[jira] [Commented] (SPARK-21509) Add a config to enable adaptive query execution only for the last query execution.

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

    [ https://issues.apache.org/jira/browse/SPARK-21509?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16097353#comment-16097353 ] 

Apache Spark commented on SPARK-21509:
--------------------------------------

User 'jinxing64' has created a pull request for this issue:
https://github.com/apache/spark/pull/18713

>  Add a config to enable adaptive query execution only for the last query execution.
> -----------------------------------------------------------------------------------
>
>                 Key: SPARK-21509
>                 URL: https://issues.apache.org/jira/browse/SPARK-21509
>             Project: Spark
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 2.2.0
>            Reporter: jin xing
>
> Feature of adaptive query execution is a good way to avoid generating too many small files on HDFS, like mentioned in SPARK-16188.
> When feature of adaptive query execution is enabled, all shuffles will be coordinated. The drawbacks:
> 1. It's hard to balance the num of reducers(this decides the processing speed) and file size on HDFS
> 2. It generates some unnecessary shuffles(https://github.com/apache/spark/blob/master/sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/EnsureRequirements.scala#L101)
> 3. It generates lots of jobs, which have extra cost for scheduling.
> We can add a config and enable adaptive query execution only for the last shuffle.



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