[SQL] Improve debug logging and toStrings.

Author: Michael Armbrust <michael@databricks.com>

Closes #2004 from marmbrus/codgenDebugging and squashes the following commits:

b7a7e41 [Michael Armbrust] Improve debug logging and toStrings.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/codegen/CodeGenerator.scala

@@ -78,7 +78,12 @@ abstract class CodeGenerator[InType <: AnyRef, OutType <: AnyRef] extends Loggin
     .build(
       new CacheLoader[InType, OutType]() {
         override def load(in: InType): OutType = globalLock.synchronized {
-           create(in)
+          val startTime = System.nanoTime()
+          val result = create(in)
+          val endTime = System.nanoTime()
+          def timeMs = (endTime - startTime).toDouble / 1000000
+          logInfo(s"Code generated expression $in in $timeMs ms")
+          result
         }
       })
 
@@ -413,7 +418,19 @@ abstract class CodeGenerator[InType <: AnyRef, OutType <: AnyRef] extends Loggin
          """.children
       }
 
-    EvaluatedExpression(code, nullTerm, primitiveTerm, objectTerm)
+    // Only inject debugging code if debugging is turned on.
+    val debugCode =
+      if (log.isDebugEnabled) {
+        val localLogger = log
+        val localLoggerTree = reify { localLogger }
+        q"""
+          $localLoggerTree.debug(${e.toString} + ": " +  (if($nullTerm) "null" else $primitiveTerm))
+        """ :: Nil
+      } else {
+        Nil
+      }
+
+    EvaluatedExpression(code ++ debugCode, nullTerm, primitiveTerm, objectTerm)
   }
 
   protected def getColumn(inputRow: TermName, dataType: DataType, ordinal: Int) = {

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

@@ -60,6 +60,8 @@ case class IsNull(child: Expression) extends Predicate with trees.UnaryNode[Expr
   override def eval(input: Row): Any = {
     child.eval(input) == null
   }
+
+  override def toString = s"IS NULL $child"
 }
 
 case class IsNotNull(child: Expression) extends Predicate with trees.UnaryNode[Expression] {