">
       <code class="structfield">dtd_identifier</code> <code class="type">sql_identifier</code>
      </p>
      <p>
       An identifier of the data type descriptor of the return data
       type of this function, unique among the data type descriptors
       pertaining to the function.  This is mainly useful for joining
       with other instances of such identifiers.  (The specific format
       of the identifier is not defined and not guaranteed to remain
       the same in future versions.)
      </