Table recovery was possible in earlier release as well. Until previous release, if we wanted to recover a table, we had following options
- Database point in time recovery (DBPITR)
- Tablespace point in time recovery (TSPITR)
- Flashback technology
In Oracle 12c, RMAN has been enhanced to perform recovery of table. We have a new command in RMAN which automates complete process of recovering the table. New process does not affect the existing objects in the database and reduces the time and space required for doing table recovery.
How table recovery works?
- RMAN uses the database backups taken previously to recovery table or table partitions to a specified time. You need to provide following inputs to RMAN
- Table name or partition names to be recovered
- Point in time until which the recovery should be done
- Whether recovered tables should be imported back to original database
- RMAN determines the correct backup based on your inputs
- RMAN creates auxiliary instance
- RMAN restores the controlfile
- RMAN restores necessary files required for obtaining the older image of table. This might include SYSTEM, SYSAUX, UNDO tablespace and tablespace which has required table.
- RMAN recovers the table or table partitions in auxiliary instance until the specified time.
- RMAN creates data pump export dump file that contains the recovered objects
- RMAN imports the recovered objects into original database
Recovery Point-in-Time Options
You can recover a table or table partition to a past point in time by specifying one of the following three clauses:
- UNTIL SCN – The system change number (SCN)
- UNTIL TIME – Time in NLS_DATE_FORMAT environment variable
- UNTIL SEQUENCE – The log sequence number and thread number
Steps for performing table recovery
Let’s take an example. I have a table T1 created in DEO database.
[oracle@advait ~]$ sqlplus advaitd/oracle@deo
SQL> select * from T1;
COL1
----------
1
2
3
We can note down the current SCN when table is available
SQL> select current_scn from v$database;
CURRENT_SCN
-----------
1243251
Following are the datafiles available in my database
SQL> select file#, name, con_id from v$datafile order by 3,1;
FILE# NAME CON_ID
---------- -------------------------------------------------------------------------------- ----------
1 /u01/app/oracle/oradata/orcl/orcl/datafiles/system-01.dbf 0
2 /u01/app/oracle/oradata/orcl/orcl/datafiles/sysaux-01.dbf 0
3 /u01/app/oracle/oradata/orcl/orcl/datafiles/undotbs1-01.dbf 0
4 /u01/app/oracle/oradata/orcl/orcl/users.dbf 0
5 /u01/app/oracle/oradata/orcl/orcl/datafiles/mgmt_depot.dbf 0
6 /u01/app/oracle/oradata/orcl/orcl/datafiles/mgmt.dbf 0
7 /u01/app/oracle/oradata/orcl/orcl/datafiles/mgmt_ad4j.dbf 0
Lets drop the table T1
SQL> drop table T1; Table dropped.
We can use below command to recover this table. Note that the SCN number we are giving below is the SCN number we noted down when table was existing.
RMAN> recover table advaitd.t1 until scn 1243251 auxiliary destination '/u01/app/oracle/oradata/aux';
Let’s see what this command does exactly.
Step 1) Command creates and mounts a new adhoc dummy instance as shown below
RMAN> recover table advaitd.t1 until scn 1243251 auxiliary destination '/u01/app/oracle/oradata/aux'; Starting recover at 26-JUL-16 using target database control file instead of recovery catalog allocated channel: ORA_DISK_1 channel ORA_DISK_1: SID=107 device type=DISK RMAN-05026: WARNING: presuming following set of tablespaces applies to specified Point-in-Time List of tablespaces expected to have UNDO segments Tablespace SYSTEM Tablespace UNDOTBS1 Creating automatic instance, with SID='jCvj'
It uses the controlfile autobackup to restore the controlfile to mount the dummy instance.
Step 2) It then restores the controlfile for the auxiliary instance
Following memory script shows the commands for restoring controlfile
contents of Memory Script:
{
# set requested point in time
set until scn 1243251;
# restore the controlfile
restore clone controlfile;
# mount the controlfile
sql clone 'alter database mount clone database';
# archive current online log
sql 'alter system archive log current';
}
Step 3) Next it restores the tablespaces required to perform tablespace PITR
Following memory script shows that it’s going to restore the required datafiles (until SCN 1243251) into auxiliary destination and switch the datafiles to copy.
When we use UNTIL SCN, RMAN restores the datafiles which are little older then this SCN.
contents of Memory Script:
{
# set requested point in time
set until scn 1243251;
# set destinations for recovery set and auxiliary set datafiles
set newname for clone datafile 1 to new;
set newname for clone datafile 3 to new;
set newname for clone datafile 2 to new;
set newname for clone tempfile 1 to new;
# switch all tempfiles
switch clone tempfile all;
# restore the tablespaces in the recovery set and the auxiliary set
restore clone datafile 1, 3, 2;
switch clone datafile all;
}
Step 4) Recover the datafiles until required SCN specified
Below memory script shows that RMAN tries to recover these datafiles until the specified SCN. Once recovered, it opens the database in read only mode.
contents of Memory Script:
{
# set requested point in time
set until scn 1243251;
# online the datafiles restored or switched
sql clone "alter database datafile 1 online";
sql clone "alter database datafile 3 online";
sql clone "alter database datafile 2 online";
# recover and open database read only
recover clone database tablespace "SYSTEM", "UNDOTBS1", "SYSAUX";
sql clone 'alter database open read only';
}
Step 5) Create SPFILE for auxiliary instance and mount the DB again
Following memory script tell us that RMAN is creating SPFILE for the auxiliary instance and including CONTROL_FILES parameter in SPFILE and mounting the auxiliary instance again
contents of Memory Script:
{
sql clone "create spfile from memory";
shutdown clone immediate;
startup clone nomount;
sql clone "alter system set control_files =
''/u01/app/oracle/oradata/aux/DEO/controlfile/o1_mf_ct8t8hyq_.ctl'' comment=
''RMAN set'' scope=spfile";
shutdown clone immediate;
startup clone nomount;
# mount database
sql clone 'alter database mount clone database';
}
Step 6) Restore USERS tablespace where the table belongs
Following memory script shows that RMAN is trying to restore USERS tablespace (datafile 0004) and switching to the copy.
contents of Memory Script:
{
# set requested point in time
set until scn 1243251;
# set destinations for recovery set and auxiliary set datafiles
set newname for datafile 4 to new;
# restore the tablespaces in the recovery set and the auxiliary set
restore clone datafile 4;
switch clone datafile all;
}
Step 7) Recovering the auxiliary database until required SCN
Following memory script shows that RMAN is trying to recover USERS tablespace and other tablespaces that it has restored previously to required SCN. It also opens the database in resetlogs.
contents of Memory Script:
{
# set requested point in time
set until scn 1243251;
# online the datafiles restored or switched
sql clone "alter database datafile 4 online";
# recover and open resetlogs
recover clone database tablespace "USERS", "SYSTEM", "UNDOTBS1", "SYSAUX" delete archivelog;
alter clone database open resetlogs;
}
Step 8) Create directory for datapump export and export the required table
Following memory script shows that RMAN is creating a directory for exporting the table and it also exports the table
contents of Memory Script:
{
# create directory for datapump import
sql "create or replace directory TSPITR_DIROBJ_DPDIR as ''
/u01/app/oracle/oradata/aux''";
# create directory for datapump export
sql clone "create or replace directory TSPITR_DIROBJ_DPDIR as ''
/u01/app/oracle/oradata/aux''";
}
executing Memory Script
sql statement: create or replace directory TSPITR_DIROBJ_DPDIR as ''/u01/app/oracle/oradata/aux''
sql statement: create or replace directory TSPITR_DIROBJ_DPDIR as ''/u01/app/oracle/oradata/aux''
Performing export of tables...
EXPDP> Starting "SYS"."TSPITR_EXP_jCvj_rjBm":
EXPDP> Estimate in progress using BLOCKS method...
EXPDP> Processing object type TABLE_EXPORT/TABLE/TABLE_DATA
EXPDP> Total estimation using BLOCKS method: 64 KB
EXPDP> Processing object type TABLE_EXPORT/TABLE/TABLE
EXPDP> Processing object type TABLE_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS
EXPDP> Processing object type TABLE_EXPORT/TABLE/STATISTICS/MARKER
EXPDP> . . exported "ADVAITD"."T1" 5.062 KB 3 rows
EXPDP> Master table "SYS"."TSPITR_EXP_jCvj_rjBm" successfully loaded/unloaded
EXPDP> ******************************************************************************
EXPDP> Dump file set for SYS.TSPITR_EXP_jCvj_rjBm is:
EXPDP> /u01/app/oracle/oradata/aux/tspitr_jCvj_52000.dmp
EXPDP> Job "SYS"."TSPITR_EXP_jCvj_rjBm" successfully completed at Tue Jul 26 19:55:13 2016 elapsed 0 00:00:21
Export completed
Step 9) Importing the table to original database
Following memory script shows, RMAN is trying to import the table to original database. You can provide additional options to prevent RMAN to import the table or you can provide import options to remap the table to different name if required or append the content to existing table.
contents of Memory Script:
{
# shutdown clone before import
shutdown clone abort
}
executing Memory Script
Oracle instance shut down
Performing import of tables...
IMPDP> Master table "SYS"."TSPITR_IMP_jCvj_tCzd" successfully loaded/unloaded
IMPDP> Starting "SYS"."TSPITR_IMP_jCvj_tCzd":
IMPDP> Processing object type TABLE_EXPORT/TABLE/TABLE
IMPDP> Processing object type TABLE_EXPORT/TABLE/TABLE_DATA
IMPDP> . . imported "ADVAITD"."T1" 5.062 KB 3 rows
IMPDP> Processing object type TABLE_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS
IMPDP> Processing object type TABLE_EXPORT/TABLE/STATISTICS/MARKER
IMPDP> Job "SYS"."TSPITR_IMP_jCvj_tCzd" successfully completed at Tue Jul 26 19:55:24 2016 elapsed 0 00:00:04
Import completed
Step 10) Finally, RMAN will clean up and remove the auxiliary instance
Removing automatic instance Automatic instance removed auxiliary instance file /u01/app/oracle/oradata/aux/DEO/datafile/o1_mf_temp_csgwmp8s_.tmp deleted auxiliary instance file /u01/app/oracle/oradata/aux/JCVJ_PITR_DEO/onlinelog/o1_mf_2_csgwo8fy_.log deleted auxiliary instance file /u01/app/oracle/oradata/aux/JCVJ_PITR_DEO/onlinelog/o1_mf_1_csgwo7ht_.log deleted auxiliary instance file /u01/app/oracle/oradata/aux/JCVJ_PITR_DEO/datafile/o1_mf_users_csgwo53j_.dbf deleted auxiliary instance file /u01/app/oracle/oradata/aux/DEO/datafile/o1_mf_sysaux_csgwkm4b_.dbf deleted auxiliary instance file /u01/app/oracle/oradata/aux/DEO/datafile/o1_mf_undotbs1_csgwkm4f_.dbf deleted auxiliary instance file /u01/app/oracle/oradata/aux/DEO/datafile/o1_mf_system_csgwkm2x_.dbf deleted auxiliary instance file /u01/app/oracle/oradata/aux/DEO/controlfile/o1_mf_csgwkf6f_.ctl deleted auxiliary instance file tspitr_jCvj_52000.dmp deleted Finished recover at 26-JUL-16 RMAN>
You can verify that you got the required table or not
SQL> select * from T1;
COL1
----------
1
2
3
As you can see all the steps are performed by RMAN automatically once we execute single RMAN command to recover the table.
This was a small test database, but in case of production databases, we have to make sure that auxiliary destination has required space available for restoring the required tablespaces. Else table recovery will fail.
Hope this helps !!
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