code: @Override
      public Set<String> getAvailableQueryAttributes() {
          if (this.queryAttributeMapping == null) {
              return Collections.emptySet();
          }
  
          return Collections.unmodifiableSet(this.queryAttributeMapping.keySet());
      }
docs: Return the list of all possible query attributes. This implementation queries the database to
  provide a list of all mapped query attribute names, plus all attribute keys currently in-use
  in the database.
  
  @return Set