@Override
  public String vectorExpressionParameters() {
    return getColumnParamString(0, inputColumnNum[0]) + ", " + getColumnParamString(1, fieldIndex);
  }
Vectorized instruction to get the field of Struct type with field name and put
the result in an output column.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/VectorUDFStructField.java