MariaDB [test]> explain select a, (select sum(X.a+B.b) from ten X, t1big B where  B.a=A.a or B.b=A.a) from ten A;
+----+--------------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+
| id | select_type        | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                           |
+----+--------------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+
|  1 | PRIMARY            | A     | ALL  | NULL          | NULL | NULL    | NULL |   10 |                                                 |
|  2 | DEPENDENT SUBQUERY | X     | ALL  | NULL          | NULL | NULL    | NULL |   10 |                                                 |
|  2 | DEPENDENT SUBQUERY | B     | ALL  | a,b           | NULL | NULL    | NULL | 1000 | Using where; Using join buffer (flat, BNL join) |
+----+--------------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------+

MariaDB [test]> create table all_numbers (a int primary key);
MariaDB [test]> insert into all_numbers select * from test.one_k;


MariaDB [test]> explain select a, (select sum(X.a+B.b) from ten X, t1big B, all_numbers where  B.a=all_numbers.a or B.b=all_numbers.a and all_numbers.a=A.a) from ten A;
+----+--------------------+-------------+-------+---------------+---------+---------+------+------+-------------------------------------------------+
| id | select_type        | table       | type  | possible_keys | key     | key_len | ref  | rows | Extra                                           |
+----+--------------------+-------------+-------+---------------+---------+---------+------+------+-------------------------------------------------+
|  1 | PRIMARY            | A           | ALL   | NULL          | NULL    | NULL    | NULL |   10 |                                                 |
|  2 | DEPENDENT SUBQUERY | X           | ALL   | NULL          | NULL    | NULL    | NULL |   10 |                                                 |
|  2 | DEPENDENT SUBQUERY | all_numbers | index | PRIMARY       | PRIMARY | 4       | NULL | 1000 | Using index; Using join buffer (flat, BNL join) |
|  2 | DEPENDENT SUBQUERY | B           | ALL   | a,b           | NULL    | NULL    | NULL | 1000 | Range checked for each record (index map: 0x3)  |
+----+--------------------+-------------+-------+---------------+---------+---------+------+------+-------------------------------------------------+