[GitHub] [spark] cloud-fan commented on a change in pull request #32014: [SPARK-34922][SQL] Use a relative cost comparison function in the CBO

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cloud-fan commented on a change in pull request #32014:
URL: https://github.com/apache/spark/pull/32014#discussion_r604879142



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File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/CostBasedJoinReorder.scala
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@@ -349,11 +349,14 @@ object JoinReorderDP extends PredicateHelper with Logging {
     }
 
     def betterThan(other: JoinPlan, conf: SQLConf): Boolean = {
-      val thisCost = BigDecimal(this.planCost.card) * conf.joinReorderCardWeight +
-        BigDecimal(this.planCost.size) * (1 - conf.joinReorderCardWeight)
-      val otherCost = BigDecimal(other.planCost.card) * conf.joinReorderCardWeight +
-        BigDecimal(other.planCost.size) * (1 - conf.joinReorderCardWeight)
-      thisCost < otherCost
+      if (other.planCost.card == 0 || other.planCost.size == 0) {
+        false
+      } else {
+        val relativeRows = BigDecimal(this.planCost.card) / BigDecimal(other.planCost.card)
+        val relativeSize = BigDecimal(this.planCost.size) / BigDecimal(other.planCost.size)
+        Math.pow(relativeRows.doubleValue(), conf.joinReorderCardWeight) *

Review comment:
       shall we update the config doc?




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