MariaDB [(none)]> create database db;

Query OK, 1 row affected (0.002 sec)

MariaDB [(none)]> use db;

Database changed

MariaDB [db]> create table t1 (id int auto_increment primary key, value varchar(10) not null)ENGINE=MyISAM;

Query OK, 0 rows affected (0.034 sec)

MariaDB [db]> insert into t1(value) values ('a'),('b'),('c'),('d'),('e');

Query OK, 5 rows affected (0.001 sec)

Records: 5  Duplicates: 0  Warnings: 0

MariaDB [db]> create index value on t1(value);

Query OK, 5 rows affected (0.064 sec)

Records: 5  Duplicates: 0  Warnings: 0

MariaDB [db]> analyze select * from t1 where value = 'c';

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

| id   | select_type | table | type | possible_keys | key   | key_len | ref   | rows | r_rows | filtered | r_filtered | Extra                 |

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

|    1 | SIMPLE      | t1    | ref  | value         | value | 32      | const |    1 |   1.00 |   100.00 |     100.00 | Using index condition |

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

1 row in set (0.001 sec)

MariaDB [db]> drop index value on t1;

Query OK, 5 rows affected (0.067 sec)

Records: 5  Duplicates: 0  Warnings: 0

MariaDB [db]> create unique index value on t1(value);

Query OK, 5 rows affected (0.044 sec)

Records: 5  Duplicates: 0  Warnings: 0

MariaDB [db]> analyze select * from t1 where value = 'c';

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

| id   | select_type | table | type  | possible_keys | key   | key_len | ref   | rows | r_rows | filtered | r_filtered | Extra |

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

|    1 | SIMPLE      | t1    | const | value         | value | 32      | const |    1 |   NULL |   100.00 |       NULL |       |

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

1 row in set (0.001 sec)