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>50.1. Foreign Data Wrapper Functions</A
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> The FDW author needs to implement a handler function, and optionally
a validator function. Both functions must be written in a compiled
language such as C, using the version-1 interface.
For details on C language calling conventions and dynamic loading,
see <A
HREF="xfunc-c.html"
>Section 35.9</A
>.
</P
><P
> The handler function simply returns a struct of function pointers to
callback functions that will be called by the planner, executor, and
various maintenance commands.
Most of the effort in writing an FDW is in implementing these callback
functions.
The handler function must be registered with
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> as taking no arguments and
returning the special pseudo-type <TT
CLASS="TYPE"
>fdw_handler</TT
>. The
callback functions are plain C functions and are not visible or
callable at the SQL level. The callback functions are described in
<A
HREF="fdw-callbacks.html"
>Section 50.2</A
>.
</P
><P
> The validator function is responsible for validating options given in
<TT
CLASS="COMMAND"
>CREATE</TT
> and <TT
CLASS="COMMAND"
>ALTER</TT
> commands for its
foreign data wrapper, as well as foreign servers, user mappings, and
foreign tables using the wrapper.
The validator function must be registered as taking two arguments, a
text array containing the options to be validated, and an OID
representing the type of object the options are associated with (in
the form of the OID of the system catalog the object would be stored
in, either
<TT
CLASS="LITERAL"
>ForeignDataWrapperRelationId</TT
>,
<TT
CLASS="LITERAL"
>ForeignServerRelationId</TT
>,
<TT
CLASS="LITERAL"
>UserMappingRelationId</TT
>,
or <TT
CLASS="LITERAL"
>ForeignTableRelationId</TT
>).
If no validator function is supplied, options are not checked at object
creation time or object alteration time.
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