Github user sethah commented on a diff in the pull request:
https://github.com/apache/spark/pull/9003#discussion_r42889558
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/functions.scala
---
@@ -930,3 +930,327 @@ object HyperLogLogPlusPlus {
)
// scalastyle:on
}
+
+/**
+ * A central moment is the expected value of a specified power of the
deviation of a random
+ * variable from the mean. Central moments are often used to characterize
the properties of about
+ * the shape of a distribution.
+ *
+ * This class implements online, one-pass algorithms for computing the
central moments of a set of
+ * points.
+ *
+ * References:
+ * - Xiangrui Meng. "Simpler Online Updates for Arbitrary-Order Central
Moments."
+ * 2015. http://arxiv.org/abs/1510.04923
+ *
+ * @see [[https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance
+ * Algorithms for calculating variance (Wikipedia)]]
+ *
+ * @param child to compute central moments of.
+ */
+abstract class CentralMomentAgg(child: Expression) extends
ImperativeAggregate with Serializable {
+
+ /**
+ * The maximum central moment order to be computed.
+ */
+ protected def momentOrder: Int
+
+ /**
+ * Array of sufficient moments need to compute the aggregate statistic.
+ */
+ protected def sufficientMoments: Array[Int]
--- End diff --
Removed this def and instead pass all moments up to the maximum moment to
the `getStatistic` function.
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