ALTER TABLE

mysql> CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL);
Query OK, 0 rows affected (0.11 sec)

mysql> INSERT INTO t1 (c1) VALUES (1),(2),(3),(4),(5);
Query OK, 5 rows affected (0.03 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
+---------------------+----------+------+-------------------------------------------------------------+
| id                  | count    | task | operator info                                               |
+---------------------+----------+------+-------------------------------------------------------------+
| TableReader_7       | 10.00    | root | data:Selection_6                                            |
| └─Selection_6       | 10.00    | cop  | eq(test.t1.c1, 3)                                           |
|   └─TableScan_5     | 10000.00 | cop  | table:t1, range:[-inf,+inf], keep order:false, stats:pseudo |
+---------------------+----------+------+-------------------------------------------------------------+
3 rows in set (0.00 sec)

mysql> ALTER TABLE t1 ADD INDEX (c1);
Query OK, 0 rows affected (0.30 sec)

mysql> EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
+-------------------+-------+------+-----------------------------------------------------------------+
| id                | count | task | operator info                                                   |
+-------------------+-------+------+-----------------------------------------------------------------+
| IndexReader_6     | 10.00 | root | index:IndexScan_5                                               |
| └─IndexScan_5     | 10.00 | cop  | table:t1, index:c1, range:[3,3], keep order:false, stats:pseudo |
+-------------------+-------+------+-----------------------------------------------------------------+
2 rows in set (0.00 sec)