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

    https://github.com/apache/spark/pull/16255#discussion_r95981143
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala
 ---
    @@ -200,6 +200,8 @@ object RemoveAliasOnlyProject extends Rule[LogicalPlan] 
{
             case plan: Project if plan eq proj => plan.child
             case plan => plan transformExpressions {
               case a: Attribute if attrMap.contains(a) => attrMap(a)
    +          case b: Alias if attrMap.exists(_._1.exprId == b.exprId)
    +            && b.child.isInstanceOf[NamedExpression] => b.child
    --- End diff --
    
    RemoveAliasOnlyProject will remove  `alias a#1 to a#2`, and `replace all 
a#2 with a#1`, so there is no `a#2` exists, If we do nothing for `alias a#1 to 
a#2`(not the same object with the removed one), it will cause the exception 
situation from `step 5` to `step 6` showed on the above comment.


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