create table t11 (a int primary key, b int);

insert into t11 values (1,1),(2,2);

create table t10 (a int, b int, c int);

insert into t10 values (10, NULL, NULL), (10, NULL, NULL);

MariaDB [j12]> explain select t11.b, max(t10.b) from t10, t11 where t10.c<1 and t11.a=2;

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

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

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

| 1 | SIMPLE | t11 | const | PRIMARY | PRIMARY | 4 | const | 1 | |

| 1 | SIMPLE | t10 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |

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

2 rows in set (2.63 sec)

MariaDB [j12]> select t11.b, max(t10.b) from t10, t11 where t10.c<1 and t11.a=2;

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

| b | max(t10.b) |

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

| 2 | NULL |

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

1 row in set (3.31 sec)

MariaDB [j12]> alter table t11 drop primary key;

Query OK, 2 rows affected (0.01 sec)

Records: 2 Duplicates: 0 Warnings: 0

MariaDB [j12]> select t11.b, max(t10.b) from t10, t11 where t10.c<1 and t11.a=2;

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

| b | max(t10.b) |

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

| NULL | NULL |

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

1 row in set (3.66 sec)