CREATE OR REPLACE FUNCTION current_trimestre () returns date as $$
    DECLARE
    dat date:= current_date;
    mese integer;
    BEGIN
    SELECT  extract('month' from current_date) into mese;
    case
    when mese between 1 and 3 then
        dat:= (extract('year' from current_date)||'-'||'1'||'-'||'1')::date;
    when mese between 4 and 6 then
        dat:= (extract('year' from current_date)||'-'||'1'||'-'||'1')::date;
    when mese between 7 and 9 then
        dat:= (extract('year' from current_date)||'-'||'1'||'-'||'1')::date;
    when mese between 10 and 12 then
        dat:= (extract('year' from current_date)||'-'||'1'||'-'||'1')::date;
    end case;

    END
$$language plpgsql;