Github user chenghao-intel commented on a diff in the pull request:

    https://github.com/apache/spark/pull/6938#discussion_r33032169
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/math.scala
 ---
    @@ -312,3 +315,90 @@ case class Logarithm(left: Expression, right: 
Expression)
         """
       }
     }
    +
    +case class Round(child: Expression, scale: Expression) extends Expression {
    +
    +  def this(child: Expression) = {
    +    this(child, Literal(0))
    +  }
    +
    +  def children: Seq[Expression] = Seq(child, scale)
    +
    +  def nullable: Boolean = true
    +
    +  private lazy val scaleV = scale.asInstanceOf[Literal].value
    +  private lazy val _scale = if (scaleV != null) scaleV.asInstanceOf[Int] 
else 0
    +
    +  override lazy val dataType: DataType = {
    +    child.dataType match {
    +      case StringType | BinaryType => DoubleType
    +      case DecimalType.Fixed(p, s) => DecimalType(p, _scale)
    +      case t => t
    +    }
    +  }
    +
    +  override def checkInputDataTypes(): TypeCheckResult = {
    +    child.dataType match {
    +      case _: NumericType | NullType | BinaryType | StringType => // 
satisfy requirement
    +      case dt =>
    +        return TypeCheckFailure(s"Only numeric, string or binary data 
types" +
    +          s" are allowed for ROUND function, got $dt")
    +    }
    +    scale match {
    +      case Literal(value, LongType) =>
    +        if (value.asInstanceOf[Long] < Int.MinValue || 
value.asInstanceOf[Long] > Int.MaxValue) {
    +          return TypeCheckFailure("ROUND scale argument out of allowed 
range")
    +        }
    +      case Literal(_, _: IntegralType) | Literal(_, NullType) => // 
satisfy requirement
    +      case child =>
    +        if (child.find { case _: AttributeReference => true; case _ => 
false } != None) {
    +          return TypeCheckFailure("Only Integral Literal or Null Literal " 
+
    +            s"are allowed for ROUND scale arguments, got 
${child.dataType}")
    +        }
    +    }
    +    TypeCheckSuccess
    +  }
    +
    +  def eval(input: InternalRow): Any = {
    +    val evalE = child.eval(input)
    +
    +    if (evalE == null || scaleV == null) return null
    +
    +    children(0).dataType match {
    --- End diff --
    
    An optimal way to this move the data type pattern matching out of `eval`, 
and create the function for the `round`, so we can avoid pattern matching for 
every row.


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