create table t1 (a int not null, b int not null) engine=innodb;
insert t1 values (1,2),(3,4);
lock table t1 write, t1 tr read;
flush status;
alter table t1 add primary key (b);
show status like 'Handler_read_rnd_next';
Variable_name	Value
Handler_read_rnd_next	0
unlock tables;
alter table t1 drop primary key;
lock table t1 write;
flush status;
alter table t1 add primary key (b);
show status like 'Handler_read_rnd_next';
Variable_name	Value
Handler_read_rnd_next	0
unlock tables;
alter table t1 drop primary key;
flush status;
alter table t1 add primary key (b);
show status like 'Handler_read_rnd_next';
Variable_name	Value
Handler_read_rnd_next	0
drop table t1;