sometimes,
    <literal>EndForeignScan</literal> may be called without this function having
    been called first.  Since the DSM segment used by parallel query is
    destroyed just after this callback is invoked, foreign data wrappers that
    wish to take some action before the DSM segment goes away should implement
    this method.
   </para>
   </sect2>

   <sect2 id="fdw-callbacks-async">
    <title>FDW Routines for Asynchronous Execution</title>
    <para>
     A <structname>ForeignScan</structname> node can, optionally, support
     asynchronous execution as described in
     <filename>src/backend/executor/README</filename>.  The following
     functions are all optional, but are all required if asynchronous
     execution is to be supported.
    </para>

    <para>
<programlisting>
bool
IsForeignPathAsyncCapable(ForeignPath *path);
</programlisting>
     Test whether a given <structname>ForeignPath</structname> path can scan
     the underlying foreign relation asynchronously.
     This function will only be called at the end of query planning when the
     given path is a direct child of an <structname>AppendPath</structname>
     path and when the planner believes that asynchronous execution improves
     performance, and should return true if the given path is able to scan the
     foreign relation asynchronously.
    </para>

    <para>
     If this function is not defined, it is assumed that the given path scans
     the foreign relation using <function>IterateForeignScan</function>.
     (This implies that the callback functions described below will never be
     called, so they need not be provided either.)
    </para>

    <para>
<programlisting>
void
ForeignAsyncRequest(AsyncRequest *areq);
</programlisting>
     Produce one tuple asynchronously from the
     <structname>ForeignScan</structname> node.  <literal>areq</literal> is
     the <structname>AsyncRequest</structname> struct describing the
     <structname>ForeignScan</structname> node and the parent
     <structname>Append</structname> node that requested the tuple from it.
     This function should store the tuple into the slot specified by
     <literal>areq-&gt;result</literal>, and set
     <literal>areq-&gt;request_complete</literal> to <literal>true</literal>;
     or if it needs to wait on an event external to the core server such as
     network I/O, and cannot produce any tuple immediately, set the flag to
     <literal>false</literal>, and set
     <literal>areq-&gt;callback_pending</literal> to <literal>true</literal>
     for the <structname>ForeignScan</structname> node to get a callback from
     the callback functions described below.  If no more tuples are available,
     set the slot to NULL or an empty slot, and the
     <literal>areq-&gt;request_complete</literal> flag to
     <literal>true</literal>.  It's recommended to use
     <function>ExecAsyncRequestDone</function> or
     <function>ExecAsyncRequestPending</function> to set the output parameters
     in the <literal>areq</literal>.
    </para>

    <para>
<programlisting>
void
ForeignAsyncConfigureWait(AsyncRequest *areq);
</programlisting>
     Configure a file descriptor event for which the
     <structname>ForeignScan</structname> node wishes to wait.
     This function will only be called when the
     <structname>ForeignScan</structname> node has the
     <literal>areq-&gt;callback_pending</literal> flag set, and should add
     the event to the <structfield>as_eventset</structfield> of the parent
     <structname>Append</structname> node described by the
     <literal>areq</literal>.  See the comments for
     <function>ExecAsyncConfigureWait</function> in
     <filename>src/backend/executor/execAsync.c</filename> for additional
     information.  When the file descriptor event occurs,
     <function>ForeignAsyncNotify</function> will be called.
    </para>

    <para>
<programlisting>
void
ForeignAsyncNotify(AsyncRequest *areq);
</programlisting>
     Process a relevant