57.3. Foreign Data Wrapper Helper Functions
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## 57.3. 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.

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