[SPARK-8948][SQL] Remove ExtractValueWithOrdinal abstract class

Also added more documentation for the file.

Author: Reynold Xin <rxin@databricks.com>

Closes #7316 from rxin/extract-value and squashes the following commits:

069cb7e [Reynold Xin] Removed ExtractValueWithOrdinal.
621b705 [Reynold Xin] Reverted a line.
11ebd6c [Reynold Xin] [Minor][SQL] Improve documentation for complex type extractors.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/ExtractValue.scala → sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/complexTypeExtractors.scala

@@ -25,6 +25,11 @@ import org.apache.spark.sql.catalyst.analysis._
 import org.apache.spark.sql.catalyst.expressions.codegen.{GeneratedExpressionCode, CodeGenContext}
 import org.apache.spark.sql.types._
 
+////////////////////////////////////////////////////////////////////////////////////////////////////
+// This file defines all the expressions to extract values out of complex types.
+// For example, getting a field out of an array, map, or struct.
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
 
 object ExtractValue {
   /**
@@ -73,11 +78,10 @@ object ExtractValue {
     }
   }
 
-  def unapply(g: ExtractValue): Option[(Expression, Expression)] = {
-    g match {
-      case o: ExtractValueWithOrdinal => Some((o.child, o.ordinal))
-      case s: ExtractValueWithStruct => Some((s.child, null))
-    }
+  def unapply(g: ExtractValue): Option[(Expression, Expression)] = g match {
+    case o: GetArrayItem => Some((o.child, o.ordinal))
+    case o: GetMapValue => Some((o.child, o.key))
+    case s: ExtractValueWithStruct => Some((s.child, null))
   }
 
   /**
@@ -117,6 +121,8 @@ abstract class ExtractValueWithStruct extends UnaryExpression with ExtractValue
 
 /**
  * Returns the value of fields in the Struct `child`.
+ *
+ * No need to do type checking since it is handled by [[ExtractValue]].
  */
 case class GetStructField(child: Expression, field: StructField, ordinal: Int)
   extends ExtractValueWithStruct {
@@ -142,6 +148,8 @@ case class GetStructField(child: Expression, field: StructField, ordinal: Int)
 
 /**
  * Returns the array of value of fields in the Array of Struct `child`.
+ *
+ * No need to do type checking since it is handled by [[ExtractValue]].
  */
 case class GetArrayStructFields(
     child: Expression,
@@ -178,25 +186,21 @@ case class GetArrayStructFields(
   }
 }
 
-abstract class ExtractValueWithOrdinal extends BinaryExpression with ExtractValue {
-  self: Product =>
+/**
+ * Returns the field at `ordinal` in the Array `child`.
+ *
+ * No need to do type checking since it is handled by [[ExtractValue]].
+ */
+case class GetArrayItem(child: Expression, ordinal: Expression)
+  extends BinaryExpression with ExtractValue {
 
-  def ordinal: Expression
-  def child: Expression
+  override def toString: String = s"$child[$ordinal]"
 
   override def left: Expression = child
   override def right: Expression = ordinal
 
   /** `Null` is returned for invalid ordinals. */
   override def nullable: Boolean = true
-  override def toString: String = s"$child[$ordinal]"
-}
-
-/**
- * Returns the field at `ordinal` in the Array `child`
- */
-case class GetArrayItem(child: Expression, ordinal: Expression)
-  extends ExtractValueWithOrdinal {
 
   override def dataType: DataType = child.dataType.asInstanceOf[ArrayType].elementType
 
@@ -227,10 +231,20 @@ case class GetArrayItem(child: Expression, ordinal: Expression)
 }
 
 /**
- * Returns the value of key `ordinal` in Map `child`
+ * Returns the value of key `ordinal` in Map `child`.
+ *
+ * No need to do type checking since it is handled by [[ExtractValue]].
  */
-case class GetMapValue(child: Expression, ordinal: Expression)
-  extends ExtractValueWithOrdinal {
+case class GetMapValue(child: Expression, key: Expression)
+  extends BinaryExpression with ExtractValue {
+
+  override def toString: String = s"$child[$key]"
+
+  override def left: Expression = child
+  override def right: Expression = key
+
+  /** `Null` is returned for invalid ordinals. */
+  override def nullable: Boolean = true
 
   override def dataType: DataType = child.dataType.asInstanceOf[MapType].valueType