Github user yhuai commented on a diff in the pull request:
https://github.com/apache/spark/pull/10454#discussion_r48386195
--- 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 --
I feel we will generate wrong results. It is not safe to push down the
limit if we are not joining on the foreign key with the primary key, right? For
example, for a left outer join, we push down the limit to the right table. It
is possible all rows returned by the right side are having the same join column
value.
am I missing anything?
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