As you can see Oracle could exploit two indexes and solve the entire query without accessing the table by rowid?
SELECT 'Scarti letture GAS' tipo, campo47 pdf, COUNT (1) n
FROM out.CONSUMI_GEE_LC_T500_00 v
WHERE stato = 'SC'
AND stato is not null
AND campo47 is not null
GROUP BY 'Scarti letture GAS', campo47;
I've made a test adding the field campo47 to the STATO index. Performance boosts from 1' 49'' to 0,6s.
Index on stato is not selective. Index on campo47 (it means field47) is really selective.