create an index on unnest

Greetings!

I need the following: create an index on multiple values for a single row. My other solution consists of a lot of
duplicatedrows in seperate tables and triggers and indexes, resulting in slower performance. 
The following fails on 9.2, it says ERROR:  index expression cannot return a set.

drop table  if exists test;
create table test (id serial, data text );
insert into test VALUES(DEFAULT, 'testdata');
drop function if exists testfct();
CREATE OR REPLACE FUNCTION testfct(text) RETURNS text[] AS $$
DECLARE
result text[];
BEGIN
result = array_fill(''::text, ARRAY[1]);
result[0] = $1 || '-one';
result[1] = $1 || '-two';
RETURN result;
END;

$$ LANGUAGE plpgsql;

create index test_idx on test (unnest(testfct(data)));

any suggestions?
TIA,
F