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create table t1(a int not null, b int, c char(10), d varchar(20), primary key (a)) engine = innodb default charset=ucs2; insert into t1 values (1,1,'ab','ab'),(2,2,'ac','ac'),(3,2,'ad','ad'),(4,4,'afe','afe'); commit; alter table t1 add unique index (b); ERROR 23000: Duplicate entry '2' for key 'b' insert into t1 values(8,9,'fff','fff'); select * from t1; a b c d 1 1 ab ab 2 2 ac ac 3 2 ad ad 4 4 afe afe 8 9 fff fff show create table t1; Table Create Table t1 CREATE TABLE `t1` ( `a` int(11) NOT NULL, `b` int(11) DEFAULT NULL, `c` char(10) DEFAULT NULL, `d` varchar(20) DEFAULT NULL, PRIMARY KEY (`a`) ) ENGINE=InnoDB DEFAULT CHARSET=ucs2 alter table t1 add index (b); insert into t1 values(10,10,'kkk','iii'); select * from t1; a b c d 1 1 ab ab 2 2 ac ac 3 2 ad ad 4 4 afe afe 8 9 fff fff 10 10 kkk iii select * from t1 force index(b) order by b; a b c d 1 1 ab ab 2 2 ac ac 3 2 ad ad 4 4 afe afe 8 9 fff fff 10 10 kkk iii explain select * from t1 force index(b) order by b; id select_type table partitions type possible_keys key key_len ref rows filtered Extra 1 SIMPLE t1 NULL index NULL b 5 NULL 6 100.00 NULL Warnings: Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c`,`test`.`t1`.`d` AS `d` from `test`.`t1` FORCE INDEX (`b`) order by `test`.`t1`.`b` show create table t1; Table Create Table t1 CREATE TABLE `t1` ( `a` int(11) NOT NULL, `b` int(11) DEFAULT NULL, `c` char(10) DEFAULT NULL, `d` varchar(20) DEFAULT NULL, PRIMARY KEY (`a`), KEY `b` (`b`) ) ENGINE=InnoDB DEFAULT CHARSET=ucs2 alter table t1 add unique index (c), add index (d); insert into t1 values(11,11,'aaa','mmm'); select * from t1; a b c d 1 1 ab ab 2 2 ac ac 3 2 ad ad 4 4 afe afe 8 9 fff fff 10 10 kkk iii 11 11 aaa mmm select * from t1 force index(b) order by b; a b c d 1 1 ab ab 2 2 ac ac 3 2 ad ad 4 4 afe afe 8 9 fff fff 10 10 kkk iii 11 11 aaa mmm select * from t1 force index(c) order by c; a b c d 11 11 aaa mmm 1 1 ab ab 2 2 ac ac 3 2 ad ad 4 4 afe afe 8 9 fff fff 10 10 kkk iii select * from t1 force index(d) order by d; a b c d 1 1 ab ab 2 2 ac ac 3 2 ad ad 4 4 afe afe 8 9 fff fff 10 10 kkk iii 11 11 aaa mmm explain select * from t1 force index(b) order by b; id select_type table partitions type possible_keys key key_len ref rows filtered Extra 1 SIMPLE t1 NULL index NULL b 5 NULL 7 100.00 NULL Warnings: Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c`,`test`.`t1`.`d` AS `d` from `test`.`t1` FORCE INDEX (`b`) order by `test`.`t1`.`b` explain select * from t1 force index(c) order by c; id select_type table partitions type possible_keys key key_len ref rows filtered Extra 1 SIMPLE t1 NULL index NULL c 21 NULL 7 100.00 NULL Warnings: Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c`,`test`.`t1`.`d` AS `d` from `test`.`t1` FORCE INDEX (`c`) order by `test`.`t1`.`c` explain select * from t1 force index(d) order by d; id select_type table partitions type possible_keys key key_len ref rows filtered Extra 1 SIMPLE t1 NULL index NULL d 43 NULL 7 100.00 NULL Warnings: Note 1003 /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b`,`test`.`t1`.`c` AS `c`,`test`.`t1`.`d` AS `d` from `test`.`t1` FORCE INDEX (`d`) order by `test`.`t1`.`d` show create table t1; Table Create Table t1 CREATE TABLE `t1` ( `a` int(11) NOT NULL, `b` int(11) DEFAULT NULL, `c` char(10) DEFAULT NULL, `d` varchar(20) DEFAULT NULL, PRIMARY KEY (`a`), UNIQUE KEY `c` (`c`), KEY `b` (`b`), KEY `d` (`d`) ) ENGINE=InnoDB DEFAULT CHARSET=ucs2 check table t1; Table Op Msg_type Msg_text test.t1 check status OK drop table t1;