從理論上來說，對於 direct insert 的 undo （rowid）實在沒有存在的必要
因爲HWM 在移動的過程中，這些block是不能被其他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，只需要把這些空間修改爲原來的狀態就可以而而不用逐個記錄去delete。
爲此我將在這裏做幾個組合實驗
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.1.0.2.0 - Prod
PL/SQL Release 10.1.0.2.0 - Production
CORE 10.1.0.2.0 Production
TNS for Linux: Version 10.1.0.2.0 - Production
NLSRTL Version 10.1.0.2.0 - Production
SQL>
SQL> drop table t;
Table dropped.
SQL> create table t as select * from dba_objects ;
Table created.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 253952 11168864
2 2220032 11335220
3 2220032 9668346
4 1171456 8887572
5 122880 84154
6 122880 0
7 1171456 234
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> insert into t select * from dba_objects;
13197 rows created.
SQL> commit;
Commit complete.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 253952 11168864
2 2220032 11336024
3 2220032 9668346
4 2220032 9810066
5 122880 85452
6 122880 0
7 1171456 234
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> select 9810066 - 8887572 from dual;
9810066-8887572
---------------
922494 正常情況下產生的回滾段信息量
SQL> select 85452 - 84152 from dual;
85452-84152
-----------
1300 undo中這個細微變化目前不詳，估計可能是空間的變化導致的
SQL> alter table t nologging;設置表爲nologging
Table altered.
SQL> insert /*+ append */ into t select * from dba_objects; direct insert
13197 rows created.
SQL> commit;
Commit complete.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 253952 11169498
2 2220032 11339548
3 2220032 9670376
4 2220032 9813076
5 122880 85452
6 122880 0
7 1171456 234
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> select 9670376 - 9668346 from dual;
9670376-9668346
---------------
2030 發現回滾段信息極少
SQL> select 9813076-9810066 from dual;
9813076-9810066
---------------
3010
SQL>
SQL> alter table t logging; 把表置回logging狀態
Table altered.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 253952 11173844
2 2220032 11343052
3 2220032 9676700
4 2220032 9816500
5 122880 89668
6 122880 0
7 1171456 234
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> insert into t select * from dba_objects; 正常插入數據
13197 rows created.
SQL> commit;
Commit complete.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 253952 11173844
2 2220032 12266342
3 2220032 9676918
4 2220032 9817356
5 122880 90478
6 122880 0
7 1171456 234
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> select 12266342 - 11343052 from dual;
12266342-11343052
-----------------
923290 產生的回滾段信息
SQL> insert /*+ append */ into t select * from dba_objects; direct insert
13197 rows created.
SQL> commit;
Commit complete.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 253952 11174528
2 2220032 12266768
3 2220032 9678420
4 2220032 9817356
5 122880 92562
6 122880 0
7 1171456 234
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL>
SQL> select 11174528 - 11173844 from dual;
11174528-11173844
-----------------
684
SQL> select 92562 - 90478 from dual;
92562-90478
-----------
2084
SQL>
在這裏我們覈對任意一個回滾段的生成量，發現幾乎很少
由此我們幾乎可以下個結論，不管表是否在nologging 下，只要是 direct insert，就不會對數據內容生成undo，也就是不會爲insert而記錄 rowid
接下來我們進一步實驗，在表同樣處於 logging 狀態下測試
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 133173584 1236385760
SQL> insert into t select * from dba_objects;
13197 rows created.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 137974492 1236385760
SQL> select 137974492 - 133173584 from dual;
137974492-133173584
-------------------
4800908 正常插入產生的日誌
SQL> roll;
Rollback complete.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 140087680 1236385760
SQL> select 140087680 - 137974492 from dual;
140087680-137974492
-------------------
2113188 正常插入後回滾所產生的日誌
SQL> insert /*+ append */ into t select * from dba_objects;
13197 rows created.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 141531644 1236385760
SQL> select 141531644 - 140087680 from dual;
141531644-140087680
-------------------
1443964 direct 插入產生的日誌
SQL> roll;
Rollback complete.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 141534344 1236385760
SQL> select 141534344 - 141531644 from dual;
141534344-141531644
-------------------
2700 direct插入後回滾產生的日誌
SQL>
從這裏的實驗可以看出，在 direct insert 後回滾數據，實際上並沒有進行數據的 刪除操作
而是僅僅對空間進行了回收。若是刪除，不可能只產生這麼少的 redo，這裏從另一個側面證明
即使 logging 下的 direct insert 對於回滾信息，也是不會對數據產生 undo 而僅僅產生空間變化的 undo
----------------------------常規插入-------direct插入
插入日誌生成量----------------8350864--------2364484
插入回滾段生成量--------------2343894--------426838
回滾日誌生成量----------------4018204--------76032
回滾本身不存在產生回滾-------------------------------------
結論是很顯然的，也許這裏有人要問，既然direct有這麼多好處，那爲什麼還用常規？
因爲920前的版本sqlldr direct導致trigger無用、函數無用
direct直接在hwm上移動而不使用delete刪除釋放的空間可能導致空間浪費
direct的時候，據說同一個extent只能由一個進程使用（未測試，目前無LMT表空間環境）
direct insert 未提交會阻塞其他dml，也會阻止進程自己對該表的查詢。
direct + nologging 由於不產生數據日誌導致恢復會出現問題
SQL> truncate table t;
Table truncated.
SQL> create index t_index on t(object_id);
Index created.
SQL> col name format a20
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 516096 14166140
2 1171456 17003930
3 2220032 13918700
4 1171456 13550540
5 122880 756246
6 122880 0
7 1171456 312
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 181757168 1236385760
SQL> insert into t select * from dba_objects;
13198 rows created.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 516096 14166140
2 4317184 19347824
3 2220032 13918700
4 1171456 13551396
5 122880 756246
6 122880 0
7 1171456 312
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 190108032 1236385760
SQL> select 19347824 - 17003930 from dual;
19347824-17003930
-----------------
2343894 存在索引，常規插入方式下產生的回滾量
SQL> select 190108032 - 181757168 from dual;
190108032-181757168
-------------------
8350864 存在索引，常規插入方式下產生的日誌量
SQL> roll;
Rollback complete.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 516096 14166140
2 4317184 19347824
3 2220032 13918700
4 1171456 13551396
5 122880 757102
6 122880 0
7 1171456 312
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 194126236 1236385760
SQL> select 194126236 - 190108032 from dual;
194126236-190108032
-------------------
4018204 存在索引，常規插入方式下然後回滾所產生的日誌量
SQL> truncate table t;
Table truncated.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 581632 14185742
2 4317184 19356862
3 2220032 13936438
4 1171456 13566936
5 122880 757102
6 122880 0
7 1171456 312
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 194287336 1236385760
SQL> insert /*+ append */ into t select * from dba_objects;
13198 rows created.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 2088960 14612580
2 4317184 19356862
3 2220032 13936438
4 1171456 13569090
5 122880 757102
6 122880 0
7 1171456 312
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 196651820 1236385760
SQL> select 14612580 - 14185742 from dual;
14612580-14185742
-----------------
426838 存在索引，direct插入方式下產生的回滾量
SQL> select 196651820 - 194287336 from dual;
196651820-194287336
-------------------
2364484 存在索引，direct插入方式下產生的日誌量
SQL> roll;
Rollback complete.
SQL> select usn,RSSIZE ,WRITES from v$rollstat;
USN RSSIZE WRITES
---------- ---------- ----------
0 385024 7620
1 2088960 14612580
2 4317184 19360722
3 2220032 13936438
4 1171456 13569946
5 122880 757908
6 122880 0
7 1171456 312
8 122880 0
9 122880 0
10 122880 0
11 rows selected.
SQL> select * from v$sysstat where name = 'redo size';
STATISTIC# NAME CLASS VALUE STAT_ID
---------- -------------------- ---------- ---------- ----------
133 redo size 2 196727852 1236385760
SQL> select 196727852 - 196651820 from dual;
196727852-196651820
-------------------
76032 存在索引，direct插入方式下回滾產生的日誌量
SQL>