A custom scan provider will typically add paths for a base relation by setting the following hook, which is called after the core code has generated all the access paths it can for the relation (except for Gather paths, which are made after this call so that they can use partial paths added by the hook):
typedef void (*set_rel_pathlist_hook_type) (PlannerInfo *root,
                                            RelOptInfo *rel,
                                            Index rti,
                                            RangeTblEntry *rte);
extern PGDLLIMPORT set_rel_pathlist_hook_type set_rel_pathlist_hook;
    Although this hook function can be used to examine, modify, or remove
    paths generated by the core system, a custom scan provider will typically
    confine itself to generating CustomPath objects and adding
    them to rel using add_path.  The custom scan
    provider is responsible for initializing the CustomPath
    object, which is declared like this:
typedef struct CustomPath
{
    Path      path;
    uint32    flags;
    List     *custom_paths;
    List     *custom_private;
    const CustomPathMethods *methods;
} CustomPath;
    path must be initialized as for any other path, including
    the row-count estimate, start and total cost, and sort ordering provided
    by this path.  flags is a bit mask, which should include
    CUSTOMPATH_SUPPORT_BACKWARD_SCAN if the custom path can support
    a backward scan and CUSTOMPATH_SUPPORT_MARK_RESTORE if it
    can support mark and restore.  Both capabilities are optional.
    An optional custom_paths is a list of Path
    nodes used by this custom-path node; these will be transformed into
    Plan nodes by planner.
    custom_private can be used to store the custom path's
    private data.  Private data should be stored in a form that can be handled
    by nodeToString, so that debugging routines that attempt to
    print the custom path will work as designed.  methods must
    point to a (usually statically allocated) object implementing the required
    custom path methods, of which there is currently only one.  The
    LibraryName and SymbolName fields must also
    be initialized so that the dynamic loader can resolve them to locate the
    method table.
  
   A custom scan provider can also provide join paths.  Just as for base
   relations, such a path must produce the same output as would normally be
   produced by the join it replaces.  To do this, the join provider should
   set the following hook, and then within the hook function,
   create CustomPath path(s) for the join relation.
typedef void (*set_join_pathlist_hook_type) (PlannerInfo *root,
                                             RelOptInfo *joinrel,
                                             RelOptInfo *outerrel,
                                             RelOptInfo *innerrel,
                                             JoinType jointype,
                                             JoinPathExtraData *extra);
extern PGDLLIMPORT set_join_pathlist_hook_type set_join_pathlist_hook;
This hook will be invoked repeatedly for the same join relation, with different combinations of inner and outer relations; it is the responsibility of the hook to minimize duplicated work.
Plan *(*PlanCustomPath) (PlannerInfo *root,
                         RelOptInfo *rel,
                         CustomPath *best_path,
                         List *tlist,
                         List *clauses,
                         List *custom_plans);
    Convert a custom path to a finished plan.  The return value will generally
    be a CustomScan object, which the callback must allocate and
    initialize.  See Section 59.2 for more details.