Postgres 13

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CREATE TABLE tbl (id int, datetime timestamp, state text, value_needed text);
    
INSERT INTO tbl VALUES
  (1, '2021-04-01 09:42:41.319000', 'incomplete', 'A')
, (2, '2021-04-04 09:42:41.319000', 'done'      , 'B')
, (3, '2021-04-05 09:42:41.319000', 'incomplete', 'C')
, (4, '2021-04-05 10:42:41.319000', 'incomplete', 'C')
, (5, '2021-04-07 09:42:41.319000', 'done'      , 'D')
, (6, '2021-04-12 09:42:41.319000', 'done'      , 'E');

CREATE TABLE



INSERT 0 6


SELECT *
FROM (
  SELECT *, 
    LEAD (state) OVER (ORDER BY datetime DESC) 
      IS DISTINCT FROM state AS first_in_group
  FROM tbl
) t
WHERE first_in_group
ORDER BY datetime DESC
LIMIT 1


id
datetime
state
value_needed
first_in_group


5
2021-04-07 09:42:41.319
done
D
t



SELECT 1


SELECT *, 
  LEAD (state) OVER (ORDER BY datetime DESC) 
    IS DISTINCT FROM state AS first_in_group
FROM tbl


id
datetime
state
value_needed
first_in_group


6
2021-04-12 09:42:41.319
done
E
f

5
2021-04-07 09:42:41.319
done
D
t

4
2021-04-05 10:42:41.319
incomplete
C
f

3
2021-04-05 09:42:41.319
incomplete
C
t

2
2021-04-04 09:42:41.319
done
B
t

1
2021-04-01 09:42:41.319
incomplete
A
t



SELECT 6

id	datetime	state	value_needed	first_in_group
6	2021-04-12 09:42:41.319	done	E	f
5	2021-04-07 09:42:41.319	done	D	t
4	2021-04-05 10:42:41.319	incomplete	C	f
3	2021-04-05 09:42:41.319	incomplete	C	t
2	2021-04-04 09:42:41.319	done	B	t
1	2021-04-01 09:42:41.319	incomplete	A	t