oralce数据库的回滚操作原理

1 打开分析参数
SQL> set autotrace traceonly statistics

2 创建测试用的表格
SQL> create table t as select * from all_objects where 1=0;

表已创建。

SQL> desc t;
名称 是否为空? 类型
----------------------------------------- -------- ----------------------------

OWNER NOT NULL VARCHAR2(30)
OBJECT_NAME NOT NULL VARCHAR2(30)
SUBOBJECT_NAME VARCHAR2(30)
OBJECT_ID NOT NULL NUMBER
DATA_OBJECT_ID NUMBER
OBJECT_TYPE VARCHAR2(19)
CREATED NOT NULL DATE
LAST_DDL_TIME NOT NULL DATE
TIMESTAMP VARCHAR2(19)
STATUS VARCHAR2(7)
TEMPORARY VARCHAR2(1)
GENERATED VARCHAR2(1)
SECONDARY VARCHAR2(1)

3 看看表格里的数据，首次读取
SQL> select * from t;

未选定行

统计信息
----------------------------------------------------------
264 recursive calls
0 db block gets
30 consistent gets
0 physical reads
0 redo size
992 bytes sent via SQL*Net to client
374 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed

4 再次查看，发现不再进行磁盘读取了
SQL> select * from t;

未选定行

统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
3 consistent gets
0 physical reads
0 redo size
992 bytes sent via SQL*Net to client
374 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed

5 插入大量的数据，伴随着大量的磁盘操作
SQL> insert into t select * from all_objects;

已创建49883行。

统计信息
----------------------------------------------------------
57484 recursive calls
4139 db block gets
67164 consistent gets
290 physical reads
5623980 redo size
670 bytes sent via SQL*Net to client
575 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
348 sorts (memory)
0 sorts (disk)
49883 rows processed

6 马上回滚
SQL> rollback;

回退已完成。

7 再次查看磁盘操作情况，发现有较大的数据读取
SQL> select * from t;

未选定行

统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
689 consistent gets
0 physical reads
0 redo size
992 bytes sent via SQL*Net to client
374 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed

8 再次查看，同样的结果
SQL> select * from t;

未选定行

统计信息
----------------------------------------------------------
0 recursive calls
0 db block gets
689 consistent gets
0 physical reads
0 redo size
992 bytes sent via SQL*Net to client
374 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed

SQL>

9 结论：
回滚并不是让磁盘恢复到以前的样子。前面的INSERT 操作将一些块增加到表的高水位线（high-water mark，HWM）之下，这些块没有因为回滚而消失，它们还在那里，而且已经格式化，只不过现在为空。全表扫描必须读取这些块，看看其中是否包含行。这说明，回滚只是一个“将数据库还原“的逻辑操作。数据库并非真的还原成原来的样子，只是逻辑上相同而已。因此杜思波得出一个结论就是，oracle的rollback只是逻辑上保持了数据库回复到了上一步。