gistfile1.txt

MariaDB []> create table test (a int, b int)
    -> ;
Query OK, 0 rows affected (0.33 sec)

MariaDB []> create index test1 on test (a)
    -> ;
Query OK, 0 rows affected (0.15 sec)
Records: 0  Duplicates: 0  Warnings: 0

MariaDB []> explain select * from test where a=1
    -> ;
+------+-------------+-------+------+---------------+-------+---------+-------+------+-------+
| id   | select_type | table | type | possible_keys | key   | key_len | ref   | rows | Extra |
+------+-------------+-------+------+---------------+-------+---------+-------+------+-------+
|    1 | SIMPLE      | test  | ref  | test1         | test1 | 5       | const |    1 |       |
+------+-------------+-------+------+---------------+-------+---------+-------+------+-------+
1 row in set (0.08 sec)

MariaDB []> explain select * from test where a=1 and b=2;
+------+-------------+-------+------+---------------+-------+---------+-------+------+-------------+
| id   | select_type | table | type | possible_keys | key   | key_len | ref   | rows | Extra       |
+------+-------------+-------+------+---------------+-------+---------+-------+------+-------------+
|    1 | SIMPLE      | test  | ref  | test1         | test1 | 5       | const |    1 | Using where |
+------+-------------+-------+------+---------------+-------+---------+-------+------+-------------+
1 row in set (0.00 sec)

MariaDB []>