Github user maropu commented on the issue: https://github.com/apache/spark/pull/21668 One of refactoring ideas is to inject the functionality of `ReorderJoin`(=`StarSchemaDetection`) into `CostBasedJoinReorder`; In [the batch rule `Join Reorder` (`Once` strategy)](https://github.com/apache/spark/blob/7c08eb6d61d55ce45229f3302e6d463e7669183d/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala#L139), if `spark.sql.cbo.starSchemaDetection` enabled (false by default), the rule applies star schema detection first. If a fact table found, [dimension tables are reordered](https://github.com/apache/spark/blob/7c08eb6d61d55ce45229f3302e6d463e7669183d/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/StarSchemaDetection.scala#L342) by the cost-based algorithm. If `spark.sql.cbo.starSchemaDetection` disabled, the rule just uses `CostBasedJoinReorder`. Currently, we have `ReorderJoin`(=`StarSchemaDetection`) in the batch rule with `fixedPoint` strategy, so, I thnk that, if we could remove this rule from there, we would skip unnecessary checks caused by `ReorderJoin` per rule iteration. @cloud-fan WDYT?
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