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https://issues.apache.org/jira/browse/HIVE-24580?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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mahesh kumar behera updated HIVE-24580:
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Description: For distinct the number of aggregation function does not
match with the number of value column and this needs special handling in the
combiner logic. (was: 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.)
> 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: Sub-task
> Components: Hive
> Reporter: mahesh kumar behera
> Assignee: mahesh kumar behera
> Priority: Major
> Labels: pull-request-available
>
> For distinct the number of aggregation function does not match with the
> number of value column and this needs special handling in the combiner logic.
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