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CREATE TABLE

INSERT 0 20
id grp chunk new_id
1 1 null null
2 2 null null
3 3 null null
4 4 null null
5 5 null null
6 6 null null
7 7 null null
8 8 null null
9 9 null null
10 10 null null
11 11 null null
12 12 null null
13 13 null null
14 14 null null
15 15 null null
16 16 null null
17 17 null null
18 18 null null
19 19 null null
20 20 null null 
SELECT 20
id grp chunk new_id
7 7 0 0
1 1 0 1
12 12 0 2
5 5 1 0
6 6 1 1
13 13 1 2
8 8 2 0
20 20 2 1
15 15 2 2
9 9 3 0
17 17 3 1
19 19 3 2
10 10 4 0
4 4 4 1
11 11 4 2
2 2 5 0
3 3 5 1
18 18 5 2
14 14 6 0
16 16 6 1 
SELECT 20

UPDATE 20
id grp chunk new_id
20 20 0 0
7 7 0 1
19 19 0 2
15 15 1 0
18 18 1 1
2 2 1 2
14 14 2 0
9 9 2 1
6 6 2 2
4 4 3 0
11 11 3 1
12 12 3 2
17 17 4 0
8 8 4 1
1 1 4 2
13 13 5 0
3 3 5 1
16 16 5 2
10 10 6 0
5 5 6 1 
SELECT 20