MariaDB [sysbench]> explain select a.* from sysbench.sbtest1 a left join sysbench2.sbtest1 as b on a.id = b.id where a.id = 1000 order by b.id;

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

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

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

|    1 | SIMPLE      | a     | const | PRIMARY       | PRIMARY | 4       | const | 1    |             |

|    1 | SIMPLE      | b     | const | PRIMARY       | PRIMARY | 4       | const | 1    | Using index |

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

2 rows in set (0.000 sec)

MariaDB [sysbench]> explain select a.* from sysbench.sbtest1 a left join sysbench2.sbtest1 as b on a.id = b.id where a.id in ( 1000, 1000) order by b.id;

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

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

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

|    1 | SIMPLE      | a     | range  | PRIMARY       | PRIMARY | 4       | NULL          | 1    | Using where; Using temporary; Using filesort |

|    1 | SIMPLE      | b     | eq_ref | PRIMARY       | PRIMARY | 4       | sysbench.a.id | 1    | Using index                                  |

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

2 rows in set (0.000 sec)