Github user heary-cao commented on a diff in the pull request:

    https://github.com/apache/spark/pull/19251#discussion_r143377744
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/statsEstimation/SizeInBytesOnlyStatsPlanVisitor.scala
 ---
    @@ -32,12 +31,14 @@ object SizeInBytesOnlyStatsPlanVisitor extends 
LogicalPlanVisitor[Statistics] {
        * same as the output row number, and compute sizes based on the column 
types.
        */
       private def visitUnaryNode(p: UnaryNode): 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 = p.child.output.map(_.dataType.defaultSize).sum + 8
    -    val outputRowSize = p.output.map(_.dataType.defaultSize).sum + 8
    --- End diff --
    
    Does 8 represent the overhead of each row of data?


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