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Posted to issues@spark.apache.org by "Apache Spark (Jira)" <ji...@apache.org> on 2021/10/13 23:19:00 UTC
[jira] [Commented] (SPARK-37001) Disable two level of map for final
hash aggregation by default
[ https://issues.apache.org/jira/browse/SPARK-37001?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17428529#comment-17428529 ]
Apache Spark commented on SPARK-37001:
--------------------------------------
User 'c21' has created a pull request for this issue:
https://github.com/apache/spark/pull/34270
> Disable two level of map for final hash aggregation by default
> --------------------------------------------------------------
>
> Key: SPARK-37001
> URL: https://issues.apache.org/jira/browse/SPARK-37001
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 3.2.0, 3.3.0
> Reporter: Cheng Su
> Priority: Minor
>
> This JIRA is to disable two level of maps for final hash aggregation by default. The feature was introduced in [#32242|https://github.com/apache/spark/pull/32242] and we found it can lead to query performance regression when the final aggregation gets rows with a lot of distinct keys. The 1st level hash map is full so a lot of rows will waste the 1st hash map lookup and inserted into 2nd hash map. This feature still benefits query with not so many distinct keys though, so introducing a config to allow query to enable the feature when seeing benefit.
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