create simple date diff function ,can return the number of years/months/days between two date

datediff.sql

--
create or replace function datediff(m text,d1 date,d2 date) returns int as $$
declare
d1_year     int;
d1_month    int;

d2_year     int;
d2_month    int;
day int;

begin
 select extract(year from d1) into d1_year;
 select extract(month from d1) into d1_month;

 select extract(year from d2) into d2_year;
 select extract(month from d2) into d2_month;

if m = 'year' then
	return (d2_year - d1_year);
elsif m = 'month' then
return (d2_year - d1_year) * 12 + (d2_month - d1_month);
elsif m = 'day' then
 select d2 - d1 into day;
 return day;
end if;
end;
$$ language plpgsql;