监控一个大事务的回滚

我们在大的事务失败时往往面临长时间的回滚，在回滚期间表会被加以TM-3 SX sub-exclusive锁，此时一般我们是无法针对表实施DDL操作的。长时间的大事务回滚可能耗尽我们的耐心，不过我们还是有办法预估何时回滚能够完成的，参考中的脚本<Script:when transaction will finish rollback>中的脚本，注意该脚本需要访问x$ktuxe内部视图，所以需要以sysdba身份方能执行。

SQL> select * from v$lock where type in ('TM','TX');

ADDR     KADDR           SID TY        ID1        ID2      LMODE    REQUEST      CTIME      BLOCK
-------- -------- ---------- -- ---------- ---------- ---------- ---------- ---------- ----------
0ED0F30C 0ED0F33C          9 TM      13865          0          3          0       3757          0
2C3975FC 2C39763C          9 TX      65557        677          6          0       3757          0

SQL> select object_name,object_type from dba_objects where object_id=13865;

OBJECT_NAM OBJECT_TYPE
---------- -------------------
SAMPLE     TABLE

因为表上存在TM-3锁，所以此时是无法对表执行需要持有TM-6 exclusive排它锁的DDL操作的

SQL> drop table sample;
drop table sample
           *
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired

SQL> select * from sample where rownum=1 for update nowait;
select * from sample where rownum=1 for update nowait
              *
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired

rollback完成之前相关行上的row level lock不会被释放，因此dml操作会被block

SQL> select xidusn,xidslot,xidsqn,status from v$transaction;

    XIDUSN    XIDSLOT     XIDSQN STATUS
---------- ---------- ---------- ----------------
         1         21        677 ACTIVE

从v$tranasction视图中可以看到事务1.21.677处于active状态

使用set time on命令显示当前时间

SQL> set time on;

20:54:26 SQL>

set serveroutput on
set feedback off
prompt
prompt Looking for transactions that are rolling back ...
prompt

declare
  cursor tx is
    select
      s.username,
      t.xidusn,
      t.xidslot,
      t.xidsqn,
      x.ktuxesiz
    from
      sys.x$ktuxe  x,
      sys.v_$transaction  t,
      sys.v_$session  s
    where
      x.inst_id = userenv('Instance') and
      x.ktuxesta = 'ACTIVE' and
      x.ktuxesiz > 1 and
      t.xidusn = x.ktuxeusn and
      t.xidslot = x.ktuxeslt and
      t.xidsqn = x.ktuxesqn and
      s.saddr = t.ses_addr;
  user_name  varchar2(30);
  xid_usn    number;
  xid_slot   number;
  xid_sqn    number;
  used_ublk1 number;
  used_ublk2 number;
begin
  open tx;
  loop
    fetch tx into user_name, xid_usn, xid_slot, xid_sqn, used_ublk1;
    exit when tx%notfound;
    if tx%rowcount = 1
    then
      sys.dbms_lock.sleep(10);
    end if;
    select
      sum(ktuxesiz)
    into
      used_ublk2
    from
      sys.x$ktuxe
    where
      inst_id = userenv('Instance') and
      ktuxeusn = xid_usn and
      ktuxeslt = xid_slot and
      ktuxesqn = xid_sqn and
      ktuxesta = 'ACTIVE';
    if used_ublk2 < used_ublk1
    then
      sys.dbms_output.put_line(
        user_name ||
        '''s transaction ' ||
        xid_usn  || '.' ||
        xid_slot || '.' ||
        xid_sqn  ||
        ' will finish rolling back at approximately ' ||
        to_char(
          sysdate + used_ublk2 / (used_ublk1 - used_ublk2) / 6 / 60 / 24,
          'HH24:MI:SS DD-MON-YYYY'
        )
      );
    end if;
  end loop;
  if user_name is null
  then
    sys.dbms_output.put_line('No transactions appear to be rolling back.');
  end if;
end;
/

MACLEAN's transaction 1.21.677 will finish rolling back at approximately 21:46:45 25-MAY-2011

以上脚本给出了估计的1.21.677事务的回滚结束时间为21:46:45，即还有50分钟，
注意这只是根据之前单位时间内rollback的进度所得到的一个估算值，所以并不精确，但还是可以做为一个参考

再次运行以上脚本，会发现2次的结果不一定一致

SQL> /
MACLEAN's transaction 1.21.677 will finish rolling back at approximately 21:35:47 25-MAY-2011

在产品数据库中应当尽可能避免出现大事务回滚的现象，因为除去lock的影响外，large transaction rollback还会导致CPU使用率大幅上升并产生往往比事务本身所产生的更多的redo重做日志记录。