diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregates.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregates.scala
index 5caf032fdf8a8..613b87ca98d97 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregates.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregates.scala
@@ -203,15 +203,6 @@ case class CollectHashSetFunction(
   @transient
   val distinctValue = new InterpretedProjection(expr)
 
-/*
-  override def merge(other: MergableAggregateFunction): MergableAggregateFunction = {
-    val otherSetIterator = other.asInstanceOf[CountDistinctFunction].seen.iterator
-    while(otherSetIterator.hasNext) {
-      seen.add(otherSetIterator.next())
-    }
-    this
-  }*/
-
   override def update(input: Row): Unit = {
     val evaluatedExpr = distinctValue(input)
     if (!evaluatedExpr.anyNull) {
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala
index d9784882de805..923a9b1445d6b 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala
@@ -97,7 +97,9 @@ case class MaxOf(left: Expression, right: Expression) extends Expression {
 
   override def dataType = left.dataType
 
-  override def eval(input: Row): Any = ???
+  override def eval(input: Row): Any = {
+    val leftEval = left.eval(input)
+  }
 
   override def toString = s"MaxOf($left, $right)"
 }
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/sets.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/sets.scala
index 14a95328c0a3a..b7433b48b82af 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/sets.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/sets.scala
@@ -20,6 +20,9 @@ package org.apache.spark.sql.catalyst.expressions
 import org.apache.spark.sql.catalyst.types._
 import org.apache.spark.util.collection.OpenHashSet
 
+/**
+ * Creates a new set of the specified type
+ */
 case class NewSet(elementType: DataType) extends LeafExpression {
   type EvaluatedType = Any
 
@@ -27,7 +30,8 @@ case class NewSet(elementType: DataType) extends LeafExpression {
 
   def nullable = false
 
-  // This is not completely accurate..
+  // We are currently only using these Expressions internally for aggregation.  However, if we ever
+  // expose these to users we'll want to create a proper type instead of hijacking ArrayType.
   def dataType = ArrayType(elementType)
 
   def eval(input: Row): Any = {
@@ -37,7 +41,10 @@ case class NewSet(elementType: DataType) extends LeafExpression {
   override def toString = s"new Set($dataType)"
 }
 
-// THIS MUTATES ITS ARUGMENTS
+/**
+ * Adds an item to a set.
+ * For performance, this expression mutates its input during evaluation.
+ */
 case class AddItemToSet(item: Expression, set: Expression) extends Expression {
   type EvaluatedType = Any
 
@@ -67,7 +74,10 @@ case class AddItemToSet(item: Expression, set: Expression) extends Expression {
   override def toString = s"$set += $item"
 }
 
-// THIS MUTATES ITS ARUGMENTS
+/**
+ * Combines the elements of two sets.
+ * For performance, this expression mutates its left input set during evaluation.
+ */
 case class CombineSets(left: Expression, right: Expression) extends BinaryExpression {
   type EvaluatedType = Any
 
@@ -97,6 +107,9 @@ case class CombineSets(left: Expression, right: Expression) extends BinaryExpres
   }
 }
 
+/**
+ * Returns the number of elements in the input set.
+ */
 case class CountSet(child: Expression) extends UnaryExpression {
   type EvaluatedType = Any
 
@@ -112,4 +125,4 @@ case class CountSet(child: Expression) extends UnaryExpression {
   }
 
   override def toString = s"$child.count()"
-}
\ No newline at end of file
+}
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/GeneratedAggregate.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/GeneratedAggregate.scala
index 39a96b2dd218f..17418726aa625 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/GeneratedAggregate.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/GeneratedAggregate.scala
@@ -108,7 +108,6 @@ case class GeneratedAggregate(
         val currentMax = AttributeReference("currentMax", expr.dataType, nullable = true)()
         val initialValue = Literal(null, expr.dataType)
         val updateMax = MaxOf(currentMax, expr)
-          //If(IsNull(currentMax), expr, If(GreaterThan(currentMax, expr), currentMax, expr))
 
         AggregateEvaluation(
           currentMax :: Nil,
@@ -128,8 +127,9 @@ case class GeneratedAggregate(
           set)
 
       case CombineSetsAndCount(inputSet) =>
+        val ArrayType(inputType) = inputSet.dataType
         val set = AttributeReference("hashSet", inputSet.dataType, nullable = false)()
-        val initialValue = NewSet(IntegerType) // NOT TRUE
+        val initialValue = NewSet(inputType)
         val collectSets = CombineSets(set, inputSet)
 
         AggregateEvaluation(
diff --git a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkSqlSerializer.scala b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkSqlSerializer.scala
index 1c5995f82d1e1..4956948e83914 100644
--- a/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkSqlSerializer.scala
+++ b/sql/core/src/main/scala/org/apache/spark/sql/execution/SparkSqlSerializer.scala
@@ -45,10 +45,13 @@ private[sql] class SparkSqlSerializer(conf: SparkConf) extends KryoSerializer(co
     kryo.register(classOf[com.clearspring.analytics.stream.cardinality.HyperLogLog],
                   new HyperLogLogSerializer)
     kryo.register(classOf[scala.math.BigDecimal], new BigDecimalSerializer)
-    // Specific hashset must come first
+
+    // Specific hashsets must come first
     kryo.register(classOf[IntegerHashSet], new IntegerHashSetSerializer)
     kryo.register(classOf[LongHashSet], new LongHashSetSerializer)
-    kryo.register(classOf[org.apache.spark.util.collection.OpenHashSet[_]], new OpenHashSetSerializer)
+    kryo.register(classOf[org.apache.spark.util.collection.OpenHashSet[_]],
+                  new OpenHashSetSerializer)
+
     kryo.setReferences(false)
     kryo.setClassLoader(Utils.getSparkClassLoader)
     new AllScalaRegistrar().apply(kryo)
@@ -188,4 +191,4 @@ private[sql] class LongHashSetSerializer extends Serializer[LongHashSet] {
     }
     set
   }
-}
\ No newline at end of file
+}