insert /*+append*/爲何會提升性能

    在上一篇的blog中 作了下使用，在歸檔和非歸檔下，作數據插入http://blog.csdn.net/guogang83/article/details/9219479。結論是在非歸檔模式下表設置爲nologging用insert /*+append*/速度最快。那爲何快呢，原理是什麼？下面咱們來一塊兒作一個實驗：

 

SQL> create or replace view m_undo_redo as
    select  v$statname.name,value
    from v$mystat, v$statname
    where v$mystat.statistic# =v$statname.statistic#
       and (v$statname.name ='redo size'
      or v$statname.name = 'undo change vector size');

視圖已建立。

SQL> create table t (x  int);

表已建立。

SQL> set timing on
SQL> select * from m_undo_redo;

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                             22644
undo change vector size                                                7484

SQL> insert into t select rownum from dual connect by level <=1000000;

已建立1000000行。

已用時間:  00: 00: 01.03
SQL> commit;

提交完成。

SQL> select * from m_undo_redo;

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                          15722456
undo change vector size                                 2380000

SQL> select (2380000-7484) undo,(15722456-22644) redo from dual;

      UNDO       REDO
---------- ----------
   2372516   15699812

SQL> truncate table t;

表被截斷。

SQL> select * from m_undo_redo;

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                          15781532
undo change vector size                                 2396672

SQL> insert /*+append*/ into t select rownum from dual connect by level <=1000000;

已建立1000000行。

已用時間:  00: 00: 00.96
SQL> commit;

提交完成。

SQL> select * from m_undo_redo;

NAME                                                                  VALUE
---------------------------------------------------------------- ----------
redo size                                                          15871640
undo change vector size                                 2419196

SQL> select (2419196-2396672) undo,(15871640-15781532) redo from dual;

      UNDO       REDO
---------- ----------
     22524      90108

兩次的對比：

 

模式	生成undo	生成redo
普通insert	2372516	15699812
insert /*+append*/	22524	90108

 

       分析結論：兩次對比的結果表示用insert /*+append*/後，數據的undo和redo沒有生成。由於HWM 在移動的過程當中，這些block是不能被其餘process使用的，那麼意味着，只要記錄下該次direct insert所涉及到的空間的redo 和 undo  ，在失敗回滾的時候，只須要把這些空間修改成原來的狀態就能夠，而不用逐個記錄去delete。

 

來源：http://blog.csdn.net/stevendbaguo/article/details/9241481