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