Kill Session

在使用drop user cascade之前，通常使用kill session来终止当前已经连接的会话，但对于一个运行中的比较长的事务尚未提交完成的情况下，kill session常需要很长的时间。而且还必须回到操作系统级别kill掉相应的会话进程，比较麻烦。否则会出现下述错误：

ORA-01940: cannot drop a user that is currently connected

Disconnect Session

如果使用disconnect session immediate，就直接相对非常方便地终止当前某个会话，并在OS级别kill掉了进程。
下面提供的procedure可以很完美地删除数据库用户。首先使用 alter user account lock锁住用户，防止其他新会话的连接。接着循环disconnect每一个已存的session连接，并等待所有session被删除，最后drop掉。

SET SERVEROUT ON

CREATE OR REPLACE PROCEDURE gracefullyDropUser(v_username IN VARCHAR2) IS
  l_cnt integer;
  sqlStmt VARCHAR2(1000);
BEGIN
  sqlStmt := 'alter user ' || v_username || ' account lock';
  EXECUTE IMMEDIATE sqlStmt;
  dbms_output.put_line(sqlStmt);
  FOR x IN (SELECT * FROM v$session WHERE username = v_username)
  LOOP
    sqlStmt := 'alter system disconnect session ''' || x.sid || ',' || x.serial# || ''' IMMEDIATE';
    EXECUTE IMMEDIATE sqlStmt;
    dbms_output.put_line(sqlStmt);
  END LOOP;

  -- Wait until all sessions are disconnected forcely, check every 2 seconds
  LOOP
    SELECT COUNT(*) INTO l_cnt FROM v$session WHERE username = v_username;
  EXIT WHEN l_cnt = 0;
    dbms_lock.sleep(2);
    dbms_output.put_line('hold on ...');
  END LOOP;
  sqlStmt := 'drop user ' || v_username || ' cascade';
  EXECUTE IMMEDIATE sqlStmt;
  dbms_output.put_line(sqlStmt);
END gracefullyDropUser;
/

下面例子调用上述过程，删除了名为'AGILE'的用户。注意用户名的大写。

SQL> execute gracefullyDropUser('AGILE');
alter user AGILE account lock
alter system disconnect session '12,97' IMMEDIATE
alter system disconnect session '20,11' IMMEDIATE
alter system disconnect session '141,118' IMMEDIATE
hold on ...
hold on ...
drop user AGILE cascade

PL/SQL procedure successfully completed.