/**
  * Make it possible to add INTO OUTFILE
  *
  * @param string $sql
  * @return null|string
  */
 protected function _renderColumns($sql)
 {
     $sql .= ' SQL_NO_CACHE ';
     $columns = parent::_renderColumns($sql);
     if ($this->outfile) {
         $columns .= sprintf(' INTO OUTFILE %s', $this->getAdapter()->quote($this->outfile));
     }
     return $columns;
 }