Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/18113#discussion_r150083215
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/aggregate/typedaggregators.scala
 ---
    @@ -26,43 +26,64 @@ import org.apache.spark.sql.expressions.Aggregator
     // This file defines internal implementations for aggregators.
     
////////////////////////////////////////////////////////////////////////////////////////////////////
     
    +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
     
    -class TypedSumDouble[IN](val f: IN => Double) extends Aggregator[IN, 
Double, Double] {
    -  override def zero: Double = 0.0
    --- End diff --
    
    so we will return 0.0 for empty input, let's not change this behavior


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