Zhan Zhang created SPARK-20215:
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             Summary: ReuseExchange is boken in SparkSQL
                 Key: SPARK-20215
                 URL: https://issues.apache.org/jira/browse/SPARK-20215
             Project: Spark
          Issue Type: Bug
          Components: SQL
    Affects Versions: 2.1.0
            Reporter: Zhan Zhang
            Priority: Minor


Currently if we have query like: A join B Union A join C... with the same join 
key. Table A will be scanned multiple times in sql. It is because the 
megastoreRelation are not shared by two joins, and ExprId is different.  
canonicalized in Expression will not be able to unify them and two Exchange 
will not compatible and cannot be reused.



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