CREATE TABLE t1 (D INT) ENGINE=innodb;
INSERT INTO t1 VALUES (10);
ALTER TABLE t1 MODIFY COLUMN d INT;
ALTER TABLE t1 ADD INDEX my_d (d);
SHOW CREATE TABLE t1;
Table	Create Table
t1	CREATE TABLE `t1` (
  `d` int(11) DEFAULT NULL,
  KEY `my_d` (`d`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
INSERT INTO t1 VALUES (1),(2),(3),(4),(5),(6),(7),(8),(9);
INSERT INTO t1 SELECT * FROM t1;
INSERT INTO t1 SELECT * FROM t1;
INSERT INTO t1 SELECT * FROM t1;
INSERT INTO t1 SELECT * FROM t1;
INSERT INTO t1 SELECT * FROM t1;
INSERT INTO t1 SELECT * FROM t1;
INSERT INTO t1 SELECT * FROM t1;
EXPLAIN SELECT d FROM t1 WHERE d = 5;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ref	my_d	my_d	5	const	128	Using index
EXPLAIN SELECT D FROM t1 WHERE D = 5;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ref	my_d	my_d	5	const	128	Using index
ALTER TABLE t1 DROP INDEX my_d;
ALTER TABLE t1 MODIFY COLUMN D INT;
ALTER TABLE t1 ADD INDEX my_d (D);
EXPLAIN SELECT d FROM t1 WHERE d = 5;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ref	my_d	my_d	5	const	128	Using index
EXPLAIN SELECT D FROM t1 WHERE D = 5;
id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
1	SIMPLE	t1	ref	my_d	my_d	5	const	128	Using index
SHOW CREATE TABLE t1;
Table	Create Table
t1	CREATE TABLE `t1` (
  `D` int(11) DEFAULT NULL,
  KEY `my_d` (`D`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 COLLATE=latin1_swedish_ci
DROP TABLE t1;