[jira] [Commented] (SPARK-8964) Use Exchange in limit operations (per partition limit -> exchange to one partition -> per partition limit)

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

    [ https://issues.apache.org/jira/browse/SPARK-8964?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14621390#comment-14621390 ] 

Apache Spark commented on SPARK-8964:
-------------------------------------

User 'JoshRosen' has created a pull request for this issue:
https://github.com/apache/spark/pull/7334

> Use Exchange in limit operations (per partition limit -> exchange to one partition -> per partition limit)
> ----------------------------------------------------------------------------------------------------------
>
>                 Key: SPARK-8964
>                 URL: https://issues.apache.org/jira/browse/SPARK-8964
>             Project: Spark
>          Issue Type: Improvement
>          Components: SQL
>            Reporter: Josh Rosen
>            Assignee: Josh Rosen
>
> Spark SQL's physical Limit operator currently performs its own shuffle rather than using Exchange to perform the shuffling.  This is less efficient since this non-exchange shuffle path won't be able to benefit from SQL-specific shuffling optimizations, such as SQLSerializer2.  It also involves additional unnecessary row copying.
> Instead, I think that we should rewrite Limit to expand into three physical operators:
> PerParititonLimit -> Exchange to one partition -> PerPartitionLimit



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