Not use index on setof from Sql Function

This simple function return setof table "DatiRegione"

CREATE OR REPLACE FUNCTION public._stato_dati_regione_rt(text)
RETURNS SETOF "DatiRegione" AS
'
SELECT DISTINCT ON ("Codice_regione") dr.* FROM "DatiRegione"
'
LANGUAGE 'sql' VOLATILE STRICT;

When I perform query on this setof, pgsql not use the indexes on table
"DatiRegion".
This is a big problem.

##################à

Query on _stato_dati_regione_rt(text) without index

QUERY PLAN
Aggregate (cost=17.50..17.51 rows=1 width=17) (actual
time=14555.295..14555.297 rows=1 loops=1)
-> Function Scan on _stato_dati_regione_rt (cost=0.00..17.50 rows=1
width=17) (actual time=14528.756..14554.753 rows=119 loops=1)
Filter: (("Categoria" = 1) AND ("Tipologia" = 'ALB'::bpchar))
Total runtime: 14561.981 ms

######################

Query on "DatiRegione" with index

QUERY PLAN
Aggregate (cost=672.10..672.10 rows=1 width=17) (actual
time=8.752..8.753 rows=1 loops=1)
-> Index Scan using idx_dr_cat_tipo on "DatiRegione"
(cost=0.00..671.64 rows=181 width=17) (actual time=0.123..6.289 rows=504
loops=1)
Index Cond: (("Categoria" = 1) AND ("Tipologia" = 'ALB'::bpchar))
Total runtime: 9.565 ms

###########################

Can I use index on setof from _stato_dati_regione_rt(text)?

Tannks in adbace.

By,Alessandro

--
-------------------------------
Alessandro Vincelli
W4B - web for business s.r.l.
Firenze
via Pellicceria 10 - 50123
E-mail: a(dot)vincelli(at)w4b(dot)it
tel: 055-2654270