Github user davies commented on a diff in the pull request:

    https://github.com/apache/spark/pull/11210#discussion_r53716771
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlan.scala
 ---
    @@ -316,6 +316,21 @@ abstract class UnaryNode extends LogicalPlan {
       override def children: Seq[LogicalPlan] = child :: Nil
     
       override protected def validConstraints: Set[Expression] = 
child.constraints
    +
    +  override def statistics: Statistics = {
    +    // There should be some overhead in Row object, the size should not be 
zero when there is
    +    // no columns, this help to prevent divide-by-zero error.
    +    val childRowSize = child.output.map(_.dataType.defaultSize).sum + 8
    +    val outputRowSize = output.map(_.dataType.defaultSize).sum + 8
    +    // Assume there will be the same number of rows as child has.
    +    var sizeInBytes = (child.statistics.sizeInBytes * outputRowSize) / 
childRowSize
    +    if (sizeInBytes == 0) {
    +      // sizeInBytes can't be zero, or sizeInBytes of BinaryNode will also 
be zero
    +      // (product of children).
    +      sizeInBytes = 1
    +    }
    --- End diff --
    
    I'd like to do that, but Statistics is an case class, I'd not to change the 
interface. Is there a good way to do that?


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