private String getLevelString(int level) {
    if (level == 0) {
      return "\n";
    }
    StringBuilder s = new StringBuilder();
    s.append("\n");
    while (level > 0) {
      s.append("  ");
      level--;
    }
    return s.toString();
  }
Returns a map of output column name to input expression map Note that
currently it returns only key columns for ReduceSink and GroupBy operators.

@return null if the operator doesn't change columns
ql/src/java/org/apache/hadoop/hive/ql/exec/Operator.java