Benefits and consequences of the NOLOGGING option

I still find confusion out there about the LOGGING and NOLOGGING clauses when performing DML and DDL operations, the reality is that the NOLOGGING clause will work only on particular conditions, but all regular inserts,updates and deletes will still log the operations.

The benefits of the NOLOGGING option are:

• Will save disk space when the archive option is enabled.
• Will largely reduce I/O on the redologs.
• Will reduce the time it takes to complete the operation.

Please note that NOLOGGING operations will only reduce -not eliminate- the logging.

Lets see an example -

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--   First: we create an empty table with the nologging clause SQL>  create table logging_example nologging as select * from dba_objects where 1=2;   Table created.   --Now let's enable the statistics and perform a couple of tests:   SQL> set autotrace on statistics SQL> set timing on   -- insert the records the traditional way SQL> alter system flush buffer_cache;  --clean the cache to compare the speeds in equal conditions   System altered.   Elapsed: 00:00:01.49   SQL> insert into logging_example select * from dba_objects;   50864 rows created.   Elapsed: 00:00:01.59   Statistics ----------------------------------------------------------  0  recursive calls  5250  db block gets  6766  consistent gets  982  physical reads 5636712  redo size --without the APPEND hint  670  bytes sent via SQL*Net to client  586  bytes received via SQL*Net from client  3  SQL*Net roundtrips to/from client  1  sorts (memory)  0  sorts (disk)  50864  rows processed   -- insert the records with the APPEND hint (nologging) SQL> alter system flush buffer_cache;  --clean the cache to compare the speeds in equal conditions   System altered.   Elapsed: 00:00:01.06   SQL> insert /*+ append */  into logging_example select * from dba_objects;   50864 rows created.   Elapsed: 00:00:00.59   Statistics ----------------------------------------------------------  0  recursive calls  743  db block gets  5374  consistent gets  944  physical reads 2200  redo size --with the APPEND hint  654  bytes sent via SQL*Net to client  604  bytes received via SQL*Net from client  3  SQL*Net roundtrips to/from client  1  sorts (memory)  0  sorts (disk)  50864  rows processed

We can see that there is a big difference on the redo size generated by each insert, there are many post and articles on the internet  that show the speed benefits when using the NOLOGGING option, but here I mainly want to clarify that a regular insert (no APPEND hint) will still generate redologs even if the table have been created with the NOLOGGING option.

What happens to the data after a restore when a nologging operation was performed on it?

I will present some scenarios to show the consequences when we need to perform a restore after a nologging transaction, this way we will know what to expect and we can better prepare ourselves in case of a disaster.

I took a full database backup, now I will create several tables with different options to see what happens after a restore, you might see some surprises here!

Scenarios:

• Table “create as select” with the nologging option (table_ctas_nologging).
• Regular table “create as select” (table_ctas_logging)
• A nologging table created empty, and a regular (logging) insert (table_ctas_nologging_insert)
• Table created with nologging, then two inserts, one with and one without logging (table_insert_mixed)
• Regular logging table, with a nologging index (table_ctas_index_nologging)

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SQL> create table table_ctas_nologging nologging as select * from dba_objects;   Table created.   SQL> create table table_ctas_logging as select * from dba_objects;   Table created.   SQL> create table table_ctas_nologging_insert nologging as select * from dba_objects where 1=2;   Table created.   SQL>  insert into table_ctas_nologging_insert  select * from dba_objects;   50864 rows created.   SQL> commit;   Commit complete.   SQL> create table table_insert_mixed nologging as select * from dba_objects where 1=2;   Table created.   SQL> insert into table_insert_mixed select * from dba_objects;   50866 rows created.   SQL> insert into table_insert_mixed select /*+ append */ * from dba_objects;   50866 rows created.   SQL> commit;   Commit complete.   SQL> select count(*) from table_insert_mixed;    COUNT(*) ----------  101732   SQL> create table table_ctas_index_nologging as select * from dba_objects;   Table created.   SQL> create index IDXNOLOG  on  table_ctas_index_nologging (object_id)  nologging;   Index created.

Now I will shutdown the database and restore the tablespace from the backup.
Next is an extract from RMAN

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rman target /   Recovery Manager: Release 10.2.0.4.0 - Production on Wed Aug 25 17:32:20 2010   Copyright (c) 1982, 2007, Oracle.  All rights reserved.   connected to target database: ORCL (DBID=1247573001)   RMAN> shutdown immediate   using target database control file instead of recovery catalog database closed database dismounted Oracle instance shut down   RMAN> startup mount;   Oracle instance started database mounted   Total System Global Area     285212672 bytes   Fixed Size                     1267068 bytes Variable Size                155191940 bytes Database Buffers             125829120 bytes Redo Buffers                   2924544 bytes   RMAN> restore tablespace users;   Starting restore at 25-AUG-10 using target database control file instead of recovery catalog allocated channel: ORA_DISK_1 channel ORA_DISK_1: sid=152 devtype=DISK   channel ORA_DISK_1: starting datafile backupset restore channel ORA_DISK_1: specifying datafile(s) to restore from backup set restoring datafile 00004 to +DATA/orcl/datafile/users.259.719792191 channel ORA_DISK_1: reading from backup piece +DATA/orcl/backupset/2010_08_25/nnndf0_tag20100825t171657_0.272.727982219 channel ORA_DISK_1: restored backup piece 1 piece handle=+DATA/orcl/backupset/2010_08_25/nnndf0_tag20100825t171657_0.272.727982219 tag=TAG20100825T171657 channel ORA_DISK_1: restore complete, elapsed time: 00:00:05 Finished restore at 25-AUG-10   RMAN> recover tablespace users;   Starting recover at 25-AUG-10 using channel ORA_DISK_1   starting media recovery media recovery complete, elapsed time: 00:00:05   Finished recover at 25-AUG-10   RMAN> alter database open;   database opened

Now lets see the status of the tables:

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SQL> select count(*) from table_ctas_nologging ;  select count(*) from table_ctas_nologging  *  ERROR at line 1:  ORA-01578: ORACLE data block corrupted (file # 4, block # 404)  ORA-01110: data file 4: '+DATA/orcl/datafile/users.259.719792191'  ORA-26040: Data block was loaded using the NOLOGGING option

That doesn’t look good, lets see the next table

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SQL> select count(*) from table_ctas_logging ;   COUNT(*) ---------- 50863

Good, no problem here, the next scenario is more interesting, the table was created with the NOLOGGING option, but the inserts were done without the APPEND hint

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SQL> select count (*) from table_ctas_nologging_insert;   COUNT(*) ---------- 50864

Good, no problem here, now let’s see our table with half data inserted with logging and half with nologging

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SQL> select count(*) from table_insert_mixed; select count(*) from table_insert_mixed * ERROR at line 1: ORA-01578: ORACLE data block corrupted (file # 4, block # 4363) ORA-01110: data file 4: '+DATA/orcl/datafile/users.259.719792191' ORA-26040: Data block was loaded using the NOLOGGING option

Wow, the whole table is unredable!

Now lets see the table with the NOLOGGING  index .

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<pre>SQL> select  count(*) from table_ctas_index_nologging;   COUNT(*) ---------- 50865

Ok, thats nice, the table is accessible, but what happend if we try to use the index?

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SQL>  select object_id from table_ctas_index_nologging  where object_id=1; select object_id from table_ctas_index_nologging  where object_id=1 * ERROR at line 1: ORA-01578: ORACLE data block corrupted (file # 4, block # 2821) ORA-01110: data file 4: '+DATA/orcl/datafile/users.259.719792191' ORA-26040: Data block was loaded using the NOLOGGING option

I tried to rebuil the index but I was still getting the same error message, at the end I was forced to drop it and recreate it.

Conclusions:

• Use the NOLOGGING option only on temporary/working/staging tables.
• Always perform a backup after a NOLOGGING operation.
• Unless explicitly indicated, DDLs like CTAS and DMLs like inserts will log all operations.

FROM ORACLE DOCUMENTATION:

NOLOGGING is supported in only a subset of the locations that support LOGGING. Only the following operations support the NOLOGGING mode:

DML:

• Direct-path INSERT (serial or parallel) resulting either from an INSERT or aMERGE statement. NOLOGGING is not applicable to any UPDATE operations resulting from the MERGE statement.
• Direct Loader (SQL*Loader)

DDL:

• CREATE TABLE … AS SELECT
• CREATE TABLE … LOB_storage_clause … LOB_parameters … NOCACHE | CACHE READS
• ALTER TABLE … LOB_storage_clause … LOB_parameters … NOCACHE | CACHE READS(to specify logging of newly created LOB columns)
• ALTER TABLE … modify_LOB_storage_clause … modify_LOB_parameters … NOCACHE |CACHE READS (to change logging of existing LOB columns)
• ALTER TABLE … MOVE
• ALTER TABLE … (all partition operations that involve data movement)
  • ALTER TABLE … ADD PARTITION (hash partition only)
  • ALTER TABLE … MERGE PARTITIONS
  • ALTER TABLE … SPLIT PARTITION
  • ALTER TABLE … MOVE PARTITION
  • ALTER TABLE … MODIFY PARTITION … ADD SUBPARTITION
  • ALTER TABLE … MODIFY PARTITION … COALESCE SUBPARTITION
• CREATE INDEX
• ALTER INDEX … REBUILD
• ALTER INDEX … REBUILD [SUB]PARTITION
• ALTER INDEX … SPLIT PARTITION

For objects other than LOBs, if you omit this clause, then the logging attribute of the object defaults to the logging attribute of the tablespace in which it resides.

For LOBs, if you omit this clause, then:

• If you specify CACHE, then LOGGING is used (because you cannot have CACHENOLOGGING).
• If you specify NOCACHE or CACHE READS, then the logging attribute defaults to the logging attribute of the tablespace in which it resides.

NOLOGGING does not apply to LOBs that are stored internally (in the table with row data). If you specify NOLOGGING for LOBs with values less than 4000 bytes and you have not disabled STORAGE IN ROW, then Oracle ignores the NOLOGGING specification and treats the LOB data the same as other table data.

 

reference : http://bit.ly/1aYFZw3