@Override
  @Explain(displayName = "Statistics")
  public String toString() {
    StringBuilder sb = new StringBuilder();
    if (runtimeStats) {
      sb.append("(RUNTIME) ");
    }
    sb.append("Num rows: ");
    sb.append(numRows);
    if (runTimeNumRows >= 0) {
      sb.append("/" + runTimeNumRows);
    }
    sb.append(" Data size: ");
    sb.append(dataSize);
    if (numErasureCodedFiles > 0) {
      sb.append(" Erasure files: ");
      sb.append(numErasureCodedFiles);
    }
    sb.append(" Basic stats: ");
    sb.append(basicStatsState);
    sb.append(" Column stats: ");
    sb.append(columnStatsState);
    return sb.toString();
  }
Statistics. Describes the output of an operator in terms of size, rows, etc
based on estimates.
ql/src/java/org/apache/hadoop/hive/ql/plan/Statistics.java