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  1. Spark
  2. SPARK-27619

MapType should be prohibited in hash expressions



    • Type: Bug
    • Status: Resolved
    • Priority: Blocker
    • Resolution: Fixed
    • Affects Version/s: 2.3.0, 2.3.4, 2.4.0, 2.4.1, 2.4.2, 2.4.3, 2.4.4, 3.0.0
    • Fix Version/s: 3.0.0
    • Component/s: SQL
    • Labels:
    • Target Version/s:


      Spark currently allows MapType expressions to be used as input to hash expressions, but I think that this should be prohibited because Spark SQL does not support map equality.

      Currently, Spark SQL's map hashcodes are sensitive to the insertion order of map elements:

      val a = spark.createDataset(Map(1->1, 2->2) :: Nil)
      val b = spark.createDataset(Map(2->2, 1->1) :: Nil)
      // Demonstration of how Scala Map equality is unaffected by insertion order:
      assert(Map(1->1, 2->2).hashCode() == Map(2->2, 1->1).hashCode())
      assert(Map(1->1, 2->2) == Map(2->2, 1->1))
      assert(a.first() == b.first())
      // In contrast, this will print two different hashcodes:
      println(Seq(a, b).map(_.selectExpr("hash(*)").first()))

      This behavior might be surprising to Scala developers.

      I think there's precedence for banning the use of MapType here because we already prohibit MapType in aggregation / joins / equality comparisons (SPARK-9415) and set operations (SPARK-19893).

      If we decide that we want this to be an error then it might also be a good idea to add a spark.sql.legacy flag as an escape-hatch to re-enable the old and buggy behavior (in case applications were relying on it in cases where it just so happens to be safe-by-accident (e.g. maps which only have one entry)).

      Alternatively, we could support hashing here if we implemented support for comparable map types (SPARK-18134).




            • Assignee:
              rakson Rakesh Raushan
              joshrosen Josh Rosen
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