Github user sethah commented on a diff in the pull request:
https://github.com/apache/spark/pull/9380#discussion_r43662570
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/functions.scala
---
@@ -1135,7 +992,76 @@ abstract class CentralMomentAgg(child: Expression)
extends ImperativeAggregate w
moments(4) = buffer.getDouble(fourthMomentOffset)
}
- getStatistic(n, mean, moments)
+ if (n == 0.0) null
+ else if (n == 1.0) 0.0
--- End diff --
I don't believe we want this behavior, since these edge cases should be
handled in the `getStatistic` implementation. If you see [previous
PR](https://github.com/apache/spark/pull/9003) we established that `Skewness`
and `Kurtosis` should yield `Double.NaN` when `n == 1.0` but other functions
like `VariancePop` should yield 0.0.
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