Github user rxin commented on a diff in the pull request: https://github.com/apache/spark/pull/10577#discussion_r48793856 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala --- @@ -595,6 +598,22 @@ object BooleanSimplification extends Rule[LogicalPlan] with PredicateHelper { } /** + * Combines all adjacent [[Union]] and [[Unions]] operators into a single [[Unions]]. + */ +object CombineUnions extends Rule[LogicalPlan] { + private def collectUnionChildren(plan: LogicalPlan): Seq[LogicalPlan] = plan match { + case Union(l, r) => collectUnionChildren(l) ++ collectUnionChildren(r) --- End diff -- you should write this without using recursion to avoid stack overflow.
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