[SPARK-11727][SQL] Split ExpressionEncoder into FlatEncoder and ProductEncoder

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/ScalaReflection.scala

@@ -75,7 +75,7 @@ trait ScalaReflection {
    *
    * @see SPARK-5281
    */
-  private def localTypeOf[T: TypeTag]: `Type` = typeTag[T].in(mirror).tpe
+  def localTypeOf[T: TypeTag]: `Type` = typeTag[T].in(mirror).tpe
 
   /**
    * Returns the Spark SQL DataType for a given scala type.  Where this is not an exact mapping

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

@@ -0,0 +1,50 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.spark.sql.catalyst.encoders
+
+import scala.reflect.ClassTag
+import scala.reflect.runtime.universe.{typeTag, TypeTag}
+
+import org.apache.spark.sql.types.StructType
+import org.apache.spark.sql.catalyst.expressions.{Literal, CreateNamedStruct, BoundReference}
+import org.apache.spark.sql.catalyst.ScalaReflection
+
+object FlatEncoder {
+  import ScalaReflection.schemaFor
+  import ScalaReflection.dataTypeFor
+
+  def apply[T : TypeTag]: ExpressionEncoder[T] = {
+    // We convert the not-serializable TypeTag into StructType and ClassTag.
+    val tpe = typeTag[T].tpe
+    val mirror = typeTag[T].mirror
+    val cls = mirror.runtimeClass(tpe)
+    assert(!schemaFor(tpe).dataType.isInstanceOf[StructType])
+
+    val input = BoundReference(0, dataTypeFor(tpe), nullable = true)
+    val toRowExpression = CreateNamedStruct(
+      Literal("value") :: ProductEncoder.extractorFor(input, tpe) :: Nil)
+    val fromRowExpression = ProductEncoder.constructorFor(tpe)
+
+    new ExpressionEncoder[T](
+      toRowExpression.dataType,
+      flat = true,
+      toRowExpression.flatten,
+      fromRowExpression,
+      ClassTag[T](cls))
+  }
+}