The FDW author needs to implement a handler function, and optionally a validator function. Both functions must be written in a compiled language such as C, using the version-1 interface. For details on C language calling conventions and dynamic loading, see Section 37.9.
     The handler function simply returns a struct of function pointers to
     callback functions that will be called by the planner, executor, and
     various maintenance commands.
     Most of the effort in writing an FDW is in implementing these callback
     functions.
     The handler function must be registered with
     PostgreSQL as taking no arguments and
     returning the special pseudo-type fdw_handler.  The
     callback functions are plain C functions and are not visible or
     callable at the SQL level.  The callback functions are described in
     Section 56.2.
    
     The validator function is responsible for validating options given in
     CREATE and ALTER commands for its
     foreign data wrapper, as well as foreign servers, user mappings, and
     foreign tables using the wrapper.
     The validator function must be registered as taking two arguments, a
     text array containing the options to be validated, and an OID
     representing the type of object the options are associated with (in
     the form of the OID of the system catalog the object would be stored
     in, either
     ForeignDataWrapperRelationId,
     ForeignServerRelationId,
     UserMappingRelationId,
     or ForeignTableRelationId).
     If no validator function is supplied, options are not checked at object
     creation time or object alteration time.