postgresql: tsearch2 indexed substrings

substr.sql

alter table towar add ftssubnpelna tsvector;
CREATE INDEX towar_ftssubnpelna_idx ON towar USING gin(ftssubnpelna);

CREATE or replace FUNCTION text_to_substrings(tekst text) RETURNS text AS $$
    substrings = []
    for tmp in tekst.split():
        for i in range(len(tmp)):
            if i >= 3:
                substrings.append(tmp[0:i])
    return ' '.join(substrings)
$$ LANGUAGE plpythonu SECURITY DEFINER;

update towar set ftssubnpelna = to_tsvector(text_to_substrings(nazwapelna));