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

    https://github.com/apache/spark/pull/23248#discussion_r239565253
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/python/ExtractPythonUDFs.scala
 ---
    @@ -131,8 +131,20 @@ object ExtractPythonUDFs extends Rule[LogicalPlan] 
with PredicateHelper {
         expressions.flatMap(collectEvaluableUDFs)
       }
     
    -  def apply(plan: LogicalPlan): LogicalPlan = plan transformUp {
    -    case plan: LogicalPlan => extract(plan)
    +  def apply(plan: LogicalPlan): LogicalPlan = plan match {
    +    // SPARK-26293: A subquery will be rewritten into join later, and will 
go through this rule
    +    // eventually. Here we skip subquery, as Python UDF only needs to be 
extracted once.
    +    case _: Subquery => plan
    --- End diff --
    
    Personally I found it a bit confusing when two seeming unrelated things are 
put together (Subquery and ExtractPythonUDFs).
    
    I wonder if it's sufficient to make ExtractPythonUDFs idempotent?


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