[SPARK-9086][SQL] Remove BinaryNode from TreeNode.

These traits are not super useful, and yet cause problems with toString in expressions due to the orders they are mixed in.

Author: Reynold Xin <rxin@databricks.com>

Closes #7433 from rxin/remove-binary-node and squashes the following commits:

1881f78 [Reynold Xin] [SPARK-9086][SQL] Remove BinaryNode from TreeNode.

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

@@ -187,8 +187,10 @@ abstract class Expression extends TreeNode[Expression] {
 /**
  * A leaf expression, i.e. one without any child expressions.
  */
-abstract class LeafExpression extends Expression with trees.LeafNode[Expression] {
+abstract class LeafExpression extends Expression {
   self: Product =>
+
+  def children: Seq[Expression] = Nil
 }
 
 
@@ -196,9 +198,13 @@ abstract class LeafExpression extends Expression with trees.LeafNode[Expression]
  * An expression with one input and one output. The output is by default evaluated to null
  * if the input is evaluated to null.
  */
-abstract class UnaryExpression extends Expression with trees.UnaryNode[Expression] {
+abstract class UnaryExpression extends Expression {
   self: Product =>
 
+  def child: Expression
+
+  override def children: Seq[Expression] = child :: Nil
+
   override def foldable: Boolean = child.foldable
   override def nullable: Boolean = child.nullable
 
@@ -271,9 +277,14 @@ abstract class UnaryExpression extends Expression with trees.UnaryNode[Expressio
  * An expression with two inputs and one output. The output is by default evaluated to null
  * if any input is evaluated to null.
  */
-abstract class BinaryExpression extends Expression with trees.BinaryNode[Expression] {
+abstract class BinaryExpression extends Expression {
   self: Product =>
 
+  def left: Expression
+  def right: Expression
+
+  override def children: Seq[Expression] = Seq(left, right)
+
   override def foldable: Boolean = left.foldable && right.foldable
 
   override def nullable: Boolean = left.nullable || right.nullable

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/LogicalPlan.scala

@@ -291,6 +291,11 @@ abstract class UnaryNode extends LogicalPlan with trees.UnaryNode[LogicalPlan] {
 /**
  * A logical plan node with a left and right child.
  */
-abstract class BinaryNode extends LogicalPlan with trees.BinaryNode[LogicalPlan] {
+abstract class BinaryNode extends LogicalPlan {
   self: Product =>
+
+  def left: LogicalPlan
+  def right: LogicalPlan
+
+  override def children: Seq[LogicalPlan] = Seq(left, right)
 }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/trees/TreeNode.scala

@@ -453,15 +453,6 @@ abstract class TreeNode[BaseType <: TreeNode[BaseType]] {
   }
 }
 
-/**
- * A [[TreeNode]] that has two children, [[left]] and [[right]].
- */
-trait BinaryNode[BaseType <: TreeNode[BaseType]] {
-  def left: BaseType
-  def right: BaseType
-
-  def children: Seq[BaseType] = Seq(left, right)
-}
 
 /**
  * A [[TreeNode]] with no children.

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala

@@ -247,6 +247,11 @@ private[sql] trait UnaryNode extends SparkPlan with trees.UnaryNode[SparkPlan] {
   override def outputPartitioning: Partitioning = child.outputPartitioning
 }
 
-private[sql] trait BinaryNode extends SparkPlan with trees.BinaryNode[SparkPlan] {
+private[sql] trait BinaryNode extends SparkPlan {
   self: Product =>
+
+  def left: SparkPlan
+  def right: SparkPlan
+
+  override def children: Seq[SparkPlan] = Seq(left, right)
 }