CREATE EXTENSION autoinc;
create sequence aitest_seq increment 10 start 0 minvalue 0;
create table aitest (
	price_id	int4,
	price_val	int4,
	price_on	int4
);
create trigger aiserial
	before insert or update on aitest
	for each row
	execute procedure
	autoinc (price_on, aitest_seq);
insert into aitest values (1, 1, null);
insert into aitest values (2, 2, 0);
insert into aitest values (3, 3, 1);
select * from aitest;
 price_id | price_val | price_on 
----------+-----------+----------
        1 |         1 |       10
        2 |         2 |       20
        3 |         3 |        1
(3 rows)

update aitest set price_on = 11;
select * from aitest;
 price_id | price_val | price_on 
----------+-----------+----------
        1 |         1 |       11
        2 |         2 |       11
        3 |         3 |       11
(3 rows)

update aitest set price_on = 0;
select * from aitest;
 price_id | price_val | price_on 
----------+-----------+----------
        1 |         1 |       30
        2 |         2 |       40
        3 |         3 |       50
(3 rows)

update aitest set price_on = null;
select * from aitest;
 price_id | price_val | price_on 
----------+-----------+----------
        1 |         1 |       60
        2 |         2 |       70
        3 |         3 |       80
(3 rows)