@Override
  public String vectorExpressionParameters() {
    return getColumnParamString(0, inputColumnNum[0]);
  }
Expression for vectorized evaluation of unary UDFs on strings.
An object of {@link IUDFUnaryString} is applied to every element of
the vector.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/StringUnaryUDF.java