[GitHub] cloud-fan commented on a change in pull request #23036: [SPARK-26065][SQL] Change query hint from a `LogicalPlan` to a field

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cloud-fan commented on a change in pull request #23036: [SPARK-26065][SQL] Change query hint from a `LogicalPlan` to a field
URL: https://github.com/apache/spark/pull/23036#discussion_r242574123
 
 

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 File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/CostBasedJoinReorder.scala
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 @@ -40,23 +40,33 @@ object CostBasedJoinReorder extends Rule[LogicalPlan] with PredicateHelper {
     if (!conf.cboEnabled || !conf.joinReorderEnabled) {
       plan
     } else {
+      // Use a map to track the hints on the join items. If a join relation turns out unchanged
 
 Review comment:
   how to define "unchanged"? If `(a join b) join c` becomes `(b join a) join c`, is there any hit we want to retain?

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