diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Projection.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Projection.scala
index 7af42295373b5..ef1d12531f109 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Projection.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/Projection.scala
@@ -156,6 +156,12 @@ class JoinedRow extends Row {
 
 /**
  * JIT HACK: Replace with macros
+ * The `JoinedRow` class is used in many performance critical situation.  Unfortunately, since there
+ * are multiple different types of `Rows` that could be stored as `row1` and `row2` most of the
+ * calls in the critical path are polymorphic.  By creating special versions of this class that are
+ * used in only a single location of the code, we increase the chance that only a single type of
+ * Row will be referenced, increasing the opportunity for the JIT to play tricks.  This sounds
+ * crazy but in benchmarks it had noticeable effects.
  */
 class JoinedRow2 extends Row {
   private[this] var row1: Row = _
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SpecificRow.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SpecificRow.scala
index 4a7a563cce5e8..75ea0e8459df8 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SpecificRow.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/SpecificRow.scala
@@ -20,42 +20,43 @@ package org.apache.spark.sql.catalyst.expressions
 import org.apache.spark.sql.catalyst.types._
 
 /**
+ * A parent class for mutable container objects that are reused when the values are changed,
+ * resulting in less garbage.  These values are held by a [[SpecificMutableRow]].
  *
+ * The following code was roughly used to generate these objects:
+ * {{{
+ * val types = "Int,Float,Boolean,Double,Short,Long,Byte,Any".split(",")
+ * types.map {tpe =>
+ * s"""
+ * final class Mutable$tpe extends MutableValue {
+ *   var value: $tpe = 0
+ *   def boxed = if (isNull) null else value
+ *   def update(v: Any) = value = {
+ *     isNull = false
+ *     v.asInstanceOf[$tpe]
+ *   }
+ *   def copy() = {
+ *     val newCopy = new Mutable$tpe
+ *     newCopy.isNull = isNull
+ *     newCopy.value = value
+ *     newCopy.asInstanceOf[this.type]
+ *   }
+ * }"""
+ * }.foreach(println)
  *
+ * types.map { tpe =>
+ * s"""
+ *   override def set$tpe(ordinal: Int, value: $tpe): Unit = {
+ *     val currentValue = values(ordinal).asInstanceOf[Mutable$tpe]
+ *     currentValue.isNull = false
+ *     currentValue.value = value
+ *   }
  *
-{{{
-val types = "Int,Float,Boolean,Double,Short,Long,Byte,Any".split(",")
-types.map {tpe =>
-s"""
-final class Mutable$tpe extends MutableValue {
-  var value: $tpe = 0
-  def boxed = if (isNull) null else value
-  def update(v: Any) = value = {
-    isNull = false
-    v.asInstanceOf[$tpe]
-  }
-  def copy() = {
-    val newCopy = new Mutable$tpe
-    newCopy.isNull = isNull
-    newCopy.value = value
-    newCopy.asInstanceOf[this.type]
-  }
-}"""
-}.foreach(println)
-
-types.map { tpe =>
-s"""
-  override def set$tpe(ordinal: Int, value: $tpe): Unit = {
-    val currentValue = values(ordinal).asInstanceOf[Mutable$tpe]
-    currentValue.isNull = false
-    currentValue.value = value
-  }
-
-  override def get$tpe(i: Int): $tpe = {
-    values(i).asInstanceOf[Mutable$tpe].value
-  }"""
-}.foreach(println)
-}}}
+ *   override def get$tpe(i: Int): $tpe = {
+ *     values(i).asInstanceOf[Mutable$tpe].value
+ *   }"""
+ * }.foreach(println)
+ * }}}
  */
 abstract class MutableValue extends Serializable {
   var isNull: Boolean = true
@@ -184,7 +185,12 @@ final class MutableAny extends MutableValue {
   }
 }
 
-class SpecificMutableRow(val values: Array[MutableValue]) extends MutableRow {
+/**
+ * A row type that holds an array specialized container objects, of type [[MutableValue]], chosen
+ * based on the dataTypes of each column.  The intent is to decrease garbage when modifying the
+ * values of primitive columns.
+ */
+final class SpecificMutableRow(val values: Array[MutableValue]) extends MutableRow {
 
   def this(dataTypes: Seq[DataType]) =
     this(
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 923a9b1445d6b..8d90614e4501a 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
@@ -93,12 +93,25 @@ case class MaxOf(left: Expression, right: Expression) extends Expression {
 
   override def children = left :: right :: Nil
 
-  override def references = (left.flatMap(_.references) ++ right.flatMap(_.references)).toSet
+  override def references = left.references ++ right.references
 
   override def dataType = left.dataType
 
   override def eval(input: Row): Any = {
     val leftEval = left.eval(input)
+    val rightEval = right.eval(input)
+    if (leftEval == null) {
+      rightEval
+    } else if (rightEval == null) {
+      leftEval
+    } else {
+      val numeric = left.dataType.asInstanceOf[NumericType].numeric.asInstanceOf[Numeric[Any]]
+      if (numeric.compare(leftEval, rightEval) < 0) {
+        rightEval
+      } else {
+        leftEval
+      }
+    }
   }
 
   override def toString = s"MaxOf($left, $right)"
diff --git a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala
index 7362d3d6b0e7c..f06b8c78a1be9 100644
--- a/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala
+++ b/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala
@@ -26,6 +26,7 @@ import org.apache.spark.sql.catalyst.expressions
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.types._
 
+// These classes are here to avoid issues with serialization and integration with quasiquotes.
 class IntegerHashSet extends org.apache.spark.util.collection.OpenHashSet[Int]
 class LongHashSet extends org.apache.spark.util.collection.OpenHashSet[Long]
 
@@ -53,6 +54,11 @@ abstract class CodeGenerator[InType <: AnyRef, OutType <: AnyRef] extends Loggin
   private val curId = new java.util.concurrent.atomic.AtomicInteger()
   private val javaSeparator = "$"
 
+  /**
+   * Can be flipped on manually in the console to add (expensive) expression evaluation trace code.
+   */
+  var debugLogging = false
+
   /**
    * Generates a class for a given input expression.  Called when there is not cached code
    * already available.
@@ -496,7 +502,7 @@ abstract class CodeGenerator[InType <: AnyRef, OutType <: AnyRef] extends Loggin
 
     // Only inject debugging code if debugging is turned on.
     val debugCode =
-      if (false) {
+      if (debugLogging) {
         val localLogger = log
         val localLoggerTree = reify { localLogger }
         q"""
diff --git a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala
index 999c9fff38d60..f1df817c41362 100644
--- a/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala
+++ b/sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ExpressionEvaluationSuite.scala
@@ -136,6 +136,16 @@ class ExpressionEvaluationSuite extends FunSuite {
     checkEvaluation(In(Literal(1), Seq(Literal(1), Literal(2))) && In(Literal(2), Seq(Literal(1), Literal(2))), true)
   }
 
+  test("MaxOf") {
+    checkEvaluation(MaxOf(1, 2), 2)
+    checkEvaluation(MaxOf(2, 1), 2)
+    checkEvaluation(MaxOf(1L, 2L), 2L)
+    checkEvaluation(MaxOf(2L, 1L), 2L)
+
+    checkEvaluation(MaxOf(Literal(null, IntegerType), 2), 2)
+    checkEvaluation(MaxOf(2, Literal(null, IntegerType)), 2)
+  }
+
   test("LIKE literal Regular Expression") {
     checkEvaluation(Literal(null, StringType).like("a"), null)
     checkEvaluation(Literal("a", StringType).like(Literal(null, StringType)), null)
diff --git a/sql/hive/src/test/scala/org/apache/spark/sql/hive/StatisticsSuite.scala b/sql/hive/src/test/scala/org/apache/spark/sql/hive/StatisticsSuite.scala
index 206f514e9de00..2653871c2a7eb 100644
--- a/sql/hive/src/test/scala/org/apache/spark/sql/hive/StatisticsSuite.scala
+++ b/sql/hive/src/test/scala/org/apache/spark/sql/hive/StatisticsSuite.scala
@@ -158,7 +158,7 @@ class StatisticsSuite extends QueryTest with BeforeAndAfterAll {
         case r if ct.runtimeClass.isAssignableFrom(r.getClass) => r.statistics.sizeInBytes
       }
       assert(sizes.size === 2 && sizes(0) <= autoBroadcastJoinThreshold,
-        s"query should contain two relations, each of which has size smaller than autoConvertSize instead ${rdd.queryExecution}")
+        s"query should contain two relations, each of which has size smaller than autoConvertSize")
 
       // Using `sparkPlan` because for relevant patterns in HashJoin to be
       // matched, other strategies need to be applied.