FlashBack-SCN-TIMESTAMP

一、基于时间（as of timestamp）的flashback
1、创建表
create table flash_tab(id,vl) as
select rownum,oname from ( select substr(object_name,1,1) oname from all_objects
group by substr(object_name,1,1) order by 1)
where rownum<=20;

2、查询表内容
SQL> select * from flash_tab;
ID VL
---------- --
1 /
2 A
3 B
4 C
5 D
6 E
7 F
8 G
9 H
10 I
11 J
12 K
13 L
14 M
15 N
16 O
17 P
18 Q
19 R
20 S
20 rows selected

3、删除表数据.
SQL> delete flash_tab where id<10;
9 rows deleted

SQL> select * from flash_tab;
ID VL
---------- --
10 I
11 J
12 K
13 L
14 M
15 N
16 O
17 P
18 Q
19 R
20 S
11 rows selected

4、执行Flashbach Query
SQL> select * from flash_tab as of timestamp sysdate-2/1440;
ID VL
---------- --
1 /
2 A
3 B
4 C
5 D
6 E
7 F
8 G
9 H
10 I
11 J
12 K
13 L
14 M
15 N
16 O
17 P
18 Q
19 R
20 S
20 rows selected
===============
说明：
AS OF TIMESTAMP表示基于时间
SYSDATE-2/1440表示当前系统时间两分钟之前；1440表示60分*24小时

5、将删除的记录恢复
insert into flash_tab
select * from flash_tab as of timestamp sysdate-2/1440
where id<10;

commit;

SQL> select * from flash_tab;
ID VL
---------- --
10 I
11 J
12 K
13 L
14 M
15 N
16 O
17 P
18 Q
19 R
20 S
1 /
2 A
3 B
4 C
5 D
6 E
7 F
8 G
9 H
20 rows selected

二、基于SCN(as of scn)的flashback
scn值的查询可使用dbms_flashback.get_system_change_number函数；或是从v$database视图中查询current_scn值。

1、授权scott用户具有查询scn值的权限
SQL> grant execute on dbms_flashback to scott;

授权成功。
SQL> grant select on v_$database to scott;

授权成功。

2、 查询当前scn值
SQL> select dbms_flashback.get_system_change_number from dual;
GET_SYSTEM_CHANGE_NUMBER
------------------------
1564606

SQL> select current_scn from v$database; --不同时间点查询的scn值不一致
CURRENT_SCN
-----------
1564631

3、查询指定scn值时对象中的记录
SQL> select * from flash_tab as of scn 1564606;
ID VL
---------- --
10 I
11 J
12 K
13 L
14 M
15 N
16 O
17 P
18 Q
19 R
20 S
1 /
2 A
3 B
4 C
5 D
6 E
7 F
8 G
9 H
20 rows selected

4、删除对象修改
SQL> delete from flash_tab;
20 rows deleted

SQL> commit;
Commit complete

5、执行flashback as of scn恢复数据
SQL> select * from flash_tab;
ID VL
---------- --

SQL> select * from flash_tab as of scn 1564606;
ID VL
---------- --
10 I
11 J
12 K
13 L
14 M
15 N
16 O
17 P
18 Q
19 R
20 S
1 /
2 A
3 B
4 C
5 D
6 E
7 F
8 G
9 H
20 rows selected

SQL> insert into flash_tab select * from flash_tab as of scn 1564606;
20 rows inserted

SQL> select * from flash_tab;
ID VL
---------- --
10 I
11 J
12 K
13 L
14 M
15 N
16 O
17 P
18 Q
19 R
20 S
1 /
2 A
3 B
4 C
5 D
6 E
7 F
8 G
9 H
20 rows selected
============================
说明：
SCN会比TIMESTAMP更精确。事实上timestamp同样是被转换成SCN。因为oracle是通过SCN来标记操作而不是时间。
每个时间点都对应一个SCN值。在10g中，系统平均每隔3秒产生一次系统时间与SCN的匹配存入sys.SMO_SCN_TIME表中。SCN和TIMESTAMP也可相互转换。

SQL> DESC SMON_SCN_TIME;
Name Type Nullable Default Comments
------------ --------- -------- ------------------------------- --------
THREAD NUMBER Y
TIME_MP NUMBER Y
TIME_DP DATE Y
SCN_WRP NUMBER Y
SCN_BAS NUMBER Y
NUM_MAPPINGS NUMBER Y
TIM_SCN_MAP RAW(1200) Y
SCN NUMBER Y 0
ORIG_THREAD NUMBER Y 0 /* for downgrade */

SQL> select timestamp_to_scn(sysdate) from dual;
TIMESTAMP_TO_SCN(SYSDATE)
-------------------------
1569124

SQL> select to_char(scn_to_timestamp(1569124),'YYYY-MM-DD HH24:MI:SS') from dual; --时间可精确到毫秒
TO_CHAR(SCN_TO_TIMESTAMP(15691
------------------------------
2015-01-05 22:55:00