PostgreSQL:

CREATE TRANSFORM

CREATE [ OR REPLACE ] TRANSFORM FOR type_name LANGUAGE lang_name (
FROM SQL WITH FUNCTION from_sql_function_name (argument_type [, ...]),
TO SQL WITH FUNCTION to_sql_function_name (argument_type [, ...])
);

Example

CREATE TYPE hstore ...;
CREATE LANGUAGE plpythonu ...;
CREATE FUNCTION hstore_to_plpython(val internal) RETURNS
internal
LANGUAGE C STRICT IMMUTABLE
AS ...;
CREATE FUNCTION plpython_to_hstore(val internal) RETURNS hstore
LANGUAGE C STRICT IMMUTABLE
AS ...;
CREATE TRANSFORM FOR hstore LANGUAGE plpythonu (
FROM SQL WITH FUNCTION hstore_to_plpython(internal),
TO SQL WITH FUNCTION plpython_to_hstore(internal)
);