<literal>true</literal> is recommended.)
    </para>

    <para>
     Note that by default, failure to acquire a row lock should result in
     raising an error; returning with an empty slot is only appropriate if
     the <literal>SKIP LOCKED</literal> option is specified
     by <literal>erm-&gt;waitPolicy</literal>.
    </para>

    <para>
     The <literal>rowid</literal> is the <structfield>ctid</structfield> value previously read
     for the row to be re-fetched.  Although the <literal>rowid</literal> value is
     passed as a <type>Datum</type>, it can currently only be a <type>tid</type>.  The
     function API is chosen in hopes that it may be possible to allow other
     data types for row IDs in future.
    </para>

    <para>
     If the <function>RefetchForeignRow</function> pointer is set to
     <literal>NULL</literal>, attempts to re-fetch rows will fail
     with an error message.
    </para>

    <para>
     See <xref linkend="fdw-row-locking"/> for more information.
    </para>

    <para>
<programlisting>
bool
RecheckForeignScan(ForeignScanState *node,
                   TupleTableSlot *slot);
</programlisting>
     Recheck that a previously-returned tuple still matches the relevant
     scan and join qualifiers, and possibly provide a modified version of
     the tuple.  For foreign data wrappers which do not perform join pushdown,
     it will typically be more convenient to set this to <literal>NULL</literal> and
     instead set <structfield>fdw_recheck_quals</structfield> appropriately.
     When outer joins are pushed down, however, it isn't sufficient to
     reapply the checks relevant to all the base tables to the result tuple,
     even if all needed attributes are present, because failure to match some
     qualifier might result in some attributes going to NULL, rather than in
     no tuple being returned.  <literal>RecheckForeignScan</literal> can recheck
     qualifiers and return true if they are still satisfied and false
     otherwise, but it can also store a replacement tuple into the supplied
     slot.
    </para>

    <para>
     To implement join pushdown, a foreign data wrapper will typically
     construct an alternative local join plan which is used only for
     rechecks; this will become the outer subplan of the
     <literal>ForeignScan</literal>.  When a recheck is required, this subplan
     can be executed and the resulting tuple can be stored in the slot.
     This plan need not be efficient since no base table will return more
     than one row; for example, it may implement all joins as nested loops.
     The function <literal>GetExistingLocalJoinPath</literal> may be used to search
     existing paths for a suitable local join path, which can be used as the
     alternative local join plan.  <literal>GetExistingLocalJoinPath</literal>
     searches for an unparameterized path in the path list of the specified
     join relation.  (If it does not find such a path, it returns NULL, in
     which case a foreign data wrapper may build the local path by itself or
     may choose not to create access paths for that join.)
    </para>
   </sect2>

   <sect2 id="fdw-callbacks-explain">
    <title>FDW Routines for <command>EXPLAIN</command></title>

    <para>
<programlisting>
void
ExplainForeignScan(ForeignScanState *node,
                   ExplainState *es);
</programlisting>

     Print additional <command>EXPLAIN</command> output for a foreign table scan.
     This function can call <function>ExplainPropertyText</function> and
     related functions to add fields to the <command>EXPLAIN</command> output.
     The flag fields in <literal>es</literal> can be used to determine what to
     print, and the state of the <structname>ForeignScanState</structname> node
     can be inspected to provide run-time statistics in the <command>EXPLAIN
     ANALYZE</command> case.
    </para>

    <para>
     If the <function>ExplainForeignScan</function> pointer is set to
     <literal>NULL</literal>,