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> 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
> Priority: Major
>
> 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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