@Override
  @Explain(displayName = "keys", explainLevels = { Level.USER })
  public Map<Byte, String> getUserLevelExplainKeysString() {
    Map<Byte, String> keyMap = new LinkedHashMap<Byte, String>();
    for (Map.Entry<Byte, List<ExprNodeDesc>> k: getKeys().entrySet()) {
      keyMap.put(k.getKey(), PlanUtils.getExprListString(k.getValue(), true));
    }
    return keyMap;
  }
@return the keys in string form
ql/src/java/org/apache/hadoop/hive/ql/plan/HashTableSinkDesc.java