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 38.10.
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 57.2.
The validator function is responsible for validating options
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
ForeignTableRelationId). If no validator
function is supplied, options are not checked at object creation
time or object alteration time.
If you see anything in the documentation that is not correct, does not match your experience with the particular feature or requires further clarification, please use this form to report a documentation issue.