transaction isolation
1. RU(read uncommitted)
2. RC(read committed)
3. RR(repeatable read)
4. Serializable
mysql提供了两种读取方式：快照度(snapshot read)和当前读(current read)
current read包含三种情况：
1. select ... lock in share mode 加S锁
2. select ... for update 加X锁
3. insert,update,delete 加X锁
