Oracle触发器不能读取表的问题

Question

12、假设有这样一张用户表表结构如下：UserInfo(id ,username,userPass)，希望向表中增加数据时，表中id列的数字自动生成。（选做）
1）第一步创建序列，要求开始的数字为1，每次递增1，按顺序产生序列值；
2）第二步创建一个触发器，向用户表中插入数据的时候触发触发器，在触发器内部调用序列并生成一个序列值赋值给表的id列。

触发器：

 create or replace trigger pro_id
  after insert on userinfo
  for each row
begin
  update userinfo set id=seq_id.nextval where username=:new.username;
end;

执行该语句时出错：

 insert into userinfo(username, userpass) values('李四',123456);

在行 1 上开始执行命令时出错:
insert into userinfo(username, userpass) values('李四',123456)
错误报告:
SQL 错误: ORA-04091: 表 SCOTT.USERINFO 发生了变化, 触发器/函数不能读它
ORA-06512: 在 "SCOTT.PRO_ID", line 2
ORA-04088: 触发器 'SCOTT.PRO_ID' 执行过程中出错
04091. 00000 - "table %s.%s is mutating, trigger/function may not see it"
*Cause: A trigger (or a user defined plsql function that is referenced in
this statement) attempted to look at (or modify) a table that was
in the middle of being modified by the statement which fired it.
*Action: Rewrite the trigger (or function) so it does not read that table.

请问哪里出错了？