Oracle Flashback Technology Detailed (flashback)

1 Overview

• In order to quickly restore an Oracle database from erroneous operation, Oracle introduced Flashback technology .
• This blog, the main share Flashback technology Flashback Query, Flashback Transaction Query, Flashback Versions Query, Flashback Table and Flashback Drop for Flashback Database and Flashback archive , the scope is too broad, is not recommended .

Query the current system timestamp and SCN

SELECT to_char(SYSDATE, 'YYYY-MM-HH hh24:mi:ss:mm') timestamp,
       timestamp_to_scn(SYSDATE) scn
  FROM dual;

2 Example description

2.1 Flashback query

• Role: query data before a certain period of time in the past
• Note: commit time

grammar:

select *
  from table_name
       [as of timestamp | scn expression]
[where condition]

Basic data: insert 3 records and perform flashback query

CREATE TABLE flashback_select_test (
  ID          NUMBER,
  NAME        VARCHAR2(30),
  create_date DATE,
  create_scn  NUMBER
);

-- 设置时间格式，查看方便
ALTER SESSION SET nls_date_format = 'YYYY/MM/DD HH24:MI:SS';

-- insert 语句，执行间隔 几秒钟，方便测试
INSERT INTO flashback_select_test
   (id, NAME, create_date, create_scn)
VALUES
   (1, 'a', SYSDATE, timestamp_to_scn(SYSDATE));
COMMIT;

INSERT INTO flashback_select_test
   (id, NAME, create_date, create_scn)
VALUES
   (2, 'b', SYSDATE, timestamp_to_scn(SYSDATE));
COMMIT;

INSERT INTO flashback_select_test
   (id, NAME, create_date, create_scn)
VALUES
   (3, 'c', SYSDATE, timestamp_to_scn(SYSDATE));
COMMIT;

data verification:

SELECT * FROM flashback_select_test;
SELECT * FROM flashback_select_test AS OF TIMESTAMP to_timestamp('2020/8/3 22:21:04', 'YYYY/MM/DD HH24:MI:SS');
SELECT * FROM flashback_select_test AS OF SCN 12614250;

Query screenshot:

2.2 Flashback version query

• When the record changes, commit time happens to be recorded, available Flashback Query (under normal circumstances, we would not deliberately go record this time)
• If there is no record, then use the Flashback Versions Query
• Role: You can query all the submitted records

grammar:

select *
  from table_name
versions between scn expr | timestamp minvalue
             and expr | maxvalue [as of scn | timestamp expr];

1. between ... and: 时间段
2. scn | timestamp：时间格式(系统变更号 | 时间戳)
3. minvalue | maxvalue：时间取值(最小值 | 最大值)
4. 可使用的伪列
  (1) versions_starttime : 基于 时间 的版本有效范围下界
  (2) versions_startscn  : 基于 SCN 的版本有效范围下界
  (3) versions_endtime   : 基于 时间 的版本有效范围上界
  (4) versions_endscn    : 基于 SCN 的版本有效范围上界
  (5) versions_xid       : 操作的事务 ID
  (6) versions_operation : 执行操作的类型. I: insert、U: delete、D: DELETE

Demonstration: (When the DML statement is executed, the interval is a few seconds, which is convenient for observation)

CREATE TABLE flashback_versions_test (
  ID   NUMBER,
  NAME VARCHAR2(30)
);

-- 下列语句执行时，间隔几秒钟，方便观察
INSERT INTO flashback_versions_test(ID, NAME) VALUES(1, '瑶瑶');
COMMIT;

INSERT INTO flashback_versions_test(ID, NAME) VALUES(2, '倩倩');
COMMIT;

UPDATE flashback_versions_test t SET t.name = '倩倩123' WHERE t.id = 2;
COMMIT;

INSERT INTO flashback_versions_test(ID, NAME) VALUES(3, '优优');
COMMIT;

Flashback version query statement:

SELECT id,
       NAME,
       to_char(versions_starttime, 'YYYY-MM-DD HH24:MI:SS') versions_starttime,
       to_char(versions_endtime, 'YYYY-MM-DD HH24:MI:SS') versions_endtime,
       versions_startscn,
       versions_endscn,
       versions_xid,
       versions_operation
  FROM flashback_versions_test versions BETWEEN TIMESTAMP minvalue AND maxvalue
 ORDER BY versions_starttime;

search result:

2.3 Flashback transaction query

• Flashback Transaction Query is actually Flashback Versions Query an expanded audit of a transaction through which you can even undo a transaction has been submitted.

Prerequisite knowledge understanding:

SELECT t.*
  FROM flashback_transaction_query t
 WHERE t.logon_user = 'SYSTEM'; -- 你登录的用户

-- GRANT SELECT ANY TRANSACTION TO scott; -- 测试用户，若权限不足，需授权

-- 添加补充日志 (若没有 undo_sql 为空)
SELECT t.supplemental_log_data_min, t.* FROM v$database t; -- YSE: 开启
ALTER DATABASE ADD SUPPLEMENTAL LOG DATA; -- 开启语句

Basic data preparation: suppose that when updating, forget to bring the where condition

CREATE TABLE flashback_transaction_query_ts (
   ID          NUMBER,
   NAME        VARCHAR2(30),
   create_date DATE
);

-- insert 语句，间隔 几秒钟，方便观察
INSERT INTO flashback_transaction_query_ts
   (id, NAME, create_date)
VALUES
   (1, '小游子', SYSDATE);
COMMIT;

INSERT INTO flashback_transaction_query_ts
   (id, NAME, create_date)
VALUES
   (2, '小优子', SYSDATE);
COMMIT;

INSERT INTO flashback_transaction_query_ts
   (id, NAME, create_date)
VALUES
   (3, '小倩子', SYSDATE);
COMMIT;

-- 假设本来想更新 id = 2 的记录，结果忘记写条件 where id = 2 
UPDATE flashback_transaction_query_ts t SET t.name = '小优子22';
COMMIT;

data verification:

1. 首先使用 闪回版本查询，查询 事务id
SELECT id,
       NAME,
       to_char(versions_starttime, 'YYYY-MM-DD HH24:MI:SS') versions_starttime,
       to_char(versions_endtime, 'YYYY-MM-DD HH24:MI:SS') versions_endtime,
       versions_startscn,
       versions_endscn,
       versions_xid,
       versions_operation
  FROM flashback_transaction_query_ts versions BETWEEN TIMESTAMP minvalue AND maxvalue
 ORDER BY versions_starttime;

2. 根据 事务id，获取 undo_sql
SELECT t.*
  FROM flashback_transaction_query t
 WHERE t.logon_user = 'SYSTEM'
   AND t.xid = '08001C00AB1F0000';

Test Results:

-- undo_sql 复制如下：
1 update "SYSTEM"."FLASHBACK_TRANSACTION_QUERY_TS" set "NAME" = '小倩子' where ROWID = 'AAAVUzAABAAAVxpAAC';
2 update "SYSTEM"."FLASHBACK_TRANSACTION_QUERY_TS" set "NAME" = '小优子' where ROWID = 'AAAVUzAABAAAVxpAAB';
3 update "SYSTEM"."FLASHBACK_TRANSACTION_QUERY_TS" set "NAME" = '小游子' where ROWID = 'AAAVUzAABAAAVxpAAA';

2.4 Flashback Table

• Restore the amount of data deleted by delete
• Need to enable row migration
• Note the time retained in undo_retention

Basic data:

CREATE TABLE flashback_table_test (
  ID   NUMBER,
  NAME VARCHAR2(30)
);

INSERT INTO flashback_table_test (ID, NAME) VALUES(1, '瑶瑶');
INSERT INTO flashback_table_test (ID, NAME) VALUES(2, '倩倩');
INSERT INTO flashback_table_test (ID, NAME) VALUES(3, '优优');
COMMIT;

Flashback table demo:

-- 先查询
SELECT t.*, ROWID FROM flashback_table_test t;

SELECT to_char(SYSDATE, 'YYYY-MM-HH hh24:mi:ss:mm') 时间,
       timestamp_to_scn(SYSDATE) scn
  FROM dual;

-- 再删除
DELETE FROM flashback_table_test t WHERE t.id = 2;
COMMIT;

-- 闪回命令 start
ALTER TABLE flashback_table_test ENABLE ROW MOVEMENT; -- 打开表的行移动(重新生成 rowid)
FLASHBACK TABLE flashback_table_test TO SCN 12442453;
-- 闪回命令 end

SELECT t.*, ROWID FROM flashback_table_test t;

2.5 Flashback delete

• Recoverable: Table deleted using drop
• Can not be restored: the table under the administrator user (such as: system, sys) or truncate table

grammar:

FLASHBACK TABLE table_name 
		TO BEFORE DROP [RENAME TO table_name2] -- 删除表同时修改 表名
		[ENABLE|DISABLE] triggers; -- 删除表的同时，启用|禁用 触发器

Basic data preparation:

CREATE TABLE scott.flashback_drop_test (
   ID   NUMBER,
   NAME VARCHAR2(30)
);

SELECT * FROM scott.flashback_drop_test;

DROP TABLE scott.flashback_drop_test;

Case 1: General flashback

SELECT * FROM dba_recyclebin t WHERE t.owner = 'SCOTT'; -- 或 user_recyclebin 

FLASHBACK TABLE scott.flashback_drop_test TO BEFORE DROP;

Case 2: The table name in the recycle bin already exists in the database (duplicate name):

• Solution: Rename

SELECT * FROM dba_recyclebin t WHERE t.owner = 'SCOTT' ORDER BY t.droptime DESC; -- 或 user_recyclebin 

方式1：
FLASHBACK TABLE scott.flashback_drop_test TO BEFORE DROP RENAME TO flashback_drop_test2;

方式2：（推荐，防止 "一张表，多次删除" 的情况）
-- SELECT * FROM scott."BIN$hr4mmwLmTAyvSvOcqvFO+A==$0";
FLASHBACK TABLE scott."BIN$hr4mmwLmTAyvSvOcqvFO+A==$0" TO BEFORE DROP RENAME TO flashback_drop_test3;

Situation 3: One table, multiple deletions

• Please note: the restored table is the last deleted table

Basic data preparation:

CREATE TABLE scott.flashback_drop_test (
   ID   NUMBER,
   NAME VARCHAR2(30)
);

DROP TABLE scott.flashback_drop_test;

-- 以上 创建、删除 至少操作 2 次，方便演示
SELECT * FROM user_recyclebin t ORDER BY t.droptime DESC;

Demo:

FLASHBACK TABLE flashback_drop_test TO BEFORE DROP;

3 expansion

1. 显示回收站（sqlplus 命令）
> show recyclebin

2. 清空回收站（sqlplus 命令）
> purge recyclebin

3. 查询回收站
select * from dba_recyclebin; -- user_recyclebin

4. 删除表并在回收站中清空
drop table table_name purge;

5. 清空表数据并在回收站中清空
truncate table table_name;