Github user hvanhovell commented on a diff in the pull request:
https://github.com/apache/spark/pull/12421#discussion_r61241988
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/execution/WindowExec.scala ---
@@ -177,6 +177,8 @@ case class WindowExec(
case e @ WindowExpression(function, spec) =>
val frame =
spec.frameSpecification.asInstanceOf[SpecifiedWindowFrame]
function match {
+ case MakeDecimal(AggregateExpression(f, _, _, _), prec, scale)
=>
--- End diff --
When does a `WindowExpression` contain a `MakeDecimal`? As I understand
your code correctly, the following things can happen:
##### Sum
The following input:
```Scala
WindowExpression(AggregateExpression(Sum(x), ...), spec)
```
is rewritten into:
```scala
MakeDecimal(WindowExpression(AggregateExpression(Sum(UnscaledValue(x)),
...), spec), prec + 10, scale)
```
##### Average
The following input:
```Scala
WindowExpression(AggregateExpression(Average(x), ...), spec)
```
is rewritten into:
```scala
Cast(Divide(WindowExpression(AggregateExpression(Average(UnscaledValue(x)),
...), spec), Literal.create(math.pow(10.0, scale), DoubleType)),
DecimalType(prec + 4, scale + 4))
```
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