1create table t1 (a int not null, b int not null) engine=innodb;
2insert t1 values (1,2),(3,4);
3lock table t1 write, t1 tr read;
4flush status;
5alter table t1 add primary key (b);
6show status like 'Handler_read_rnd_next';
7Variable_name	Value
8Handler_read_rnd_next	0
9unlock tables;
10alter table t1 drop primary key;
11lock table t1 write;
12flush status;
13alter table t1 add primary key (b);
14show status like 'Handler_read_rnd_next';
15Variable_name	Value
16Handler_read_rnd_next	0
17unlock tables;
18alter table t1 drop primary key;
19flush status;
20alter table t1 add primary key (b);
21show status like 'Handler_read_rnd_next';
22Variable_name	Value
23Handler_read_rnd_next	0
24drop table t1;
25