Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/17286#discussion_r106582668
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/CostBasedJoinReorder.scala
 ---
    @@ -185,11 +184,14 @@ object JoinReorderDP extends PredicateHelper {
               // Should not join two overlapping item sets.
               if 
(oneSidePlan.itemIds.intersect(otherSidePlan.itemIds).isEmpty) {
                 val joinPlan = buildJoin(oneSidePlan, otherSidePlan, conf, 
conditions, topOutput)
    -            // Check if it's the first plan for the item set, or it's a 
better plan than
    -            // the existing one due to lower cost.
    -            val existingPlan = nextLevel.get(joinPlan.itemIds)
    -            if (existingPlan.isEmpty || 
joinPlan.cost.lessThan(existingPlan.get.cost)) {
    -              nextLevel.update(joinPlan.itemIds, joinPlan)
    +            if (joinPlan.isDefined) {
    --- End diff --
    
    when will this condition be false?


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