MySQL 8.0

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select version();

version()

8.0.20

CREATE TABLE a (x INT, f char(255) default 'xxxxxxxxxxx', INDEX(x));
INSERT INTO a (x) VALUES (4), (3), (1), (2), (0) ;
select x from a ; -- Field x is a key, so MySQL sorts by that key when a SELECT is done
                  -- on that field only!

x

0

1

2

3

4

SELECT * FROM a;  -- Compare with this where the sort is by the implicit PK,
                  -- i.e. INSERT order if no ORDER BY is specified!

x
f

4
xxxxxxxxxxx

3
xxxxxxxxxxx

1
xxxxxxxxxxx

2
xxxxxxxxxxx

0
xxxxxxxxxxx

x	f
4	xxxxxxxxxxx
3	xxxxxxxxxxx
1	xxxxxxxxxxx
2	xxxxxxxxxxx
0	xxxxxxxxxxx