50.3. 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.