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><A
NAME="FDW-HELPERS"
>52.3. Foreign Data Wrapper Helper Functions</A
></H1
><P
>     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
     <TT
CLASS="FILENAME"
>foreign/foreign.h</TT
> in your source file.
     That header also defines the struct types that are returned by
     these functions.
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>ForeignDataWrapper *
GetForeignDataWrapper(Oid fdwid);</PRE
><P>

     This function returns a <TT
CLASS="STRUCTNAME"
>ForeignDataWrapper</TT
>
     object for the foreign-data wrapper with the given OID.  A
     <TT
CLASS="STRUCTNAME"
>ForeignDataWrapper</TT
> object contains properties
     of the FDW (see <TT
CLASS="FILENAME"
>foreign/foreign.h</TT
> for details).
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>ForeignServer *
GetForeignServer(Oid serverid);</PRE
><P>

     This function returns a <TT
CLASS="STRUCTNAME"
>ForeignServer</TT
> object
     for the foreign server with the given OID.  A
     <TT
CLASS="STRUCTNAME"
>ForeignServer</TT
> object contains properties
     of the server (see <TT
CLASS="FILENAME"
>foreign/foreign.h</TT
> for details).
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>UserMapping *
GetUserMapping(Oid userid, Oid serverid);</PRE
><P>

     This function returns a <TT
CLASS="STRUCTNAME"
>UserMapping</TT
> 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
     <TT
CLASS="LITERAL"
>PUBLIC</TT
>, or throw error if there is none.)  A
     <TT
CLASS="STRUCTNAME"
>UserMapping</TT
> object contains properties of the
     user mapping (see <TT
CLASS="FILENAME"
>foreign/foreign.h</TT
> for details).
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>ForeignTable *
GetForeignTable(Oid relid);</PRE
><P>

     This function returns a <TT
CLASS="STRUCTNAME"
>ForeignTable</TT
> object for
     the foreign table with the given OID.  A
     <TT
CLASS="STRUCTNAME"
>ForeignTable</TT
> object contains properties of the
     foreign table (see <TT
CLASS="FILENAME"
>foreign/foreign.h</TT
> for details).
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>List *
GetForeignColumnOptions(Oid relid, AttrNumber attnum);</PRE
><P>

     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
     <TT
CLASS="STRUCTNAME"
>DefElem</TT
>.  NIL is returned if the column has no
     options.
    </P
><P
>     Some object types have name-based lookup functions in addition to the
     OID-based ones:
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>ForeignDataWrapper *
GetForeignDataWrapperByName(const char *name, bool missing_ok);</PRE
><P>

     This function returns a <TT
CLASS="STRUCTNAME"
>ForeignDataWrapper</TT
>
     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.
    </P
><P
></P><PRE
CLASS="PROGRAMLISTING"
>ForeignServer *
GetForeignServerByName(const char *name, bool missing_ok);</PRE
><P>

     This function returns a <TT
CLASS="STRUCTNAME"
>ForeignServer</TT
> 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.
    </P
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