update inside function does not use the index

Dear List,

I have problems with a self written function, which does not use the
index, which takes very long (500 ms per update).

The pl/pgsql function iterates over a select resultset with a cursor.
In every loop I execute an update with a where LIKE condition, which
relates to my current cursor position:

FOR i IN SELECT id, level_ids, path_names||'%' as path_names from x LOOP
update x set path_ids[i.level] = id where path_names like i.path_names;
RAISE NOTICE 'path_names : %', i.path_names;
END LOOP;

Calling the updates outside the function, they are very fast because
like 'a.b%' uses the index of the path field ( ~ 15 ms ).

Does anyone know how to fix that?
Thanks, Johannes