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

    https://github.com/apache/spark/pull/18113#discussion_r118821077
  
    --- 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 --
    
    Hi, thanks for having a look. This is actually not an issue because on an 
empty dataset, nothing is returned. For more details, you could have a look a 
the existing tests: the 'agg' function is called on a 'KeyValueGroupedDataset' 
object, which is returned by the 'groupByKey'. This ensures it's only done per 
key.
    
    I have added an additional unit test to demonstrate.
    
    Regarding Double.PositiveInfinity, I could change it to Double.Max, to be 
in line with Long.Max if you'd prefer that. I personally think Infinity makes 
more sense, although that is inconsistent with Long.Max because 
Long.PositiiveInfinity is not available 


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