Postgres 13

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CREATE TABLE orders
    (id int, store_id varchar(9), updated_at date)
;
    
INSERT INTO orders VALUES
    (0, 'zzz', '2020-01-01'),
    (1, 'store-1', '2021-01-01'),
    (2, 'store-2', '2021-01-02'),
    (3, 'store-3', '2021-01-03'),
    (4, 'store-1', '2021-01-04'),
    (5, 'store-2', '2021-01-05'),
    (6, 'store-3', '2021-01-06'),
    (7, 'store-1', '2021-01-07'),
    (8, 'store-2', '2021-01-08'),
    (9, 'store-3', '2021-01-09'),
    (10, 'store-1', '2021-01-10'),
    (11, 'store-2', '2021-01-11'),
    (12, 'store-3', '2021-01-12'),
    (13, 'store-1', '2021-01-13'),
    (14, 'store-2', '2021-01-14'),
    (15, 'store-3', '2021-01-15');
    
SELECT * FROM orders
16 rows affected



id
store_id
updated_at


0
zzz
2020-01-01

1
store-1
2021-01-01

2
store-2
2021-01-02

3
store-3
2021-01-03

4
store-1
2021-01-04

5
store-2
2021-01-05

6
store-3
2021-01-06

7
store-1
2021-01-07

8
store-2
2021-01-08

9
store-3
2021-01-09

10
store-1
2021-01-10

11
store-2
2021-01-11

12
store-3
2021-01-12

13
store-1
2021-01-13

14
store-2
2021-01-14

15
store-3
2021-01-15


     SELECT
         store_id
     FROM (
         SELECT DISTINCT ON (store_id)
             store_id, updated_at
         FROM orders
         ORDER BY store_id, updated_at
     ) s
     ORDER BY updated_at
     LIMIT 2


store_id


zzz

store-1


-- Example for 2 stores out of 3 and 4 orders

    SELECT
        o.*
    FROM (
        SELECT
            store_id
        FROM (
            SELECT DISTINCT ON (store_id)
                store_id, updated_at
            FROM orders
            ORDER BY store_id, updated_at
        ) s
        ORDER BY updated_at
        LIMIT 2
    ) s
    CROSS JOIN LATERAL (
        SELECT 
            *
        FROM orders o
        WHERE o.store_id = s.store_id
        ORDER BY updated_at
        LIMIT 4
    ) o


id
store_id
updated_at


0
zzz
2020-01-01

1
store-1
2021-01-01

4
store-1
2021-01-04

7
store-1
2021-01-07

10
store-1
2021-01-10

id	store_id	updated_at
0	zzz	2020-01-01
1	store-1	2021-01-01
2	store-2	2021-01-02
3	store-3	2021-01-03
4	store-1	2021-01-04
5	store-2	2021-01-05
6	store-3	2021-01-06
7	store-1	2021-01-07
8	store-2	2021-01-08
9	store-3	2021-01-09
10	store-1	2021-01-10
11	store-2	2021-01-11
12	store-3	2021-01-12
13	store-1	2021-01-13
14	store-2	2021-01-14
15	store-3	2021-01-15