[SPARK-15308][SQL] RowEncoder should preserve nested column name.

## What changes were proposed in this pull request?

The following code generates wrong schema:

```
val schema = new StructType().add(
  "struct",
  new StructType()
    .add("i", IntegerType, nullable = false)
    .add(
      "s",
      new StructType().add("int", IntegerType, nullable = false),
      nullable = false),
  nullable = false)
val ds = sqlContext.range(10).map(l => Row(l, Row(l)))(RowEncoder(schema))
ds.printSchema()
```

This should print as follows:

```
root
 |-- struct: struct (nullable = false)
 |    |-- i: integer (nullable = false)
 |    |-- s: struct (nullable = false)
 |    |    |-- int: integer (nullable = false)
```

but the result is:

```
root
 |-- struct: struct (nullable = false)
 |    |-- col1: integer (nullable = false)
 |    |-- col2: struct (nullable = false)
 |    |    |-- col1: integer (nullable = false)
```

This PR fixes `RowEncoder` to preserve nested column name.

## How was this patch tested?

Existing tests and I added a test to check if `RowEncoder` preserves nested column name.

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

Closes #13090 from ueshin/issues/SPARK-15308.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/encoders/RowEncoder.scala

@@ -62,7 +62,7 @@ object RowEncoder {
     new ExpressionEncoder[Row](
       schema,
       flat = false,
-      serializer.asInstanceOf[CreateStruct].children,
+      serializer.asInstanceOf[CreateNamedStruct].flatten,
       deserializer,
       ClassTag(cls))
   }
@@ -148,28 +148,30 @@ object RowEncoder {
         dataType = t)
 
     case StructType(fields) =>
-      val convertedFields = fields.zipWithIndex.map { case (f, i) =>
+      val nonNullOutput = CreateNamedStruct(fields.zipWithIndex.flatMap { case (field, index) =>
         val fieldValue = serializerFor(
-          GetExternalRowField(inputObject, i, f.name, externalDataTypeForInput(f.dataType)),
-          f.dataType
+          GetExternalRowField(
+            inputObject, index, field.name, externalDataTypeForInput(field.dataType)),
+          field.dataType
         )
-        if (f.nullable) {
+        val convertedField = if (field.nullable) {
           If(
-            Invoke(inputObject, "isNullAt", BooleanType, Literal(i) :: Nil),
-            Literal.create(null, f.dataType),
+            Invoke(inputObject, "isNullAt", BooleanType, Literal(index) :: Nil),
+            Literal.create(null, field.dataType),
             fieldValue
           )
         } else {
           fieldValue
         }
-      }
+        Literal(field.name) :: convertedField :: Nil
+      })
 
       if (inputObject.nullable) {
         If(IsNull(inputObject),
           Literal.create(null, inputType),
-          CreateStruct(convertedFields))
+          nonNullOutput)
       } else {
-        CreateStruct(convertedFields)
+        nonNullOutput
       }
   }
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/encoders/RowEncoderSuite.scala

@@ -185,6 +185,28 @@ class RowEncoderSuite extends SparkFunSuite {
     assert(encoder.serializer.head.nullable == false)
   }
 
+  test("RowEncoder should preserve nested column name") {
+    val schema = new StructType().add(
+      "struct",
+      new StructType()
+        .add("i", IntegerType, nullable = false)
+        .add(
+          "s",
+          new StructType().add("int", IntegerType, nullable = false),
+          nullable = false),
+      nullable = false)
+    val encoder = RowEncoder(schema)
+    assert(encoder.serializer.length == 1)
+    assert(encoder.serializer.head.dataType ==
+      new StructType()
+      .add("i", IntegerType, nullable = false)
+      .add(
+        "s",
+        new StructType().add("int", IntegerType, nullable = false),
+        nullable = false))
+    assert(encoder.serializer.head.nullable == false)
+  }
+
   test("RowEncoder should support array as the external type for ArrayType") {
     val schema = new StructType()
       .add("array", ArrayType(IntegerType))