解决方法如下：

=========================================================

SQL> select session_id from v$locked_object;

SESSION_ID

----------

56

SQL> SELECT sid, serial#, username, osuser FROM v$session where sid = 56;

SID    SERIAL# USERNAME                       OSUSER

---------- ---------- ------------------------------ ------------------------------

56         2088 ghb                          fy

SQL> ALTER SYSTEM KILL SESSION '56,2088';

System altered

执行完上述命令后，提示会话断开。重新连接数据库，然后执行truncate操作，成功！