OID of the system catalog the object
     would be stored in, one of:
     <itemizedlist spacing="compact">
      <listitem><para><literal>AttributeRelationId</literal></para></listitem>
      <listitem><para><literal>ForeignDataWrapperRelationId</literal></para></listitem>
      <listitem><para><literal>ForeignServerRelationId</literal></para></listitem>
      <listitem><para><literal>ForeignTableRelationId</literal></para></listitem>
      <listitem><para><literal>UserMappingRelationId</literal></para></listitem>
     </itemizedlist>
     If no validator function is supplied, options are not checked at object
     creation time or object alteration time.
    </para>

   </sect1>

   <sect1 id="fdw-callbacks">
    <title>Foreign Data Wrapper Callback Routines</title>

    <para>
     The FDW handler function returns a palloc'd <structname>FdwRoutine</structname>
     struct containing pointers to the callback functions described below.
     The scan-related functions are required, the rest are optional.
    </para>

    <para>
     The <structname>FdwRoutine</structname> struct type is declared in
     <filename>src/include/foreign/fdwapi.h</filename>, which see for additional
     details.
    </para>

   <sect2 id="fdw-callbacks-scan">
    <title>FDW Routines for Scanning Foreign Tables</title>

    <para>
<programlisting>
void
GetForeignRelSize(PlannerInfo *root,
                  RelOptInfo *baserel,
                  Oid foreigntableid);
</programlisting>

     Obtain relation size estimates for a foreign table.  This is called
     at the beginning of planning for a query that scans a foreign table.
     <literal>root</literal> is the planner's global information about the query;
     <literal>baserel</literal> is the planner's information about this table; and
     <literal>foreigntableid</literal> is the <structname>pg_class</structname> OID of the
     foreign table.  (<literal>foreigntableid</literal> could be obtained from the
     planner data structures, but it's passed explicitly to save effort.)
    </para>

    <para>
     This function should update <literal>baserel-&gt;rows</literal> to be the
     expected number of rows returned by the table scan, after accounting for
     the filtering done by the restriction quals.  The initial value of
     <literal>baserel-&gt;rows</literal> is just a constant default estimate, which
     should be replaced if at all possible.  The function may also choose to
     update <literal>baserel-&gt;width</literal> if it can compute a better estimate
     of the average result row width.
     (The initial value is based on column data types and on column
     average-width values measured by the last <command>ANALYZE</command>.)
     Also, this function may update <literal>baserel-&gt;tuples</literal> if
     it can compute a better estimate of the foreign table's total row count.
     (The initial value is
     from <structname>pg_class</structname>.<structfield>reltuples</structfield>
     which represents the total row count seen by the
     last <command>ANALYZE</command>; it will be <literal>-1</literal> if
     no <command>ANALYZE</command> has been done on this foreign table.)
    </para>

    <para>
     See <xref linkend="fdw-planning"/> for additional information.
    </para>

    <para>
<programlisting>
void
GetForeignPaths(PlannerInfo *root,
                RelOptInfo *baserel,
                Oid foreigntableid);
</programlisting>

     Create possible access paths for a scan on a foreign table.
     This is called during query planning.
     The parameters are the same as for <function>GetForeignRelSize</function>,
     which has already been called.
    </para>

    <para>
     This function must generate at least one access path
     (<structname>ForeignPath</structname> node) for a scan on the foreign table and
     must call <function>add_path</function> to add each such path to
     <literal>baserel-&gt;pathlist</literal>.  It's recommended to use
     <function>create_foreignscan_path</function>