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

["jin xing (JIRA)" <ji...@apache.org>]

jin xing created SPARK-21509:
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             Summary:  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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