mahesh kumar behera created HIVE-24580:
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Summary: Add support for combiner in hash mode group aggregation
(Support for distinct)
Key: HIVE-24580
URL: https://issues.apache.org/jira/browse/HIVE-24580
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 (with sorting also being
performed before flushing). 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 improve performance both in Hash and Streaming mode, 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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