Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

with x(data, towar, stan1, stan2, roznica) as
(values
('2020-01-01'::date,'towar1','10'::int,'9'::int,'1'::int),
('2020-01-02','towar1','11','9','2'),
('2020-01-03','towar1','15','14','1'),
('2020-01-04','towar1','12','12','0'),
('2020-01-05','towar1','10','10','0'),
('2020-01-06','towar1','10','8','2'),
('2020-01-07','towar1','5','5','0'),
('2020-01-08','towar1','4','4','0'),
('2020-01-09','towar1','3','3','0'),
('2020-01-10','towar1','1','1','0'),

('2020-01-03','towar2','15','14','1'),
('2020-01-04','towar2','12','12','0'),
('2020-01-05','towar2','10','10','0'),
('2020-01-06','towar2','10','8','2'),
('2020-01-07','towar2','5','5','0'),
('2020-01-08','towar2','4','4','0'),
('2020-01-07','towar2','5','3','2'),
('2020-01-08','towar2','4','4','0'),

('2020-01-07','towar3','5','5','0'),
('2020-01-08','towar3','8','8','0'),
('2020-01-07','towar3','5','3','2'),
('2020-01-08','towar3','4','3','1')

)
, w as
(
select 
	*, 
	sum(roznica) over(partition by towar order by data) s
from x 
)
, z as
(
select *, lag(s) over(partition by towar order by data) c from w
)
select towar, min(data) data_od, max(data) data_do, count(*) ilosc from z where s=c group by towar, s
order by 1,2

towar	data_od	data_do	ilosc
towar1	2020-01-04	2020-01-05	2
towar1	2020-01-07	2020-01-10	4
towar2	2020-01-04	2020-01-05	2
towar2	2020-01-07	2020-01-08	3
towar3	2020-01-07	2020-01-07	1
towar3	2020-01-08	2020-01-08	1