select for update

select for update的两个选项
nowait
wait [time]

实验如下：
SessionB>select * from test_serializable for update;

         A
----------
       199
       299

切换到另一个session
SQL> select * from test_serializable for update wait 180;
select * from test_serializable for update wait 180
              *
ERROR at line 1:
ORA-30006: resource busy; acquire with WAIT timeout expired
由上可见，wait 180 等待180s后如果还是无法获取表的锁就报错

SQL> select * from test_serializable for update nowait;
select * from test_serializable for update nowait
              *
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified
由上可见，no wait判断无法立即获取表的锁就报错