Github user hvanhovell commented on a diff in the pull request:
https://github.com/apache/spark/pull/15317#discussion_r81433195
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -538,6 +539,19 @@ object CollapseRepartition extends Rule[LogicalPlan] {
}
/**
+ * Collapse Adjacent Window Expression.
+ * - Collapse only if they have the same partition specs and order specs.
+ */
+object CollapseWindow extends Rule[LogicalPlan] {
+ def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
+ case w @ Window(we1, ps1, os1, Window(we2, ps2, os2, grandChild))
+ if ps1.equals(ps2) && os1.equals(os2) =>
--- End diff --
`equals`? That is only needed in Java, `==` is the same in Scala.
You can be a bit more lenient with respect to the sort order. The sort
order of the window collapsed window must be match the prefix of the window in
which it is collapsed, for instance a window with sort order `a desc`, can be
collapsed into a window with sort order `a desc`, `b asc`.
Note that if we widen the ordering constraint, then we should also support
the inverse case, by collapsing the parent window in to the child window.
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