Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/17899#discussion_r115346396
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -609,6 +610,19 @@ object CollapseWindow extends Rule[LogicalPlan] {
}
/**
+ * Flip Adjacent Window Expressions.
+ * - If the partition spec of the parent Window expression is a subset of
the partition spec
+ * of the child window expression, flip them.
+ */
+object FlipWindow extends Rule[LogicalPlan] {
+ def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
+ case w1 @ Window(we1, ps1, os1, w2 @ Window(we2, ps2, os2, grandChild))
+ if ps2.containsSlice(ps1) =>
--- End diff --
This condition might not be enough. `we1` might depend on the outputs of
`we2`, right?
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