update code according to the code review comment

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

@@ -222,6 +222,11 @@ abstract class Expression extends TreeNode[Expression] {
   }
 }
 
+/**
+ * Root class for rewritten 2 operands UDF expression. By default, we assume it produces Null if 
+ * either one of its operands is null. Exceptional case requires to update the optimization rule 
+ * at [[optimizer.ConstantFolding ConstantFolding]]
+ */
 abstract class BinaryExpression extends Expression with trees.BinaryNode[Expression] {
   self: Product =>
 
@@ -238,6 +243,11 @@ abstract class LeafExpression extends Expression with trees.LeafNode[Expression]
   self: Product =>
 }
 
+/**
+ * Root class for rewritten single operand UDF expression. By default, we assume it produces Null if 
+ * its operand is null. Exceptional case requires to update the optimization rule 
+ * at [[optimizer.ConstantFolding ConstantFolding]]
+ */
 abstract class UnaryExpression extends Expression with trees.UnaryNode[Expression] {
   self: Product =>
 

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

@@ -18,6 +18,7 @@
 package org.apache.spark.sql.catalyst.expressions
 
 import org.apache.spark.sql.catalyst.errors.TreeNodeException
+import org.apache.spark.sql.catalyst.trees
 
 abstract sealed class SortDirection
 case object Ascending extends SortDirection
@@ -27,7 +28,10 @@ case object Descending extends SortDirection
  * An expression that can be used to sort a tuple.  This class extends expression primarily so that
  * transformations over expression will descend into its child.
  */
-case class SortOrder(child: Expression, direction: SortDirection) extends UnaryExpression {
+case class SortOrder(child: Expression, direction: SortDirection) extends Expression 
+    with trees.UnaryNode[Expression] {
+
+  override def references = child.references
   override def dataType = child.dataType
   override def nullable = child.nullable
 

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -26,6 +26,7 @@ import org.apache.spark.sql.catalyst.types._
 object Optimizer extends RuleExecutor[LogicalPlan] {
   val batches =
     Batch("ConstantFolding", Once,
+      NullPropagation,
       ConstantFolding,
       BooleanSimplification,
       SimplifyFilters,
@@ -87,23 +88,18 @@ object ColumnPruning extends Rule[LogicalPlan] {
 
 /**
  * Replaces [[catalyst.expressions.Expression Expressions]] that can be statically evaluated with
- * equivalent [[catalyst.expressions.Literal Literal]] values.
+ * equivalent [[catalyst.expressions.Literal Literal]] values. This rule is more specific with 
+ * Null value propagation from bottom to top of the expression tree.
  */
-object ConstantFolding extends Rule[LogicalPlan] {
+object NullPropagation extends Rule[LogicalPlan] {
   def apply(plan: LogicalPlan): LogicalPlan = plan transform {
     case q: LogicalPlan => q transformExpressionsUp {
       // Skip redundant folding of literals.
       case l: Literal => l
-      // if it's foldable
-      case e if e.foldable => Literal(e.eval(null), e.dataType)
       case e @ Count(Literal(null, _)) => Literal(null, e.dataType)
       case e @ Sum(Literal(null, _)) => Literal(null, e.dataType)
       case e @ Average(Literal(null, _)) => Literal(null, e.dataType)
-      case e @ IsNull(Literal(null, _)) => Literal(true, BooleanType)
-      case e @ IsNull(Literal(_, _)) => Literal(false, BooleanType)
       case e @ IsNull(c @ Rand) => Literal(false, BooleanType)
-      case e @ IsNotNull(Literal(null, _)) => Literal(false, BooleanType)
-      case e @ IsNotNull(Literal(_, _)) => Literal(true, BooleanType)
       case e @ IsNotNull(c @ Rand) => Literal(true, BooleanType)
       case e @ GetItem(Literal(null, _), _) => Literal(null, e.dataType)
       case e @ GetItem(_, Literal(null, _)) => Literal(null, e.dataType)
@@ -113,10 +109,10 @@ object ConstantFolding extends Rule[LogicalPlan] {
           case Literal(null, _) => false
           case _ => true
         })
-        if(newChildren.length == null) {
+        if(newChildren.length == 0) {
           Literal(null, e.dataType)
-        } else if(newChildren.length == children.length){
-          e
+        } else if(newChildren.length == 1) {
+          newChildren(0)
         } else {
           Coalesce(newChildren)
         }
@@ -126,9 +122,8 @@ object ConstantFolding extends Rule[LogicalPlan] {
           case Literal(candidate, _) if(candidate == v) => true
           case _ => false
         })) => Literal(true, BooleanType)
-
-      case e @ SortOrder(_, _) => e
-      // put exceptional cases(Unary & Binary Expression) before here.
+      // Put exceptional cases(Unary & Binary Expression if it doesn't produce null with constant 
+      // null operand) before here.
       case e: UnaryExpression => e.child match {
         case Literal(null, _) => Literal(null, e.dataType)
         case _ => e
@@ -141,6 +136,19 @@ object ConstantFolding extends Rule[LogicalPlan] {
     }
   }
 }
+/**
+ * Replaces [[catalyst.expressions.Expression Expressions]] that can be statically evaluated with
+ * equivalent [[catalyst.expressions.Literal Literal]] values.
+ */
+object ConstantFolding extends Rule[LogicalPlan] {
+  def apply(plan: LogicalPlan): LogicalPlan = plan transform {
+    case q: LogicalPlan => q transformExpressionsDown {
+      // Skip redundant folding of literals.
+      case l: Literal => l
+      case e if e.foldable => Literal(e.eval(null), e.dataType)
+    }
+  }
+}
 
 /**
  * Simplifies boolean expressions where the answer can be determined without evaluating both sides.