code: @Nullable
      @ThriftField(value = 4, requiredness = OPTIONAL)
      public List<Set<String>> getIndexableKeys()
      {
          return indexableKeys;
      }
docs: Returns a list of key sets which can be used for index lookups.
  The list is expected to have only unique key sets.
  {@code set<set<string>>} is not used here because some languages (like php) don't support it.