Postgres 14

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CREATE TABLE tsrs (
  id int
, customer_id int
, timestamp timestamp
, licensekeys_checksum varchar(32)
);

INSERT INTO tsrs VALUES
  (1, 2, '2014-08-21 16:03:35', '3FF2561A')
, (2, 2, '2014-08-22 10:00:41', '3FF2561A')
, (2, 2, '2014-06-10 10:00:41', '081AB3CA')
, (3, 5, '2014-02-01 12:03:23', '299AFF90')
, (4, 5, '2013-12-13 08:14:26', '299AFF90')
, (5, 6, '2013-09-09 18:21:53', '49FFA891')
;

CREATE TABLE



INSERT 0 6


SELECT DISTINCT ON (licensekeys_checksum) *
FROM   tsrs
ORDER  BY licensekeys_checksum, timestamp DESC NULLS LAST;


id
customer_id
timestamp
licensekeys_checksum


2
2
2014-06-10 10:00:41
081AB3CA

3
5
2014-02-01 12:03:23
299AFF90

2
2
2014-08-22 10:00:41
3FF2561A

5
6
2013-09-09 18:21:53
49FFA891



SELECT 4


-- typically slower
WITH cte AS (
   SELECT id, customer_id, timestamp, licensekeys_checksum
        , row_number() OVER (PARTITION BY licensekeys_checksum ORDER BY timestamp DESC NULLS LAST) AS rn
   FROM  tsrs
   )
SELECT *
FROM   cte
WHERE  rn = 1
ORDER  BY licensekeys_checksum;


id
customer_id
timestamp
licensekeys_checksum
rn


2
2
2014-06-10 10:00:41
081AB3CA
1

3
5
2014-02-01 12:03:23
299AFF90
1

2
2
2014-08-22 10:00:41
3FF2561A
1

5
6
2013-09-09 18:21:53
49FFA891
1



SELECT 4

id	customer_id	timestamp	licensekeys_checksum
2	2	2014-06-10 10:00:41	081AB3CA
3	5	2014-02-01 12:03:23	299AFF90
2	2	2014-08-22 10:00:41	3FF2561A
5	6	2013-09-09 18:21:53	49FFA891