gistfile1.txt

root@localhost [test] > create table t (a int, b int, c varchar(255), index (a, b));
Query OK, 0 rows affected (0.03 sec)

root@localhost [test] > insert into t values
    -> (200, 200, 'aaa'),
    -> (199, 199, 'aaa'),
    -> (198, 198, 'aaa'),
    -> (197, 197, 'aaa'),
    -> (196, 196, 'aaa'),
    -> (195, 195, 'aaa'),
    -> (194, 194, 'aaa'),
    -> (193, 193, 'aaa'),
    -> (192, 192, 'aaa'),
    -> (191, 191, 'aaa'),
    -> (190, 190, 'aaa'),
    -> (189, 189, 'aaa'),
    -> (188, 188, 'aaa'),
    -> (187, 187, 'aaa'),
    -> (186, 186, 'aaa'),
    -> (185, 185, 'aaa'),
    -> (184, 184, 'aaa'),
    -> (183, 183, 'aaa'),
    -> (182, 182, 'aaa'),
    -> (181, 181, 'aaa'),
    -> (180, 180, 'aaa'),
    -> (179, 179, 'aaa'),
    -> (178, 178, 'aaa'),
    -> (177, 177, 'aaa'),
    -> (176, 176, 'aaa'),
    -> (175, 175, 'aaa'),
    -> (174, 174, 'aaa'),
    -> (173, 173, 'aaa'),
    -> (172, 172, 'aaa'),
    -> (171, 171, 'aaa'),
    -> (170, 170, 'aaa');
Query OK, 31 rows affected (0.00 sec)
Records: 31  Duplicates: 0  Warnings: 0

root@localhost [test] > select a, b from t;
+------+------+
| a    | b    |
+------+------+
|  170 |  170 |
|  171 |  171 |
|  172 |  172 |
|  173 |  173 |
|  174 |  174 |
|  175 |  175 |
|  176 |  176 |
|  177 |  177 |
|  178 |  178 |
|  179 |  179 |
|  180 |  180 |
|  181 |  181 |
|  182 |  182 |
|  183 |  183 |
|  184 |  184 |
|  185 |  185 |
|  186 |  186 |
|  187 |  187 |
|  188 |  188 |
|  189 |  189 |
|  190 |  190 |
|  191 |  191 |
|  192 |  192 |
|  193 |  193 |
|  194 |  194 |
|  195 |  195 |
|  196 |  196 |
|  197 |  197 |
|  198 |  198 |
|  199 |  199 |
|  200 |  200 |
+------+------+
31 rows in set (0.00 sec)

root@localhost [test] > explain select a, b from t;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | t     | NULL       | index | NULL          | a    | 10      | NULL |   31 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)