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

    https://github.com/apache/spark/pull/17330#discussion_r107331403
  
    --- Diff: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/subquery.scala
 ---
    @@ -61,6 +63,37 @@ abstract class SubqueryExpression(
       }
     }
     
    +/**
    + * This expression is used to represent any form of subquery expression 
namely
    + * ListQuery, Exists and ScalarSubquery. This is only used to make sure the
    + * expression equality works properly when LogicalPlan.sameResult is called
    + * on plans containing SubqueryExpression(s). This is only a transient 
expression
    + * that only lives in the scope of sameResult function call. In other 
words, analyzer,
    + * optimizer or planner never sees this expression type during 
transformation of
    + * plans.
    + */
    +case class CanonicalizedSubqueryExpr(expr: SubqueryExpression)
    --- End diff --
    
    Do we really need this new abstraction?
    
    Actually I don't think we should compare `SubqueryExpression`s and wonder 
how to canonicalize them.
    
    Basically, in `sameResult`, by adding new condition `(left.subqueries, 
right.subqueries).zipped.forall(_ sameResult _)`, we can achieve the same goal 
and all added tests are passed.
    
    
    
    
    



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