Github user rxin commented on the issue:
https://github.com/apache/spark/pull/17711
I feel both are pretty complicated. Can we just do something similar to
CombineUnion:
```
/**
* Combines all adjacent [[Union]] operators into a single [[Union]].
*/
object CombineUnions extends Rule[LogicalPlan] {
def apply(plan: LogicalPlan): LogicalPlan = plan transformDown {
case u: Union => flattenUnion(u, false)
case Distinct(u: Union) => Distinct(flattenUnion(u, true))
}
private def flattenUnion(union: Union, flattenDistinct: Boolean): Union =
{
val stack = mutable.Stack[LogicalPlan](union)
val flattened = mutable.ArrayBuffer.empty[LogicalPlan]
while (stack.nonEmpty) {
stack.pop() match {
case Distinct(Union(children)) if flattenDistinct =>
stack.pushAll(children.reverse)
case Union(children) =>
stack.pushAll(children.reverse)
case child =>
flattened += child
}
}
Union(flattened)
}
}```
It's going to be simpler because you don't need to handle distinct here.
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