[SPARK-8218][SQL] Binary log math function update.

Some minor updates based on after merging #6725.

Author: Reynold Xin <rxin@databricks.com>

Closes #6871 from rxin/log and squashes the following commits:

ab51542 [Reynold Xin] Use JVM log
76fc8de [Reynold Xin] Fixed arg.
a7c1522 [Reynold Xin] [SPARK-8218][SQL] Binary log math function update.

python/pyspark/sql/functions.py

@@ -404,18 +404,23 @@ def when(condition, value):
     return Column(jc)
 
 
-@since(1.4)
-def log(col, base=math.e):
+@since(1.5)
+def log(arg1, arg2=None):
     """Returns the first argument-based logarithm of the second argument.
 
-    >>> df.select(log(df.age, 10.0).alias('ten')).map(lambda l: str(l.ten)[:7]).collect()
+    If there is only one argument, then this takes the natural logarithm of the argument.
+
+    >>> df.select(log(10.0, df.age).alias('ten')).map(lambda l: str(l.ten)[:7]).collect()
     ['0.30102', '0.69897']
 
     >>> df.select(log(df.age).alias('e')).map(lambda l: str(l.e)[:7]).collect()
     ['0.69314', '1.60943']
     """
     sc = SparkContext._active_spark_context
-    jc = sc._jvm.functions.log(base, _to_java_column(col))
+    if arg2 is None:
+        jc = sc._jvm.functions.log(_to_java_column(arg1))
+    else:
+        jc = sc._jvm.functions.log(arg1, _to_java_column(arg2))
     return Column(jc)
 
 

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

@@ -260,6 +260,10 @@ case class Pow(left: Expression, right: Expression)
 
 case class Logarithm(left: Expression, right: Expression)
   extends BinaryMathExpression((c1, c2) => math.log(c2) / math.log(c1), "LOG") {
+
+  /**
+   * Natural log, i.e. using e as the base.
+   */
   def this(child: Expression) = {
     this(EulerNumber(), child)
   }