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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.
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