[jira] [Commented] (SPARK-22408) RelationalGroupedDataset's distinct pivot value calculation launches unnecessary stages

["Apache Spark (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-22408?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16234295#comment-16234295 ] 

Apache Spark commented on SPARK-22408:
--------------------------------------

User 'pwoody' has created a pull request for this issue:
https://github.com/apache/spark/pull/19629

> RelationalGroupedDataset's distinct pivot value calculation launches unnecessary stages
> ---------------------------------------------------------------------------------------
>
>                 Key: SPARK-22408
>                 URL: https://issues.apache.org/jira/browse/SPARK-22408
>             Project: Spark
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 2.2.0
>            Reporter: Patrick Woody
>
> When calculating the distinct values for a pivot in RelationalGroupedDataset (https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/RelationalGroupedDataset.scala#L322), we sort before doing a take(maxValues + 1).
> We should be able to improve this by adding a global limit before the sort, which should reduce the work of the sort, and by simply doing a collect to avoid multiple launching multiple stages as a part of the take. 



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