[jira] [Created] (SPARK-21998) SortMergeJoinExec should calculate its outputOrdering independent of its children's outputOrdering

["Maryann Xue (JIRA)" <ji...@apache.org>]

Maryann Xue created SPARK-21998:
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             Summary: SortMergeJoinExec should calculate its outputOrdering independent of its children's outputOrdering
                 Key: SPARK-21998
                 URL: https://issues.apache.org/jira/browse/SPARK-21998
             Project: Spark
          Issue Type: Bug
          Components: SQL
    Affects Versions: 2.2.0
            Reporter: Maryann Xue
            Priority: Minor


Right now SortMergeJoinExec calculates its outputOrdering based on its children's outputOrdering, thus oftentimes the SortMergeJoinExec's outputOrdering is NOT correct until after EnsureRequirements, which happens at a rather late stage. As a result, potential optimizations that rely on the required/output orderings, like SPARK-18591, will not work for SortMergeJoinExec.
Unlike operators like Project or Filter, which simply preserve the ordering of their inputs, the SortMergeJoinExec has a behavior that generates a new ordering in its output regardless of the orderings of its children. I think the code below together with its comment is buggy.
{code}
  /**
   * For SMJ, child's output must have been sorted on key or expressions with the same order as
   * key, so we can get ordering for key from child's output ordering.
   */
  private def getKeyOrdering(keys: Seq[Expression], childOutputOrdering: Seq[SortOrder])
    : Seq[SortOrder] = {
    keys.zip(childOutputOrdering).map { case (key, childOrder) =>
      SortOrder(key, Ascending, childOrder.sameOrderExpressions + childOrder.child - key)
    }
  }
{code}



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