---
title: "Foreign Data Wrapper Helper Functions"
id: fdw-helpers
pg_version: "20devel"
---

<a id="fdw-helpers"></a>

## 58.3. 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, uint16 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, uint16 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.