[SPARK-22141][FOLLOWUP][SQL] Add comments for the order of batches

## What changes were proposed in this pull request?
Add comments for specifying the position of  batch "Check Cartesian Products", as rxin suggested in apache#19362 .

## How was this patch tested?
Unit test

Author: Wang Gengliang <[email protected]>

Closes apache#19379 from gengliangwang/SPARK-22141-followup.

Author: gengliangwang

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -134,6 +134,7 @@ abstract class Optimizer(sessionCatalog: SessionCatalog)
     Batch("LocalRelation", fixedPoint,
       ConvertToLocalRelation,
       PropagateEmptyRelation) ::
+    // The following batch should be executed after batch "Join Reorder" and "LocalRelation".
     Batch("Check Cartesian Products", Once,
       CheckCartesianProducts) ::
     Batch("OptimizeCodegen", Once,
@@ -1089,6 +1090,9 @@ object CombineLimits extends Rule[LogicalPlan] {
  * SELECT * from R, S where R.r = S.s,
  * the join between R and S is not a cartesian product and therefore should be allowed.
  * The predicate R.r = S.s is not recognized as a join condition until the ReorderJoin rule.
+ *
+ * This rule must be run AFTER the batch "LocalRelation", since a join with empty relation should
+ * not be a cartesian product.
  */
 object CheckCartesianProducts extends Rule[LogicalPlan] with PredicateHelper {
   /**