join vs subquery in searches between ranges - postgresql

Question

subquery vs query's performance is an inaccurate science, google shows cases where both are advantageous, it depends on the data structure, it's necessary to test both to arrive at your truth.
I have a subquery that I couldn't replace by join and be able to test its performance.
Assume that you have a price history table, you add records every time the price or its characteristics change, take this simple example: sql fiddle simple sample!
create table price_hist
( hid serial,
product int,
start_day date,
price numeric,
max_discount numeric,
promo_code character(4) );
create table deliveries
( del_id serial,
del_date date,
product int,
quantity int,
u_price numeric);
insert into price_hist (product, start_day,price,max_discount,promo_code)
values
(21,'2018-03-14',56.22, .022, 'Sam2'),
(18,'2018-02-24',11.25, .031, 'pax3'),
(21,'2017-12-28',50.12, .019, 'titi'),
(21,'2017-12-01',51.89, .034, 'any7'),
(18,'2017-12-26',11.52, .039, 'jun3'),
(18,'2017-12-10',10.99, .029, 'sep9');
insert into deliveries(del_date, product, quantity)
values
('2017-12-05',21,4),
('2017-12-20',18,3),
('2017-12-28',21,2),
('2018-05-08',18,1),
('2018-08-20',21,5);
select d.del_id, d.del_date, d.product, d.quantity,
(select price from price_hist h where h.product=d.product order by h.start_day desc limit 1) u_price,
(select max_discount from price_hist h where h.product=d.product order by h.start_day desc limit 1) max_discount,
(select price from price_hist h where h.product=d.product order by h.start_day desc limit 1)*d.quantity total
from deliveries d;
subqueries find values between date ranges, I have not been able to do in postgresql the join that does the same

Answer 1

You can use distinct on to get values from price_hist for the latest start_day:
select distinct on(product)
product, price, max_discount
from price_hist h
order by product, start_day desc
product | price | max_discount
---------+-------+--------------
18 | 11.25 | 0.031
21 | 56.22 | 0.022
(2 rows)
Use it as a derived table to join it with deliveries:
select
d.del_id, d.del_date, d.product, d.quantity,
h.price as u_price, h.max_discount, h.price * d.quantity as total
from deliveries d
join (
select distinct on(product)
product, price, max_discount
from price_hist
order by product, start_day desc
) h using(product)
SqlFiddle.