@Override
  public String aggregateStats(String keyPrefix, String statType) {
    SessionState ss = SessionState.get();
    // Have to use the length instead of the actual prefix because the prefix is location dependent
    // 17 is 16 (16 byte MD5 hash) + 1 for the path separator
    // Can be less than 17 due to unicode characters
    ss.out.println("Stats prefix is hashed: " + new Boolean(keyPrefix.length() <= 17));
    return null;
  }
An test implementation for StatsAggregator.
aggregateStats prints the length of the keyPrefix to SessionState's out stream
All other methods are no-ops.
itests/util/src/main/java/org/apache/hadoop/hive/ql/stats/KeyVerifyingStatsAggregator.java