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

    https://github.com/apache/flink/pull/4471#discussion_r132230160
  
    --- Diff: 
flink-libraries/flink-table/src/main/scala/org/apache/flink/table/plan/util/UpdatingPlanChecker.scala
 ---
    @@ -56,17 +59,20 @@ object UpdatingPlanChecker {
       }
     
       /** Identifies unique key fields in the output of a RelNode. */
    -  private class UniqueKeyExtractor extends RelVisitor {
    -
    -    var keys: Option[Array[String]] = None
    +  private class UniqueKeyExtractor {
     
    -    override def visit(node: RelNode, ordinal: Int, parent: RelNode): Unit 
= {
    +    // visit() function will return a tuple, the first element of tuple is 
the key, the second is
    +    // the key's corresponding ancestor. Ancestors are used to identify 
same keys, for example:
    +    // select('pk as pk1, 'pk as pk2), both pk1 and pk2 have the same 
ancestor, i.e., pk.
    +    // A node having keys means: 1.it generates keys by itself 2.all 
ancestors from it's upstream
    --- End diff --
    
    `A node can have keys if it generates the keys by itself or it forwards 
keys from its input(s).`?


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