Github user gatorsmile commented on a diff in the pull request:
https://github.com/apache/spark/pull/10451#discussion_r48579360
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -153,6 +153,17 @@ object SetOperationPushDown extends Rule[LogicalPlan]
with PredicateHelper {
)
)
+ // Adding extra Limit below UNION ALL iff both left and right childs
are not Limit and no Limit
+ // was pushed down before. This heuristic is valid assuming there does
not exist any Limit
+ // push-down rule that is unable to infer the value of maxRows. Any
operator that a Limit can
+ // be pushed passed should override this function.
+ case Limit(exp, Union(left, right))
+ if left.maxRows.isEmpty || right.maxRows.isEmpty =>
--- End diff --
Below is the example. If one side has a limit child/descendant, we still
can push it down to reduce the number of returned rows.
https://github.com/gatorsmile/spark/blob/unionLimit/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/optimizer/PushdownLimitsSuite.scala#L50-L57
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