Github user mgaido91 commented on a diff in the pull request:

    https://github.com/apache/spark/pull/22518#discussion_r232733022
  
    --- Diff: sql/core/src/test/scala/org/apache/spark/sql/SubquerySuite.scala 
---
    @@ -1268,4 +1269,16 @@ class SubquerySuite extends QueryTest with 
SharedSQLContext {
           assert(getNumSortsInQuery(query5) == 1)
         }
       }
    +
    +  test("SPARK-25482: Reuse same Subquery in order to execute it only 
once") {
    +    withTempView("t1", "t2") {
    +      sql("create temporary view t1(a int) using parquet")
    +      sql("create temporary view t2(b int) using parquet")
    +      val plan = sql("select * from t2 where b > (select max(a) from t1)")
    --- End diff --
    
    > we could execute scan and subquery at the same time (
    
    is this really possible? My understanding is that subqueries are executed 
before the plan they belong to (in `SparkPlan.executeQuery`). So my 
understanding is that when a subquery is running, the rest of the query is not.


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