乐观锁

乐观锁的常规操作
select rev from table where id = $id
update table set name=$name, rev=rev+1 where id = $id and rev=$rev
此时有两个线程a和b来修改用户名
分成4中情况
1、

a	b
select rev from table where id = $id	select rev from table where id = $id
update table set name=$name, rev=rev+1 where id = $id and rev=$rev  rev=2
	update table set name=$name, rev=rev+1 where id = $id and rev=$rev  失败

a	b
select rev from table where id = $id	select rev from table where id = $id
	update table set name=$name, rev=rev+1 where id = $id and rev=$rev  rev=2
update table set name=$name, rev=rev+1 where id = $id and rev=$rev  失败

a	b
select rev from table where id = $id
update table set name=$name, rev=rev+1 where id = $id and rev=$rev rev=2
	select rev from table where id = $id
	update table set name=$name, rev=rev+1 where id = $id and rev=$rev rev=3

a	b
	select rev from table where id = $id
	update table set name=$name, rev=rev+1 where id = $id and rev=$rev rev=2
select rev from table where id = $id
update table set name=$name, rev=rev+1 where id = $id and rev=$rev rev=3