public DoubleWritable evaluate(HiveDecimalWritable baseWritable, HiveDecimalWritable writable) {
    if (baseWritable == null || writable == null) {
      return null;
    }

    double base = baseWritable.doubleValue();
    double d = writable.doubleValue();
    return log(base, d);
  }
Get the logarithm of the given decimal input with the given decimal base.
ql/src/java/org/apache/hadoop/hive/ql/udf/UDFLog.java