[SPARK-18284][SQL] Make ExpressionEncoder.serializer.nullable precise

## What changes were proposed in this pull request?

This PR makes `ExpressionEncoder.serializer.nullable` for flat encoder for a primitive type `false`. Since it is `true` for now, it is too conservative.
While `ExpressionEncoder.schema` has correct information (e.g. `<IntegerType, false>`), `serializer.head.nullable` of `ExpressionEncoder`, which got from `encoderFor[T]`, is always false. It is too conservative.

This is accomplished by checking whether a type is one of primitive types. If it is `true`, `nullable` should be `false`.

## How was this patch tested?

Added new tests for encoder and dataframe

Author: Kazuaki Ishizaki <ishizaki@jp.ibm.com>

Closes #15780 from kiszk/SPARK-18284.

6 files changed, 44 insertions, 19 deletions

diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/JavaTypeInference.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/JavaTypeInference.scala
index 04f0cfce88..7e8e4dab72 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/JavaTypeInference.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/JavaTypeInference.scala
@@ -396,12 +396,14 @@ object JavaTypeInference {
 
         case _ if mapType.isAssignableFrom(typeToken) =>
           val (keyType, valueType) = mapKeyValueType(typeToken)
+
           ExternalMapToCatalyst(
             inputObject,
             ObjectType(keyType.getRawType),
             serializerFor(_, keyType),
             ObjectType(valueType.getRawType),
-            serializerFor(_, valueType)
+            serializerFor(_, valueType),
+            valueNullable = true
           )
 
         case other =>
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala
index 0aa21b9347..6e20096901 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala
@@ -498,7 +498,8 @@ object ScalaReflection extends ScalaReflection {
           dataTypeFor(keyType),
           serializerFor(_, keyType, keyPath),
           dataTypeFor(valueType),
-          serializerFor(_, valueType, valuePath))
+          serializerFor(_, valueType, valuePath),
+          valueNullable = !valueType.typeSymbol.asClass.isPrimitive)
 
       case t if t <:< localTypeOf[String] =>
         StaticInvoke(
@@ -590,7 +591,9 @@ object ScalaReflection extends ScalaReflection {
               "cannot be used as field name\n" + walkedTypePath.mkString("\n"))
           }
 
-          val fieldValue = Invoke(inputObject, fieldName, dataTypeFor(fieldType))
+          val fieldValue = Invoke(
+            AssertNotNull(inputObject, walkedTypePath), fieldName, dataTypeFor(fieldType),
+            returnNullable = !fieldType.typeSymbol.asClass.isPrimitive)
           val clsName = getClassNameFromType(fieldType)
           val newPath = s"""- field (class: "$clsName", name: "$fieldName")""" +: walkedTypePath
           expressions.Literal(fieldName) :: serializerFor(fieldValue, fieldType, newPath) :: Nil
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoder.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoder.scala
index 9c4818db63..3757eccfa2 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoder.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoder.scala
@@ -60,7 +60,7 @@ object ExpressionEncoder {
     val cls = mirror.runtimeClass(tpe)
     val flat = !ScalaReflection.definedByConstructorParams(tpe)
 
-    val inputObject = BoundReference(0, ScalaReflection.dataTypeFor[T], nullable = true)
+    val inputObject = BoundReference(0, ScalaReflection.dataTypeFor[T], nullable = !cls.isPrimitive)
     val nullSafeInput = if (flat) {
       inputObject
     } else {
@@ -71,10 +71,7 @@ object ExpressionEncoder {
     val serializer = ScalaReflection.serializerFor[T](nullSafeInput)
     val deserializer = ScalaReflection.deserializerFor[T]
 
-    val schema = ScalaReflection.schemaFor[T] match {
-      case ScalaReflection.Schema(s: StructType, _) => s
-      case ScalaReflection.Schema(dt, nullable) => new StructType().add("value", dt, nullable)
-    }
+    val schema = serializer.dataType
 
     new ExpressionEncoder[T](
       schema,
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/ReferenceToExpressions.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/ReferenceToExpressions.scala
index 6c75a7a502..2ca77e8394 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/ReferenceToExpressions.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/ReferenceToExpressions.scala
@@ -74,7 +74,7 @@ case class ReferenceToExpressions(result: Expression, children: Seq[Expression])
         ctx.addMutableState("boolean", classChildVarIsNull, "")
 
         val classChildVar =
-          LambdaVariable(classChildVarName, classChildVarIsNull, child.dataType)
+          LambdaVariable(classChildVarName, classChildVarIsNull, child.dataType, child.nullable)
 
         val initCode = s"${classChildVar.value} = ${childGen.value};\n" +
           s"${classChildVar.isNull} = ${childGen.isNull};"
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala
index e517ec18eb..a8aa1e7255 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/objects/objects.scala
@@ -171,15 +171,18 @@ case class StaticInvoke(
  * @param arguments An optional list of expressions, whos evaluation will be passed to the function.
  * @param propagateNull When true, and any of the arguments is null, null will be returned instead
  *                      of calling the function.
+ * @param returnNullable When false, indicating the invoked method will always return
+ *                       non-null value.
  */
 case class Invoke(
     targetObject: Expression,
     functionName: String,
     dataType: DataType,
     arguments: Seq[Expression] = Nil,
-    propagateNull: Boolean = true) extends InvokeLike {
+    propagateNull: Boolean = true,
+    returnNullable : Boolean = true) extends InvokeLike {
 
-  override def nullable: Boolean = true
+  override def nullable: Boolean = targetObject.nullable || needNullCheck || returnNullable
   override def children: Seq[Expression] = targetObject +: arguments
 
   override def eval(input: InternalRow): Any =
@@ -405,13 +408,15 @@ case class WrapOption(child: Expression, optType: DataType)
  * A place holder for the loop variable used in [[MapObjects]].  This should never be constructed
  * manually, but will instead be passed into the provided lambda function.
  */
-case class LambdaVariable(value: String, isNull: String, dataType: DataType) extends LeafExpression
+case class LambdaVariable(
+    value: String,
+    isNull: String,
+    dataType: DataType,
+    nullable: Boolean = true) extends LeafExpression
   with Unevaluable with NonSQLExpression {
 
-  override def nullable: Boolean = true
-
   override def genCode(ctx: CodegenContext): ExprCode = {
-    ExprCode(code = "", value = value, isNull = isNull)
+    ExprCode(code = "", value = value, isNull = if (nullable) isNull else "false")
   }
 }
 
@@ -592,7 +597,8 @@ object ExternalMapToCatalyst {
       keyType: DataType,
       keyConverter: Expression => Expression,
       valueType: DataType,
-      valueConverter: Expression => Expression): ExternalMapToCatalyst = {
+      valueConverter: Expression => Expression,
+      valueNullable: Boolean): ExternalMapToCatalyst = {
     val id = curId.getAndIncrement()
     val keyName = "ExternalMapToCatalyst_key" + id
     val valueName = "ExternalMapToCatalyst_value" + id
@@ -601,11 +607,11 @@ object ExternalMapToCatalyst {
     ExternalMapToCatalyst(
       keyName,
       keyType,
-      keyConverter(LambdaVariable(keyName, "false", keyType)),
+      keyConverter(LambdaVariable(keyName, "false", keyType, false)),
       valueName,
       valueIsNull,
       valueType,
-      valueConverter(LambdaVariable(valueName, valueIsNull, valueType)),
+      valueConverter(LambdaVariable(valueName, valueIsNull, valueType, valueNullable)),
       inputMap
     )
   }
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoderSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoderSuite.scala
index 4d896c2e38..080f11b769 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoderSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/encoders/ExpressionEncoderSuite.scala
@@ -24,7 +24,7 @@ import java.util.Arrays
 import scala.collection.mutable.ArrayBuffer
 import scala.reflect.runtime.universe.TypeTag
 
-import org.apache.spark.sql.Encoders
+import org.apache.spark.sql.{Encoder, Encoders}
 import org.apache.spark.sql.catalyst.{OptionalData, PrimitiveData}
 import org.apache.spark.sql.catalyst.analysis.AnalysisTest
 import org.apache.spark.sql.catalyst.dsl.plans._
@@ -300,6 +300,11 @@ class ExpressionEncoderSuite extends PlanTest with AnalysisTest {
   encodeDecodeTest(
     ReferenceValueClass(ReferenceValueClass.Container(1)), "reference value class")
 
+  encodeDecodeTest(Option(31), "option of int")
+  encodeDecodeTest(Option.empty[Int], "empty option of int")
+  encodeDecodeTest(Option("abc"), "option of string")
+  encodeDecodeTest(Option.empty[String], "empty option of string")
+
   productTest(("UDT", new ExamplePoint(0.1, 0.2)))
 
   test("nullable of encoder schema") {
@@ -338,6 +343,18 @@ class ExpressionEncoderSuite extends PlanTest with AnalysisTest {
     }
   }
 
+  test("nullable of encoder serializer") {
+    def checkNullable[T: Encoder](nullable: Boolean): Unit = {
+      assert(encoderFor[T].serializer.forall(_.nullable === nullable))
+    }
+
+    // test for flat encoders
+    checkNullable[Int](false)
+    checkNullable[Option[Int]](true)
+    checkNullable[java.lang.Integer](true)
+    checkNullable[String](true)
+  }
+
   test("null check for map key") {
     val encoder = ExpressionEncoder[Map[String, Int]]()
     val e = intercept[RuntimeException](encoder.toRow(Map(("a", 1), (null, 2))))