code: protected function buildHint()/*# : array */
      {
          $res = [];
          foreach ($this->clause_hints as $name => $status) {
              if ($status) {
                  $res[] = $name;
              }
          }
          if (!empty($res)) {
              return [join(' ', $res)];
          } else {
              return [];
          }
      }
docs: Build HINT
  
  @return array
  @access protected