56.3. 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 * GetForeignDataWrapperExtended(Oid fdwid, bits16 flags);
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).
flags
is a bitwise-or'd bit mask indicating
an extra set of options. It can take the value
FDW_MISSING_OK
, in which case a
NULL
result is returned to the caller instead of an error for an undefined
object.
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 * GetForeignServerExtended(Oid serverid, bits16 flags);
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).
flags
is a bitwise-or'd bit mask indicating
an extra set of options. It can take the value
FSV_MISSING_OK
, in which case a
NULL
result is returned to the caller instead of an error for an undefined
object.
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.