Github user mengxr commented on a diff in the pull request:
https://github.com/apache/spark/pull/10960#discussion_r51533703
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/CentralMomentAgg.scala
---
@@ -57,173 +55,170 @@ abstract class CentralMomentAgg(child: Expression)
extends ImperativeAggregate w
override def dataType: DataType = DoubleType
- override def inputTypes: Seq[AbstractDataType] = Seq(NumericType)
+ override def inputTypes: Seq[AbstractDataType] = Seq(DoubleType)
+
+ protected val count = AttributeReference("count", DoubleType, nullable =
false)()
+ protected val avg = AttributeReference("avg", DoubleType, nullable =
false)()
+ protected val m2 = AttributeReference("m2", DoubleType, nullable =
false)()
+ protected val m3 = AttributeReference("m3", DoubleType, nullable =
false)()
+ protected val m4 = AttributeReference("m4", DoubleType, nullable =
false)()
+
+ override val aggBufferAttributes = Seq(count, avg, m2, m3,
m4).take(momentOrder + 1)
+
+ override val initialValues: Seq[Expression] = Seq(
+ /* count = */ Literal(0.0),
+ /* avg = */ Literal(0.0),
+ /* m2 = */ Literal(0.0),
+ /* m3 = */ Literal(0.0),
+ /* m4 = */ Literal(0.0)
+ ).take(momentOrder + 1)
+
+ override lazy val updateExpressions: Seq[Expression] = {
+ val n = count + Literal(1.0)
+ val delta = child - avg
+ val deltaN = delta / n
+ val newAvg = avg + deltaN
+ val newM2 = m2 + delta * (delta - deltaN)
+
+ val delta2 = delta * delta
+ val deltaN2 = deltaN * deltaN
+ val newM3 = m3 - Literal(3.0) * deltaN * newM2 + delta * (delta2 -
deltaN2)
+
+ val newM4 = m4 - Literal(4.0) * deltaN * newM3 - Literal(6.0) *
deltaN2 * newM2 +
+ delta * (delta * delta2 - deltaN * deltaN2)
+
+ if (child.nullable) {
+ Seq(
+ /* count = */ If(IsNull(child), count, n),
--- End diff --
The comment `/* count = */` seems unnecessary to me because it is clear
from the variable name.
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