Foreign Data Wrapper Helper Functions

Several helper functions are exported from the core server so that authors of foreign data wrappers can get easy access to attributes of FDW-related objects, such as FDW options. To use any of these functions, you need to include the header file foreign/foreign.h in your source file. That header also defines the struct types that are returned by these functions.

ForeignDataWrapper *
GetForeignDataWrapper(Oid fdwid);

This function returns a ForeignDataWrapper object for the foreign-data wrapper with the given OID. A ForeignDataWrapper object contains properties of the FDW (see foreign/foreign.h for details).

ForeignServer *
GetForeignServer(Oid serverid);

This function returns a ForeignServer object for the foreign server with the given OID. A ForeignServer object contains properties of the server (see foreign/foreign.h for details).

UserMapping *
GetUserMapping(Oid userid, Oid serverid);

This function returns a UserMapping object for the user mapping of the given role on the given server. (If there is no mapping for the specific user, it will return the mapping for PUBLIC , or throw error if there is none.) A UserMapping object contains properties of the user mapping (see foreign/foreign.h for details).

ForeignTable *
GetForeignTable(Oid relid);

This function returns a ForeignTable object for the foreign table with the given OID. A ForeignTable object contains properties of the foreign table (see foreign/foreign.h for details).

List *
GetForeignColumnOptions(Oid relid, AttrNumber attnum);

This function returns the per-column FDW options for the column with the given foreign table OID and attribute number, in the form of a list of DefElem . NIL is returned if the column has no options.

Some object types have name-based lookup functions in addition to the OID-based ones:

ForeignDataWrapper *
GetForeignDataWrapperByName(const char *name, bool missing_ok);

This function returns a ForeignDataWrapper object for the foreign-data wrapper with the given name. If the wrapper is not found, return NULL if missing_ok is true, otherwise raise an error.

ForeignServer *
GetForeignServerByName(const char *name, bool missing_ok);

This function returns a ForeignServer object for the foreign server with the given name. If the server is not found, return NULL if missing_ok is true, otherwise raise an error.