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

    https://github.com/apache/spark/pull/17302#discussion_r107090057
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/ExistingRDD.scala ---
    @@ -70,7 +70,20 @@ object RDDConversions {
     object ExternalRDD {
     
       def apply[T: Encoder](rdd: RDD[T], session: SparkSession): LogicalPlan = 
{
    -    val externalRdd = ExternalRDD(CatalystSerde.generateObjAttr[T], 
rdd)(session)
    +    val attr = {
    +      val attr = CatalystSerde.generateObjAttr[T]
    --- End diff --
    
    When we called `deserializer.nullable` at `CatalystSerde.generateObjAttr` 
with the following change in the calling context of `ExternalRDD.apply()`, the 
following error occurs. I think that `ExternalRDD.apply()` calls 
`CatalystSerde.generateObjAttr` at very early phase where a plan has not been 
resolved yet.
    
    ```java
    object CatalystSerde {
      def deserialize[T : Encoder](child: LogicalPlan): DeserializeToObject = {
        val deserializer = UnresolvedDeserializer(encoderFor[T].deserializer)
        DeserializeToObject(deserializer, generateObjAttr[T], child)
      }
    
      def serialize[T : Encoder](child: LogicalPlan): SerializeFromObject = {
        SerializeFromObject(encoderFor[T].namedExpressions, child)
      }
    
      def generateObjAttr[T : Encoder]: Attribute = {
        val deserializer = encoderFor[T].deserializer
        AttributeReference("obj", deserializer.dataType, 
deserializer.nullable)()
      }
    }
    ```
    
    ```java
    Invalid call to nullable on unresolved object, tree: getcolumnbyordinal(0, 
IntegerType)
    org.apache.spark.sql.catalyst.analysis.UnresolvedException: Invalid call to 
nullable on unresolved object, tree: getcolumnbyordinal(0, IntegerType)
        at 
org.apache.spark.sql.catalyst.analysis.GetColumnByOrdinal.nullable(unresolved.scala:399)
        at 
org.apache.spark.sql.catalyst.expressions.UnaryExpression.nullable(Expression.scala:314)
        at 
org.apache.spark.sql.catalyst.expressions.objects.InvokeLike$$anonfun$needNullCheck$1.apply(objects.scala:44)
        at 
org.apache.spark.sql.catalyst.expressions.objects.InvokeLike$$anonfun$needNullCheck$1.apply(objects.scala:44)
        at 
scala.collection.LinearSeqOptimized$class.exists(LinearSeqOptimized.scala:93)
        at scala.collection.immutable.List.exists(List.scala:84)
        at 
org.apache.spark.sql.catalyst.expressions.objects.InvokeLike$class.needNullCheck(objects.scala:44)
        at 
org.apache.spark.sql.catalyst.expressions.objects.NewInstance.needNullCheck$lzycompute(objects.scala:290)
        at 
org.apache.spark.sql.catalyst.expressions.objects.NewInstance.needNullCheck(objects.scala:290)
        at 
org.apache.spark.sql.catalyst.expressions.objects.NewInstance.nullable(objects.scala:298)
        at 
org.apache.spark.sql.catalyst.plans.logical.CatalystSerde$.generateObjAttr(object.scala:45)
        at 
org.apache.spark.sql.execution.ExternalRDD$.apply(ExistingRDD.scala:76)
        at 
org.apache.spark.sql.SparkSession.createDataset(SparkSession.scala:471)
        at org.apache.spark.sql.SQLContext.createDataset(SQLContext.scala:393)
        at 
org.apache.spark.sql.SQLImplicits.rddToDatasetHolder(SQLImplicits.scala:238)
        at 
org.apache.spark.sql.DataFrameImplicitsSuite$$anonfun$6.apply$mcV$sp(DataFrameImplicitsSuite.scala:56)
        at 
org.apache.spark.sql.DataFrameImplicitsSuite$$anonfun$6.apply(DataFrameImplicitsSuite.scala:55)
        at 
org.apache.spark.sql.DataFrameImplicitsSuite$$anonfun$6.apply(DataFrameImplicitsSuite.scala:55)
        at 
org.scalatest.Transformer$$anonfun$apply$1.apply$mcV$sp(Transformer.scala:22)
        at org.scalatest.OutcomeOf$class.outcomeOf(OutcomeOf.scala:85)
        at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
        at org.scalatest.Transformer.apply(Transformer.scala:22)
        at org.scalatest.Transformer.apply(Transformer.scala:20)
        at org.scalatest.FunSuiteLike$$anon$1.apply(FunSuiteLike.scala:166)
        at org.apache.spark.SparkFunSuite.withFixture(SparkFunSuite.scala:68)
        at 
org.scalatest.FunSuiteLike$class.invokeWithFixture$1(FunSuiteLike.scala:163)
        at 
org.scalatest.FunSuiteLike$$anonfun$runTest$1.apply(FunSuiteLike.scala:175)
        at 
org.scalatest.FunSuiteLike$$anonfun$runTest$1.apply(FunSuiteLike.scala:175)
        at org.scalatest.SuperEngine.runTestImpl(Engine.scala:306)
        at org.scalatest.FunSuiteLike$class.runTest(FunSuiteLike.scala:175)
        at 
org.apache.spark.sql.DataFrameImplicitsSuite.org$scalatest$BeforeAndAfterEach$$super$runTest(DataFrameImplicitsSuite.scala:22)
        at 
org.scalatest.BeforeAndAfterEach$class.runTest(BeforeAndAfterEach.scala:255)
        at 
org.apache.spark.sql.DataFrameImplicitsSuite.runTest(DataFrameImplicitsSuite.scala:22)
    ...
    ```
    
    Here is an example program with plans.
    ```java
    val dfInt = sparkContext.parallelize(Seq[java.lang.Integer](0, null, 2), 
1).toDF
    dfInt.explain(true)
    assert(dfInt.collect === Array(Row(0), Row(null), Row(2)))
    ```
    
    ```
    
    == Parsed Logical Plan ==
    SerializeFromObject [input[0, java.lang.Integer, true].intValue AS value#2]
    +- ExternalRDD [obj#1]
    
    == Analyzed Logical Plan ==
    value: int
    SerializeFromObject [input[0, java.lang.Integer, true].intValue AS value#2]
    +- ExternalRDD [obj#1]
    
    == Optimized Logical Plan ==
    SerializeFromObject [input[0, java.lang.Integer, true].intValue AS value#2]
    +- ExternalRDD [obj#1]
    
    == Physical Plan ==
    *SerializeFromObject [input[0, java.lang.Integer, true].intValue AS value#2]
    +- Scan ExternalRDDScan[obj#1]
    ```


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