locking</firstterm> (as described
     in <xref linkend="fdw-row-locking"/>), it must provide the following
     callback functions:
    </para>

    <para>
<programlisting>
RowMarkType
GetForeignRowMarkType(RangeTblEntry *rte,
                      LockClauseStrength strength);
</programlisting>

     Report which row-marking option to use for a foreign table.
     <literal>rte</literal> is the <structname>RangeTblEntry</structname> node for the table
     and <literal>strength</literal> describes the lock strength requested by the
     relevant <literal>FOR UPDATE/SHARE</literal> clause, if any.  The result must be
     a member of the <literal>RowMarkType</literal> enum type.
    </para>

    <para>
     This function is called during query planning for each foreign table that
     appears in an <command>UPDATE</command>, <command>DELETE</command>, or <command>SELECT
     FOR UPDATE/SHARE</command> query and is not the target of <command>UPDATE</command>
     or <command>DELETE</command>.
    </para>

    <para>
     If the <function>GetForeignRowMarkType</function> pointer is set to
     <literal>NULL</literal>, the <literal>ROW_MARK_COPY</literal> option is always used.
     (This implies that <function>RefetchForeignRow</function> will never be called,
     so it need not be provided either.)
    </para>

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

    <para>
<programlisting>
void
RefetchForeignRow(EState *estate,
                  ExecRowMark *erm,
                  Datum rowid,
                  TupleTableSlot *slot,
                  bool *updated);
</programlisting>

     Re-fetch one tuple slot from the foreign table, after locking it if required.
     <literal>estate</literal> is global execution state for the query.
     <literal>erm</literal> is the <structname>ExecRowMark</structname> struct describing
     the target foreign table and the row lock type (if any) to acquire.
     <literal>rowid</literal> identifies the tuple to be fetched.
     <literal>slot</literal> contains nothing useful upon call, but can be used to
     hold the returned tuple. <literal>updated</literal> is an output parameter.
    </para>

    <para>
     This function should store the tuple into the provided slot, or clear it if
     the row lock couldn't be obtained.  The row lock type to acquire is
     defined by <literal>erm-&gt;markType</literal>, which is the value
     previously returned by <function>GetForeignRowMarkType</function>.
     (<literal>ROW_MARK_REFERENCE</literal> means to just re-fetch the tuple
     without acquiring any lock, and <literal>ROW_MARK_COPY</literal> will
     never be seen by this routine.)
    </para>

    <para>
     In addition, <literal>*updated</literal> should be set to <literal>true</literal>
     if what was fetched was an updated version of the tuple rather than
     the same version previously obtained.  (If the FDW cannot be sure about
     this, always returning <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.