the
   responsibility of the hook to minimize duplicated work.
  </para>

  <para>
   Note also that the set of join clauses to apply to the join,
   which is passed as <literal>extra-&gt;restrictlist</literal>, varies
   depending on the combination of inner and outer relations.  A
   <structname>CustomPath</structname> path generated for the
   <literal>joinrel</literal> must contain the set of join clauses it uses,
   which will be used by the planner to convert the
   <structname>CustomPath</structname> path into a plan, if it is selected
   by the planner as the best path for the <literal>joinrel</literal>.
  </para>

  <sect2 id="custom-scan-path-callbacks">
  <title>Custom Scan Path Callbacks</title>

  <para>
<programlisting>
Plan *(*PlanCustomPath) (PlannerInfo *root,
                         RelOptInfo *rel,
                         CustomPath *best_path,
                         List *tlist,
                         List *clauses,
                         List *custom_plans);
</programlisting>
    Convert a custom path to a finished plan.  The return value will generally
    be a <literal>CustomScan</literal> object, which the callback must allocate and
    initialize.  See <xref linkend="custom-scan-plan"/> for more details.
   </para>

   <para>
<programlisting>
List *(*ReparameterizeCustomPathByChild) (PlannerInfo *root,
                                          List *custom_private,
                                          RelOptInfo *child_rel);
</programlisting>
    This callback is called while converting a path parameterized by the
    top-most parent of the given child relation <literal>child_rel</literal>
    to be parameterized by the child relation.  The callback is used to
    reparameterize any paths or translate any expression nodes saved in the
    given <literal>custom_private</literal> member of a
    <structname>CustomPath</structname>.  The callback may use
    <literal>reparameterize_path_by_child</literal>,
    <literal>adjust_appendrel_attrs</literal> or
    <literal>adjust_appendrel_attrs_multilevel</literal> as required.
   </para>
  </sect2>
 </sect1>

 <sect1 id="custom-scan-plan">
  <title>Creating Custom Scan Plans</title>

  <para>
    A custom scan is represented in a finished plan tree using the following
    structure:
<programlisting>
typedef struct CustomScan
{
    Scan      scan;
    uint32    flags;
    List     *custom_plans;
    List     *custom_exprs;
    List     *custom_private;
    List     *custom_scan_tlist;
    Bitmapset *custom_relids;
    const CustomScanMethods *methods;
} CustomScan;
</programlisting>
  </para>

  <para>
    <structfield>scan</structfield> must be initialized as for any other scan, including
    estimated costs, target lists, qualifications, and so on.
    <structfield>flags</structfield> is a bit mask with the same meaning as in
    <structname>CustomPath</structname>.
    <structfield>custom_plans</structfield> can be used to store child
    <structname>Plan</structname> nodes.
    <structfield>custom_exprs</structfield> should be used to
    store expression trees that will need to be fixed up by
    <filename>setrefs.c</filename> and <filename>subselect.c</filename>, while
    <structfield>custom_private</structfield> should be used to store other private data
    that is only used by the custom scan provider itself.
    <structfield>custom_scan_tlist</structfield> can be NIL when scanning a base
    relation, indicating that the custom scan returns scan tuples that match
    the base relation's row type.  Otherwise it is a target list describing
    the actual scan tuples.  <structfield>custom_scan_tlist</structfield> must be
    provided for joins, and could be provided for scans if the custom scan
    provider can compute some non-Var expressions.
    <structfield>custom_relids</structfield> is set by the core code to the set of
    relations (range table indexes) that this scan node handles; except when
    this scan is replacing a join, it