Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/10451#discussion_r48517911
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -153,6 +153,15 @@ object SetOperationPushDown extends Rule[LogicalPlan]
with PredicateHelper {
)
)
+ // Adding extra Limit below UNION ALL if both left and right childs
are not Limit.
+ // This heuristic is valid assuming there does not exist any Limit
push-down rule.
+ case Limit(exp, Union(left, right))
+ if left.maxRows.isEmpty || right.maxRows.isEmpty =>
--- End diff --
is `left.maxRows.isEmpty` equal to `check if left is a Limit`?
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