[SPARK-1819] [SQL] Fix GetField.nullable.

`GetField.nullable` should be `true` not only when `field.nullable` is `true` but also when `child.nullable` is `true`.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #757 from ueshin/issues/SPARK-1819 and squashes the following commits:

8781a11 [Takuya UESHIN] Modify a test to use named parameters.
5bfc77d [Takuya UESHIN] Fix GetField.nullable.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/complexTypes.scala

@@ -74,7 +74,7 @@ case class GetField(child: Expression, fieldName: String) extends UnaryExpressio
   type EvaluatedType = Any
 
   def dataType = field.dataType
-  override def nullable = field.nullable
+  override def nullable = child.nullable || field.nullable
   override def foldable = child.foldable
 
   protected def structType = child.dataType match {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala

@@ -364,6 +364,19 @@ class ExpressionEvaluationSuite extends FunSuite {
 
     checkEvaluation(GetField(BoundReference(2, AttributeReference("c", typeS)()), "a"), "aa", row)
     checkEvaluation(GetField(Literal(null, typeS), "a"), null, row)
+
+    val typeS_notNullable = StructType(
+      StructField("a", StringType, nullable = false)
+        :: StructField("b", StringType, nullable = false) :: Nil
+    )
+
+    assert(GetField(BoundReference(2,
+      AttributeReference("c", typeS)()), "a").nullable === true)
+    assert(GetField(BoundReference(2,
+      AttributeReference("c", typeS_notNullable, nullable = false)()), "a").nullable === false)
+
+    assert(GetField(Literal(null, typeS), "a").nullable === true)
+    assert(GetField(Literal(null, typeS_notNullable), "a").nullable === true)
   }
 
   test("arithmetic") {