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

    https://github.com/apache/spark/pull/10577#discussion_r49048219
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicOperators.scala
 ---
    @@ -123,6 +115,43 @@ case class Except(left: LogicalPlan, right: 
LogicalPlan) extends SetOperation(le
       override def output: Seq[Attribute] = left.output
     }
     
    +/** Factory for constructing new `Union` nodes. */
    +object Union {
    +  def apply(left: LogicalPlan, right: LogicalPlan): Union = {
    +    Union (left :: right :: Nil)
    +  }
    +}
    +
    +case class Union(children: Seq[LogicalPlan]) extends LogicalPlan {
    +
    +  override def output: Seq[Attribute] = {
    +    // updating nullability to make all the children consistent
    +    children.tail.foldLeft(children.head.output) { case (currentOutput, 
child) =>
    +      currentOutput.zip(child.output).map { case (a1, a2) =>
    +        a1.withNullability(a1.nullable || a2.nullable)
    +      }
    --- End diff --
    
    Thank you!


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