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05 November, 2011

CTAS in a NOARCHIVELOG database is a NOLOGGING operation

Normally, a CTAS (CREATE TABLE ... AS SELECT ....) operation generates redo and logging. However, if the database is in NOARCHIVELOG mode, Oracle silently converts it to a NOLOGGING operation.

This also means that a CTAS in a Test database (that is NOARCHIVELOG mode) will run faster than a CTAS in a Production database (that is ARCHIVELOG mode) , allowing for differences in server and I/O performance.

Here is a demonstration :
I first create a Tablespace and run a Backup, with the database in NOARCHIVELOG mode :

SQL> archive log list
Database log mode No Archive Mode
Automatic archival Disabled
Archive destination USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence 295
Current log sequence 297
SQL>
SQL> create tablespace CTAS_NOARCH ;

Tablespace created.

SQL> select file_name from dba_data_files where tablespace_name = 'CTAS_NOARCH';

FILE_NAME
--------------------------------------------------------------------------------------------------------------
/home/oracle/app/oracle/oradata/ORCL/datafile/o1_mf_ctas_noa_7c84n1w9_.dbf

SQL>
SQL> -- startup in MOUNT mode for an RMAN Backup
SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area 535662592 bytes
Fixed Size 1337720 bytes
Variable Size 213911176 bytes
Database Buffers 314572800 bytes
Redo Buffers 5840896 bytes
Database mounted.
SQL> archive log list
Database log mode No Archive Mode
Automatic archival Disabled
Archive destination USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence 295
Current log sequence 297
SQL> pause Run an RMAN Backup of the tablespace
Run an RMAN Backup of the tablespace

The RMAN backup is executed :


RMAN> backup tablespace CTAS_NOARCH;

Starting backup at 05-NOV-11
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=20 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=21 device type=DISK
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00017 name=/home/oracle/app/oracle/oradata/ORCL/datafile/o1_mf_ctas_noa_7c84n1w9_.dbf
channel ORA_DISK_1: starting piece 1 at 05-NOV-11
channel ORA_DISK_1: finished piece 1 at 05-NOV-11
piece handle=/home/oracle/app/oracle/flash_recovery_area/ORCL/backupset/2011_11_05/o1_mf_nnndf_TAG20111105T003023_7c84s28w_.bkp tag=TAG20111105T003023 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:01
Finished backup at 05-NOV-11

Starting Control File and SPFILE Autobackup at 05-NOV-11
piece handle=/home/oracle/app/oracle/flash_recovery_area/ORCL/autobackup/2011_11_05/o1_mf_s_766369695_7c84s6ks_.bkp comment=NONE
Finished Control File and SPFILE Autobackup at 05-NOV-11

RMAN>


I then run a CTAS in the Tablespace :


SQL> alter database open;

Database altered.

SQL>
SQL> connect hemant/hemant
Connected.
SQL> -- get current SCN
SQL> select current_scn from v$database;

CURRENT_SCN
-----------
5479486

SQL> select to_char(sysdate,'DD-MON HH24:MI') from dual;

TO_CHAR(SYSDATE,'DD-M
---------------------
05-NOV 00:31

SQL>
SQL> -- run CTAS
SQL> create table CTAS_NOARCH_TBL tablespace CTAS_NOARCH as select * from dba_objects;

Table created.

SQL>
SQL> -- check unrecoverable stamps on the datafile
SQL> select unrecoverable_change#, to_char(unrecoverable_time,'DD-MON HH24:MI')
2 from v$datafile, dba_data_files
3 where file#=file_id
4 and tablespace_name = 'CTAS_NOARCH'
5 /

UNRECOVERABLE_CHANGE# TO_CHAR(UNRECOVERABLE
--------------------- ---------------------
0

SQL>
SQL> connect / as sysdba
Connected.
SQL> -- set tablespace offline
SQL> alter tablespace CTAS_NOARCH offline;

Tablespace altered.

SQL>
SQL> pause Remove datafile and restore and recover datafile
Remove datafile and restore and recover datafile

SQL>


I remove the datafile and RESTORE and RECOVER the tablespace :
[root@localhost ~]# rm /home/oracle/app/oracle/oradata/ORCL/datafile/o1_mf_ctas_noa_7c84n1w9_.dbf
rm: remove regular file `/home/oracle/app/oracle/oradata/ORCL/datafile/o1_mf_ctas_noa_7c84n1w9_.dbf'? yes
[root@localhost ~]#


RMAN> restore tablespace CTAS_NOARCH;

Starting restore at 05-NOV-11
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=33 device type=DISK
allocated channel: ORA_DISK_2
channel ORA_DISK_2: SID=20 device type=DISK

channel ORA_DISK_1: starting datafile backup set restore
channel ORA_DISK_1: specifying datafile(s) to restore from backup set
channel ORA_DISK_1: restoring datafile 00017 to /home/oracle/app/oracle/oradata/ORCL/datafile/o1_mf_ctas_noa_7c84n1w9_.dbf
channel ORA_DISK_1: reading from backup piece /home/oracle/app/oracle/flash_recovery_area/ORCL/backupset/2011_11_05/o1_mf_nnndf_TAG20111105T003023_7c84s28w_.bkp
channel ORA_DISK_1: piece handle=/home/oracle/app/oracle/flash_recovery_area/ORCL/backupset/2011_11_05/o1_mf_nnndf_TAG20111105T003023_7c84s28w_.bkp tag=TAG20111105T003023
channel ORA_DISK_1: restored backup piece 1
channel ORA_DISK_1: restore complete, elapsed time: 00:00:16
Finished restore at 05-NOV-11

RMAN> recover tablespace CTAS_NOARCH;

Starting recover at 05-NOV-11
using channel ORA_DISK_1
using channel ORA_DISK_2

starting media recovery
media recovery complete, elapsed time: 00:00:03

Finished recover at 05-NOV-11

RMAN> sql 'alter tablespace CTAS_NOARCH online';

sql statement: alter tablespace CTAS_NOARCH online

RMAN>


These are the alert.log messages :
Sat Nov 05 00:31:45 2011
alter tablespace CTAS_NOARCH offline
Completed: alter tablespace CTAS_NOARCH offline
Sat Nov 05 00:34:40 2011
Full restore complete of datafile 17 /home/oracle/app/oracle/oradata/ORCL/datafile/o1_mf_ctas_noa_7c850q6y_.dbf. Elapsed time: 0:00:08
checkpoint is 5479138
Sat Nov 05 00:34:41 2011
Checker run found 3 new persistent data failures
Sat Nov 05 00:34:59 2011
alter database recover datafile list clear
Completed: alter database recover datafile list clear
alter database recover if needed
tablespace CTAS_NOARCH
Media Recovery Start
Serial Media Recovery started
Recovery of Online Redo Log: Thread 1 Group 3 Seq 297 Reading mem 0
Mem# 0: /home/oracle/app/oracle/oradata/orcl/redo03.log
Media Recovery Complete (orcl)
Completed: alter database recover if needed
tablespace CTAS_NOARCH
Sat Nov 05 00:35:13 2011
alter tablespace CTAS_NOARCH online


I then attempt to access the table :

SQL> connect hemant/hemant
Connected.
SQL> -- query the table
SQL> select count(*) from CTAS_NOARCH_TBL;
select count(*) from CTAS_NOARCH_TBL
*
ERROR at line 1:
ORA-01578: ORACLE data block corrupted (file # 17, block # 131)
ORA-01110: data file 17: '/home/oracle/app/oracle/oradata/ORCL/datafile/o1_mf_ctas_noa_7c850q6y_.dbf'
ORA-26040: Data block was loaded using the NOLOGGING option


SQL>
SQL>
SQL>
SQL> spool off
SQL> !oerr ora 26040
26040, 00000, "Data block was loaded using the NOLOGGING option\n"
//* Cause: Trying to access data in block that was loaded without
//* redo generation using the NOLOGGING/UNRECOVERABLE option
//* Action: Drop the object containing the block.

SQL>


So, Oracle now finds that the blocks for the table have been loaded via a NOLOGGING operation. I did not create the tablespace explicitly as NOLOGGING, I did not create the table as NOLOGGING. Yet, the CTAS was (silently) executed as a NOLOGGING operation.

.
.
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5 comments:

Joel Garry said...

Hmmmmmm.... are you sure you can do media recovery without doing the whole database backup and losing any changes since the backup? I wonder if the 26040 is just slightly misleading, rather than showing a silent conversion, though the end result is the same.

In other words, proving something works by success is not commutative with proving something doesn't work with failure.

word: glorming

Hemant K Chitale said...

Joel,
See my next post http://hemantoracledba.blogspot.com/2011/11/tablespace-recovery-in-noarchivelog.html

The documentation shows a RECOVER DATABASE.
I do a RESTORE TABLESPACE and RECOVER TABLESPACE and this is possibly only if the Online Redo Logs have all the redo generated since the Backup Tablespace.


You may ask why I showed a RESTORE and RECOVER. I could have demonstrated the case with a query on V$MYSTAT for 'redo size'. But I deliberately wanted to show the ORA-26040 error to prove that Oracle finds the blocks as having been loaded in a NOLOGGING manner.

Hemant

Thomas said...

Hemanth, technically i will not agree with this test as we can not restore a data file and recover in NOARCHIVELOG mode. But, for the sake of showing the NOLOGGING option it is good enough.

Thanks
Thomas

Hemant K Chitale said...

Thomas,
My next post demonstrates RESTORE and RECOVERy of a Tablespace in NOARCHIVELOG mode. You can do so *if* you have all the redo still available in the Online Redo Logs.

Hemant

Hemant K Chitale said...

See the "Followup April 18, 2006 - 2pm Central time zone" entry at http://asktom.oracle.com/pls/asktom/f?p=100:11:3325444209131519::::P11_QUESTION_ID:485221567528

"create table as select - skips redo".
The exceptions are :
1. An Index would generate redo
2. FORCE LOGGING would generate redo even in NOARCHIVELOG mode


In NOARCHIVELOG mode, a CTAS is a Direct Path Insert and that is how it avoids generating redo for the table.

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