@Override
  protected DoubleWritable doEvaluate(DoubleWritable a) {
    if (a.get() <= 0.0) {
      return null;
    } else {
      result.set(StrictMath.log10(a.get()));
      return result;
    }
  }
Returns the logarithm of "a" with base 10.
ql/src/java/org/apache/hadoop/hive/ql/udf/UDFLog10.java