Github user yhuai commented on a diff in the pull request:
https://github.com/apache/spark/pull/10454#discussion_r48387575
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
---
@@ -858,6 +859,30 @@ object PushPredicateThroughJoin extends
Rule[LogicalPlan] with PredicateHelper {
}
/**
+ * Push [[Limit]] operators through [[Join]] operators, iff the join type
is outer joins.
+ * Adding extra [[Limit]] operators on top of the outer-side
child/children.
+ */
+object PushLimitThroughOuterJoin extends Rule[LogicalPlan] with
PredicateHelper {
--- End diff --
Can you explain why the full outer join is good?
Yes, the returned results of a limit will not be deterministic unless there
is a sort.
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