@Override
  protected Object _evaluate(Object row, int version) throws HiveException {
    if (simpleCase) {
      return inspector.getStructFieldData(row, field);
    }
    Object o = row;
    for (int i = 0; i < fields.length; i++) {
      o = inspectors[i].getStructFieldData(o, fields[i]);
      if (unionField[i]) {
        o = ((StandardUnion)o).getObject();
      }
    }
    return o;
  }
This evaluator gets the column from the row object.
ql/src/java/org/apache/hadoop/hive/ql/exec/ExprNodeColumnEvaluator.java