Flashback technologies by examples

Preamble

At which level can you flashback figures in Oracle:

Database level Data will be modified ? Technology
Database Yes (uses flashback logs and archived logs) Flashback logs
Table Yes Undo
Drop Table Yes (obviously you accidentally dropped the table) Recycle Bin
Query No Undo
Version Query No Undo
Transaction Query No Undo
Archive Yes Flashback Data Archive

All testing has been done on Oracle 11.2.0.2.0 on Red Hat Enterprise Linux Server release 5.5 (Tikanga).

To avoid any non-expected results Undo tablespace retention has been set to guaranteed (and so preserved for UNDO_RETENTION seconds, set to one hour):

SQL> SET lines 200
SQL> SELECT tablespace_name,retention FROM dba_tablespaces;
 
TABLESPACE_NAME                RETENTION
------------------------------ -----------
SYSTEM                         NOT APPLY
SYSAUX                         NOT APPLY
UNDOTBS1                       NOGUARANTEE
TEMP                           NOT APPLY
USERS                          NOT APPLY
TESTTBS                        NOT APPLY
 
6 ROWS selected.
 
SQL> ALTER TABLESPACE UNDOTBS1 retention guarantee;
 
TABLESPACE altered.
 
SQL> SELECT tablespace_name,retention FROM dba_tablespaces;
 
TABLESPACE_NAME                RETENTION
------------------------------ -----------
SYSTEM                         NOT APPLY
SYSAUX                         NOT APPLY
UNDOTBS1                       GUARANTEE
TEMP                           NOT APPLY
USERS                          NOT APPLY
TESTTBS                        NOT APPLY
 
6 ROWS selected.
 
SQL> ALTER SYSTEM SET undo_retention=3600;
 
SYSTEM altered.

All testing have been done with below table (created in locally managed tablespace):

SQL> CREATE TABLE test1(id NUMBER, descr VARCHAR2(30));
 
TABLE created.
 
SQL> INSERT INTO test1 VALUES(1,'One');
 
1 ROW created.
 
SQL> INSERT INTO test1 VALUES(2,'Two');
 
1 ROW created.
 
SQL> INSERT INTO test1 VALUES(3,'Three');
 
1 ROW created.
 
SQL> COMMIT;
 
COMMIT complete.

Flashback Database

This flashback technology is a complete rewind of the database i.e. no granularity here, the whole database is re-winded…

You must activate it with:

SQL> shutdown IMMEDIATE
DATABASE closed.
DATABASE dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.
 
Total SYSTEM Global Area 1068937216 bytes
Fixed SIZE                  2233336 bytes
Variable SIZE             796920840 bytes
DATABASE Buffers          260046848 bytes
Redo Buffers                9736192 bytes
DATABASE mounted.
SQL> ALTER DATABASE flashback ON;
 
DATABASE altered.
 
SQL> ALTER DATABASE OPEN;
 
DATABASE altered.

Can be then controlled with:

SQL> SELECT flashback_on FROM v$database;
 
FLASHBACK_ON
------------------
YES

You define the flashback database retention with:

SQL> show parameter db_flashback_retention_target
 
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
db_flashback_retention_target        INTEGER     1440

You are now able to rewind your database at maximum db_flashback_retention_target minutes in the past, if you look into your Fast Recovery Area you see creation of below files:

[oracle@server1 fast_recovery_area]$ pwd
/oracle/fast_recovery_area
[oracle@server1 fast_recovery_area]$ ll
total 20
drwx------ 2 root   root 16384 May 11 17:05 lost+found
drwxr-x--- 7 oracle dba   4096 Aug 15 14:23 TEST
[oracle@server1 fast_recovery_area]$ ll TEST
total 20
drwxr-x--- 30 oracle dba 4096 Aug 15 00:11 archivelog
drwxr-x---  2 oracle dba 4096 Jul 13 11:22 autobackup
drwxr-----  5 oracle dba 4096 Jul 25 17:07 backupset
drwxr-x---  2 oracle dba 4096 Aug 15 14:23 flashback
drwxr-x---  2 oracle dba 4096 Jul 20 12:07 onlinelog
[oracle@server1 fast_recovery_area]$ ll TEST/flashback
total 16040
-rw-r----- 1 oracle dba 8200192 Aug 15 14:31 o1_mf_74l3ynbt_.flb
-rw-r----- 1 oracle dba 8200192 Aug 15 14:23 o1_mf_74l3yoth_.flb

Remark:
Activating flashback logs is not completely transparent as apparently performance overhead is around 2% (but mainly depends on your workload so expected to be more from what I heard around)…

Let’s test it with a DDL statement:

SQL> SET lines 200
SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn) FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)
----------- ---------------------------------------------------------------------------
   29034095 15-AUG-11 02.35.27.000000000 PM
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> ALTER TABLE test1 ADD (column1 VARCHAR2(20));
 
TABLE altered.
 
SQL> UPDATE test1 SET column1='Temporary';
 
3 ROWS updated.
 
SQL> COMMIT;
 
COMMIT complete.
 
SQL> SELECT * FROM test1;
 
        ID DESCR                          COLUMN1
---------- ------------------------------ --------------------
         1 One                            TEMPORARY
         2 Two                            TEMPORARY
         3 Three                          TEMPORARY
 
SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn) FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)
----------- ---------------------------------------------------------------------------
   29034142 15-AUG-11 02.36.11.000000000 PM

Database must be in mount state to flashback it:

SQL> shutdown IMMEDIATE;
DATABASE closed.
DATABASE dismounted.
ORACLE instance shut down.
SQL> startup mount;
ORACLE instance started.
 
Total SYSTEM Global Area 1068937216 bytes
Fixed SIZE                  2233336 bytes
Variable SIZE             796920840 bytes
DATABASE Buffers          260046848 bytes
Redo Buffers                9736192 bytes
DATABASE mounted.
SQL> flashback DATABASE TO scn 29034095;
 
Flashback complete.
 
SQL> ALTER DATABASE OPEN resetlogs;
 
DATABASE altered.
 
SQL> SELECT * FROM yjaquier.test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three

Remark:

  • You may use SCN, timestamp or restore points to flashback a database.
  • You can open the database read only instead of resetlogs to only export figures.

Flashback Table

This technology allows you to flashback a table to a previous state after “wrong” DML statements, DDL statements are not flashback-able…

Enabling row movement for your test table is mandatory for flashback table:

SQL> ALTER TABLE test1 enable ROW movement;
 
TABLE altered.

Inserting few test rows and performing a “wrong” update:

SQL> SET lines 200
SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn) FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)
----------- ---------------------------------------------------------------------------
   28947307 15-AUG-11 09.35.57.000000000 AM
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> INSERT INTO test1 VALUES(4,'Five');
 
1 ROW created.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
         4 Five
 
SQL> UPDATE test1 SET descr='Four';
 
4 ROWS updated.
 
SQL> COMMIT;
 
COMMIT complete.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 Four
         2 Four
         3 Four
         4 Four

Flashing back table to original good state (SCN or timestamp taken in previous step, Oracle suggests to record current SCN before issuing such command):

SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn) FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)
----------- ---------------------------------------------------------------------------
   28947426 15-AUG-11 09.38.03.000000000 AM
 
SQL> flashback TABLE test1 TO scn 28947307;
 
Flashback complete.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three

You can also work with restore point (refer to official documentation for retention policies):

SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> CREATE restore point before_upgrade;
 
Restore point created.
 
SQL> UPDATE test1 SET descr='Temporary';
 
3 ROWS updated.
 
SQL> COMMIT;
 
COMMIT complete.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 TEMPORARY
         2 TEMPORARY
         3 TEMPORARY
 
SQL> SET lines 200
SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn) FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)
----------- ---------------------------------------------------------------------------
   28949800 15-AUG-11 10.03.02.000000000 AM
 
SQL> flashback TABLE test1 TO restore point before_upgrade;
 
Flashback complete.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> DROP restore point before_upgrade;
 
Restore point dropped.

Remark:
Using flasback technology will create SYS_TEMP_FBT in your schema:

SQL> SELECT table_name FROM user_tables;
 
TABLE_NAME
------------------------------
SYS_TEMP_FBT
TEST1

Flashback Drop

This technology is no more no less than Windows recycle bin and allows you to restore a wrongly dropped table. In fact objects are not really dropped but just renamed and they remain in tablespace until you need space to create other objects:

SQL> SELECT table_name FROM user_tables;
 
TABLE_NAME
------------------------------
SYS_TEMP_FBT
TEST1
 
SQL> DROP TABLE test1;
 
TABLE dropped.
 
SQL> SELECT table_name FROM user_tables;
 
TABLE_NAME
------------------------------
SYS_TEMP_FBT
 
SQL> show recyclebin;
ORIGINAL NAME    RECYCLEBIN NAME                OBJECT TYPE  DROP TIME
---------------- ------------------------------ ------------ -------------------
TEST1            BIN$qogQry1iu6TgQEsKbCUKuA==$0 TABLE        2011-08-15:10:29:44
 
SQL> SELECT * FROM recyclebin;
 
OBJECT_NAME                    ORIGINAL_NAME                    OPERATION TYPE                      TS_NAME                        CREATETIME          DROPTIME               DROPSCN
------------------------------ -------------------------------- --------- ------------------------- ------------------------------ ------------------- ------------------- ----------
PARTITION_NAME                   CAN CAN    RELATED BASE_OBJECT PURGE_OBJECT      SPACE
-------------------------------- --- --- ---------- ----------- ------------ ----------
BIN$qogQry1iu6TgQEsKbCUKuA==$0 TEST1                            DROP      TABLE                     USERS                          2011-08-15:09:35:12 2011-08-15:10:29:44   28951308
                                 YES YES      71097       71097        71097          8

Once the object is dropped you cannot access it but you can still query its recyclebin counterpart and/or restore it:

SQL> SELECT * FROM test1;
SELECT * FROM test1
              *
ERROR AT line 1:
ORA-00942: TABLE OR VIEW does NOT exist
 
 
SQL> SELECT * FROM "BIN$qogQry1iu6TgQEsKbCUKuA==$0";
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> flashback TABLE test1 TO before DROP;
 
Flashback complete.
 
SQL> show recyclebin;
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three

You can also purge it with:

SQL> purge recyclebin;
 
Recyclebin purged.

Flashback Query

This flashback technology has the taste of flashback table except that you can see the past figures without actually restoring them.

Let’s use our test table and update it with “wrong” figures:

SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> SELECT table_name FROM user_tables;
 
TABLE_NAME
------------------------------
SYS_TEMP_FBT
TEST1
 
SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn),TO_CHAR(SYSDATE,'dd-mon-yyyy hh24:mi:ss') AS current_time FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)                                               CURRENT_TIME
----------- --------------------------------------------------------------------------- -----------------------------
   28954179 15-AUG-11 11.03.48.000000000 AM                                             15-aug-2011 11:03:48
 
SQL> UPDATE test1 SET descr='Temporary';
 
3 ROWS updated.
 
SQL> COMMIT;
 
COMMIT complete.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 TEMPORARY
         2 TEMPORARY
         3 TEMPORARY

The flashback query feature works with AS OF SCN and AS OF TIMESTAMP in SELECT statement:

SQL> SELECT * FROM test1
     AS OF scn 28954179;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> SELECT * FROM test1
     AS OF TIMESTAMP TO_TIMESTAMP('15-AUG-11 11.03.48.000000000 AM');
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
SQL> SELECT * FROM test1
     AS OF TIMESTAMP SYSTIMESTAMP - INTERVAL '10'  MINUTE
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three

You can also use DBMS_FLASHBACK package for flashback query. This can as well been done with SCN using ENABLE_AT_SYSTEM_CHANGE_NUMBER procedure or timestamp using ENABLE_AT_TIME procedure:

SQL> EXEC dbms_flashback.enable_at_system_change_number(28954179);
 
PL/SQL PROCEDURE successfully completed.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> EXEC dbms_flashback.disable;
 
PL/SQL PROCEDURE successfully completed.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 TEMPORARY
         2 TEMPORARY
         3 TEMPORARY

Finally you can flashback your table using flashback table technology (or insert using AS SELECT in a subquery):

SQL> flashback TABLE test1 TO scn 28954179;
 
Flashback complete.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three

Remark:
This flashback query technology can also be used in export utility (exp and expdp) using FLASHBACK_SCN and FLASHBACK_TIME parameters.

Flashback Versions Query

This flashback technology is similar to flashback query but you can see different past figures of a column over an interval. Let’s first perform few updates on our test table:

SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn) FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)
----------- ---------------------------------------------------------------------------
   29006118 15-AUG-11 12.51.21.000000000 PM
 
SQL> UPDATE test1 SET descr='The one' WHERE id=1;
 
1 ROW updated.
 
SQL> COMMIT;
 
COMMIT complete.
 
SQL> UPDATE test1 SET descr='The only one' WHERE id=1;
 
1 ROW updated.
 
SQL> COMMIT;
 
COMMIT complete.
 
SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn) FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)
----------- ---------------------------------------------------------------------------
   29006142 15-AUG-11 12.52.03.000000000 PM

Remark:
Please note the commit after each update to generate multiple transactions.

You can now see past figures:

SQL> SELECT versions_startscn, versions_starttime, versions_endscn, versions_endtime, versions_xid, versions_operation, id, descr
     FROM test1
     VERSIONS BETWEEN SCN 29006118 AND 29006142
     WHERE id = 1;
 
VERSIONS_STARTSCN VERSIONS_STARTTIME       VERSIONS_ENDSCN VERSIONS_ENDTIME         VERSIONS_XID     V         ID DESCR
----------------- ------------------------ --------------- ------------------------ ---------------- - ---------- ------------------------------
         29006133 15-AUG-11 12.51.45 PM                                             0A001500B99F0000 U          1 The only one
         29006128 15-AUG-11 12.51.36 PM           29006133 15-AUG-11 12.51.45 PM    060017000A100000 U          1 The one
                                                  29006128 15-AUG-11 12.51.36 PM                                1 One
 
SQL> SELECT * FROM test1 AS OF scn 29006128;
 
        ID DESCR
---------- ------------------------------
         1 The one
         2 Two
         3 Three
 
SQL> SELECT * FROM test1 AS OF scn 29006118;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 The only one
         2 Two
         3 Three

Remark:
Same as DBMS_FLASHBACK package you may work with timestamp or SCN.

Flashback Transaction

Nothing special in this flashback technology except an extension of flashback query to get extra information of transactions. To use it you must have activated database minimal supplemental logging with:

SQL> ALTER DATABASE ADD SUPPLEMENTAL LOG DATA;
 
DATABASE altered.

If we use flashback query example:

SQL> SELECT xid, operation, start_scn, commit_scn, logon_user, undo_sql
FROM flashback_transaction_query
WHERE xid = HEXTORAW('0A001500B99F0000');
 
no ROWS selected
 
SQL> SELECT xid, operation, start_scn, commit_scn, logon_user, undo_sql
FROM flashback_transaction_query
WHERE xid = HEXTORAW('060017000A100000');
 
XID              OPERATION                         START_SCN COMMIT_SCN LOGON_USER
---------------- -------------------------------- ---------- ---------- ------------------------------
UNDO_SQL
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
060017000A100000 UPDATE                             29006126   29006128 YJAQUIER
UPDATE "YJAQUIER"."TEST1" SET "DESCR" = 'One' WHERE ROWID = 'AAARW7AAEAABn6GAAA';
 
060017000A100000 BEGIN                              29006126   29006128 YJAQUIER

First strange thing is missing information for one of the DML statement… After a while if you again select the existing one:

SQL> SELECT xid, operation, start_scn, commit_scn, logon_user, undo_sql
FROM flashback_transaction_query
WHERE xid = HEXTORAW('060017000A100000');
 
no ROWS selected

Behavior is really erratic and this is explained by bugs, apparently corrected in 11.2.0.2.2, so not really mature so far:
Bug 10358019 – Queries against FLASHBACK_TRANSACTION_QUERY return wrong results [ID 10358019.8]

Remark:
Oracle 11gR2 extend this functionality with flashback transaction backout by flashing back a specific transactions and all its dependent transactions.

Flashback Data Archive

Flashback Data Archive (FDA) technology is an extension of flashback versions query technology and extend Undo functionality (bypassing UNDO_RETENTION parameter) by keeping figures for fixed pre-defined period (if enough available space obviously). On the paper it’s perfect: transparent, efficient, old figures stored in compressed format, answers to SOX requirements. Small drawback is the cost as you must purchase Oracle Total Recall option to use it…

First start by creating a dedicated tablespace (you may instead use an existing one):

SQL> CREATE TABLESPACE fda datafile '/oracle/data01/test/fda01.dbf' SIZE 100m
     extent management local SEGMENT SPACE management auto;
 
TABLESPACE created.

Then create a default flashback data archive (then no need to specify one when activating FDA on your tables), no quota to use whole tablespace and retention set to one month:

SQL> CREATE flashback archive DEFAULT fla1
     TABLESPACE fda
     retention 1 MONTH;
 
Flashback archive created.

Then modify your table:

SQL> ALTER TABLE test1 flashback archive;
 
TABLE altered.

Remark:
To see what’s activated and your FDA you may use the following queries (USER and ALL counterparts may also be used)

SELECT * FROM dba_flashback_archive;
SELECT * FROM dba_flashback_archive_ts;
SELECT * FROM dba_flashback_archive_tables;

Let’s modify the table and see past figures, same as versions query and you may use SCN or timestamp. From pure SQL standpoint you don’t see difference with versions query, except in retention policy…

SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn) FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)
----------- ---------------------------------------------------------------------------
   29045024 15-AUG-11 04.22.20.000000000 PM
 
SQL> UPDATE test1 SET descr='The one' WHERE id=1;
 
1 ROW updated.
 
SQL> COMMIT;
 
COMMIT complete.
 
SQL> SELECT * FROM test1;
 
        ID DESCR
---------- ------------------------------
         1 The one
         2 Two
         3 Three
 
SQL> SELECT * FROM test1 AS OF scn 29045024;
 
        ID DESCR
---------- ------------------------------
         1 One
         2 Two
         3 Three
 
SQL> SELECT current_scn, SCN_TO_TIMESTAMP(current_scn) FROM v$database;
 
CURRENT_SCN SCN_TO_TIMESTAMP(CURRENT_SCN)
----------- ---------------------------------------------------------------------------
   29045079 15-AUG-11 04.23.29.000000000 PM
 
SQL> UPDATE test1 SET descr='The only one' WHERE id=1;
 
1 ROW updated.
 
SQL> COMMIT;
 
COMMIT complete.
 
SQL> SELECT * FROM test1 AS OF scn 29045079;
 
        ID DESCR
---------- ------------------------------
         1 The one
         2 Two
         3 Three

In this example past version of the table will be kept for one month and then automatically deleted.

With only few exceptions DDLs are supported and for unsupported ones you may use DBMS_FLASHBACK_ARCHIVE PL/SQL package:

SQL> ALTER TABLE test1 ADD (column1 VARCHAR2(20));
 
TABLE altered.
 
SQL> SELECT * FROM test1;
 
        ID DESCR                          COLUMN1
---------- ------------------------------ --------------------
         1 The only one
         2 Two
         3 Three
 
SQL> SELECT * FROM test1 AS OF scn 29045024;
 
        ID DESCR
---------- ------------------------------
         1 One
         3 Three
         2 Two

References

8 thoughts on “Flashback technologies by examples

  1. very nice article.I practice by seeing these example.lots of thing i have learn,but little bit more explanation you need to add here,such as when i was practicing i logged in as “SYS” user and try to do flashback table ,but it didn’t allow me.So i think it is confusing for new bee or novice user.So you need to explain here that some command will not work for sys user and why it will not work for sys user that also you need to mention.Rest of the article is very nice.Keep it up your good work.

    • Thanks to you for reading !

      Honestly I was not aware of:
      ORA-08185: Flashback not supported for user SYS

      But I think it’s a general best practice to not use SYS user when performing testing. Your customers will not connect to database using SYS right ? Then coming back to FLASHBACK TABLE, the prerequisites section of the official documentation well described needed grant to use this feature as a “normal user”…

  2. This has been a very useful information. Examples are very clear. I have noted this page to my reference pages. Thanks for the effort.

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