Github user hvanhovell commented on a diff in the pull request:
https://github.com/apache/spark/pull/8587#discussion_r43162271
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/functions.scala
---
@@ -524,6 +525,133 @@ case class Sum(child: Expression) extends
DeclarativeAggregate {
override val evaluateExpression = Cast(currentSum, resultType)
}
+/**
+ * Compute Pearson correlation between two expressions.
+ * When applied on empty data (i.e., count is zero), it returns NaN.
+ *
+ * Definition of Pearson correlation can be found at
+ *
http://en.wikipedia.org/wiki/Pearson_product-moment_correlation_coefficient
+ *
+ * @param left one of the expressions to compute correlation with.
+ * @param right another expression to compute correlation with.
+ */
+case class Corr(
+ left: Expression,
+ right: Expression,
+ mutableAggBufferOffset: Int = 0,
+ inputAggBufferOffset: Int = 0)
+ extends ImperativeAggregate {
+
+ def children: Seq[Expression] = Seq(left, right)
+
+ def nullable: Boolean = false
+
+ def dataType: DataType = DoubleType
+
+ def inputTypes: Seq[AbstractDataType] = Seq(DoubleType)
+
+ def aggBufferSchema: StructType =
StructType.fromAttributes(aggBufferAttributes)
+
+ def inputAggBufferAttributes: Seq[AttributeReference] =
aggBufferAttributes.map(_.newInstance())
+
+ val aggBufferAttributes: Seq[AttributeReference] = Seq(
+ AttributeReference("xAvg", DoubleType)(),
+ AttributeReference("yAvg", DoubleType)(),
+ AttributeReference("Ck", DoubleType)(),
+ AttributeReference("MkX", DoubleType)(),
+ AttributeReference("MkY", DoubleType)(),
+ AttributeReference("count", LongType)())
+
+ override def withNewMutableAggBufferOffset(newMutableAggBufferOffset:
Int): ImperativeAggregate =
+ copy(mutableAggBufferOffset = newMutableAggBufferOffset)
+
+ override def withNewInputAggBufferOffset(newInputAggBufferOffset: Int):
ImperativeAggregate =
+ copy(inputAggBufferOffset = newInputAggBufferOffset)
+
+ override def initialize(buffer: MutableRow): Unit = {
+ (0 until 5).map(idx => buffer.setDouble(mutableAggBufferOffset + idx,
0.0))
+ buffer.setLong(mutableAggBufferOffset + 5, 0L)
+ }
+
+ override def update(buffer: MutableRow, input: InternalRow): Unit = {
+ val x = left.eval(input).asInstanceOf[Double]
+ val y = right.eval(input).asInstanceOf[Double]
+
+ var xAvg = buffer.getDouble(mutableAggBufferOffset)
+ var yAvg = buffer.getDouble(mutableAggBufferOffset + 1)
+ var Ck = buffer.getDouble(mutableAggBufferOffset + 2)
+ var MkX = buffer.getDouble(mutableAggBufferOffset + 3)
+ var MkY = buffer.getDouble(mutableAggBufferOffset + 4)
+ var count = buffer.getLong(mutableAggBufferOffset + 5)
+
+ val deltaX = x - xAvg
+ val deltaY = y - yAvg
+ count += 1
+ xAvg += deltaX / count
+ yAvg += deltaY / count
+ Ck += deltaX * (y - yAvg)
+ MkX += deltaX * (x - xAvg)
+ MkY += deltaY * (y - yAvg)
+
+ buffer.setDouble(mutableAggBufferOffset, xAvg)
+ buffer.setDouble(mutableAggBufferOffset + 1, yAvg)
+ buffer.setDouble(mutableAggBufferOffset + 2, Ck)
+ buffer.setDouble(mutableAggBufferOffset + 3, MkX)
+ buffer.setDouble(mutableAggBufferOffset + 4, MkY)
+ buffer.setLong(mutableAggBufferOffset + 5, count)
+ }
+
+ // Merge counters from other partitions. Formula can be found at:
+ // http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance
+ override def merge(buffer1: MutableRow, buffer2: InternalRow): Unit = {
+ val count2 = buffer2.getLong(inputAggBufferOffset + 5)
+
+ if (count2 > 0) {
--- End diff --
Is it safe to assume that the ```count2``` in ```buffer1``` is non zero?
There is - currently - no documentation on this.
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