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Posted to issues@hive.apache.org by "ASF GitHub Bot (Jira)" <ji...@apache.org> on 2021/03/07 00:53:00 UTC
[jira] [Work logged] (HIVE-24471) Add support for combiner in hash
mode group aggregation
[ https://issues.apache.org/jira/browse/HIVE-24471?focusedWorklogId=561887&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-561887 ]
ASF GitHub Bot logged work on HIVE-24471:
-----------------------------------------
Author: ASF GitHub Bot
Created on: 07/Mar/21 00:52
Start Date: 07/Mar/21 00:52
Worklog Time Spent: 10m
Work Description: github-actions[bot] commented on pull request #1736:
URL: https://github.com/apache/hive/pull/1736#issuecomment-792135255
This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
Feel free to reach out on the dev@hive.apache.org list if the patch is in need of reviews.
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Issue Time Tracking
-------------------
Worklog Id: (was: 561887)
Time Spent: 2h (was: 1h 50m)
> 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
> Labels: pull-request-available
> Time Spent: 2h
> Remaining Estimate: 0h
>
> 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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