CREATE TABLE t1 (a INT, INDEX(a)) ENGINE=InnoDB;

INSERT INTO t1 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

MariaDB [test]> SET OPTIMIZER_SWITCH="index_merge_sort_union=ON";

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> explain SELECT * FROM t1 WHERE a > 5;

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | t1    | range | a             | a    | 5       | NULL |    4 | Using where; Using index |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

1 row in set (0.00 sec)

MariaDB [test]> SET OPTIMIZER_SWITCH="index_merge_sort_union=OFF";

Query OK, 0 rows affected (0.00 sec)

MariaDB [test]> explain SELECT * FROM t1 WHERE a > 5;

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

| id   | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

|    1 | SIMPLE      | t1    | index | a             | a    | 5       | NULL |   10 | Using where; Using index |

+------+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+

1 row in set (0.00 sec)