Github user jiangxb1987 commented on a diff in the pull request: https://github.com/apache/spark/pull/20345#discussion_r163113728 --- Diff: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/joins.scala --- @@ -84,19 +84,46 @@ object ReorderJoin extends Rule[LogicalPlan] with PredicateHelper { } } + private def extractLeftDeepInnerJoins(plan: LogicalPlan): Seq[LogicalPlan] = plan match { --- End diff -- It took me a little while to understand that this can handle `(a join b) join c` versus `a join (b join c)` correctly. Would be great if we can explain how it works in the function comment.
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