run all queries against
       that server sequentially even if there are multiple foreign tables
       involved, unless those tables are subject to different user mappings.
       In such a case, it may be more performant to disable this option to
       eliminate the overhead associated with running queries asynchronously.
      </p><p>
       Asynchronous execution is applied even when an
       <code class="structname">Append</code> node contains subplan(s) executed
       synchronously as well as subp