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Posted to issues@spark.apache.org by "Cheng Su (Jira)" <ji...@apache.org> on 2021/10/13 23:17:00 UTC

[jira] [Created] (SPARK-37001) Disable two level of map for final hash aggregation by default

Cheng Su created SPARK-37001:
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             Summary: 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


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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