@Explain(displayName = "keys")
  @Signature
  public Map<String, String> getKeysString() {
    if (joinKeys == null) {
      return null;
    }

    Map<String, String> keyMap = new LinkedHashMap<String, String>();
    for (byte i = 0; i < joinKeys.length; i++) {
      keyMap.put(String.valueOf(i), PlanUtils.getExprListString(Arrays.asList(joinKeys[i])));
    }
    return keyMap;
  }
@return the keys in string form
ql/src/java/org/apache/hadoop/hive/ql/plan/JoinDesc.java