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

    https://github.com/apache/spark/pull/18113#discussion_r118932092
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/typedaggregators.scala
 ---
    @@ -99,3 +97,67 @@ class TypedAverage[IN](val f: IN => Double) extends 
Aggregator[IN, (Double, Long
         toColumn.asInstanceOf[TypedColumn[IN, java.lang.Double]]
       }
     }
    +
    +class TypedMinDouble[IN](val f: IN => Double) extends Aggregator[IN, 
Double, Double] {
    +  override def zero: Double = Double.PositiveInfinity
    --- End diff --
    
    A modified version of existing `TypedSumDouble` to use `java.lang.Double` 
as `BUF` and `OUT` looks like below. It returns `null` when applying on an 
empty dataset.
    
        class TypedSumDouble[IN](val f: IN => Double)
              extends Aggregator[IN, java.lang.Double, java.lang.Double] {
          override def zero: java.lang.Double = null
          override def reduce(b: java.lang.Double, a: IN): java.lang.Double = {
            if (b == null) {
              f(a)
            } else {
              b + f(a)
            }
          }
          override def merge(b1: java.lang.Double, b2: java.lang.Double): 
java.lang.Double = {
            if (b1 == null) {
              b2
            } else if (b2 == null) {
              b1
            } else {
              b1 + b2
            }
          }
          override def finish(reduction: java.lang.Double): java.lang.Double = {
            reduction
          }
          override def bufferEncoder: Encoder[java.lang.Double] = 
ExpressionEncoder[java.lang.Double]()
          override def outputEncoder: Encoder[java.lang.Double] = 
ExpressionEncoder[java.lang.Double]()
    
          // Java api support
          def this(f: MapFunction[IN, java.lang.Double]) = this(x => f.call(x))
    
          def toColumnJava: TypedColumn[IN, java.lang.Double] = {
            toColumn.asInstanceOf[TypedColumn[IN, java.lang.Double]]
          }
        }



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