Github user gatorsmile commented on a diff in the pull request:

    https://github.com/apache/spark/pull/17286#discussion_r106775016
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/CostBasedJoinReorder.scala
 ---
    @@ -19,31 +19,33 @@ package org.apache.spark.sql.catalyst.optimizer
     
     import scala.collection.mutable
     
    -import org.apache.spark.sql.catalyst.CatalystConf
     import org.apache.spark.sql.catalyst.expressions.{And, Attribute, 
AttributeSet, Expression, PredicateHelper}
     import org.apache.spark.sql.catalyst.plans.{Inner, InnerLike}
     import org.apache.spark.sql.catalyst.plans.logical.{BinaryNode, Join, 
LogicalPlan, Project}
     import org.apache.spark.sql.catalyst.rules.Rule
    +import org.apache.spark.sql.internal.SQLConf
     
     
     /**
      * Cost-based join reorder.
      * We may have several join reorder algorithms in the future. This class 
is the entry of these
      * algorithms, and chooses which one to use.
      */
    -case class CostBasedJoinReorder(conf: CatalystConf) extends 
Rule[LogicalPlan] with PredicateHelper {
    +case class CostBasedJoinReorder(conf: SQLConf) extends Rule[LogicalPlan] 
with PredicateHelper {
       def apply(plan: LogicalPlan): LogicalPlan = {
         if (!conf.cboEnabled || !conf.joinReorderEnabled) {
           plan
         } else {
    -      val result = plan transform {
    -        case p @ Project(projectList, j @ Join(_, _, _: InnerLike, _)) =>
    -          reorder(p, p.outputSet)
    -        case j @ Join(_, _, _: InnerLike, _) =>
    +      val result = plan transformDown {
    +        // Start reordering with a joinable item, which is an InnerLike 
join with conditions.
    +        case j @ Join(_, _, _: InnerLike, Some(cond)) =>
    --- End diff --
    
    I have a very general question about how to exclude cartesian product. See 
my old PR: #16762
    
    Having a join condition does not mean it is not a Cartesian product. : )
    
    Only having non-equal predicates in join conditions for an inner join. 
e.g., tab1.a < tab2.b


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