Tuesday, April 29, 2008

Force logging/nologging mode

You can create tables and indexes specifying that the database create them with the NOLOGGING option. When you create a table or index as NOLOGGING, the database does not generate redo log records for the operation. Thus, you cannot recover objects created with NOLOGGING, even if you are running in ARCHIVELOG mode. With respect to the NOLOGGING option, you can get three benefits:

1. Space is saved in the redo log files
2. The time it takes to create the table is decreased
3. Performance improves for parallel creation of large tables


Note: NOLOGGING can be overriden at tablespace level using alter tablespace ... force logging. NOLOGGING has no effect if the database is in force logging mode which can be controlled with (alter database force [no] logging mode).

SQL> set timing on;
SQL> create table sales_logging as select * from sales;
Table created.
Elapsed: 00:00:25.24
SQL> create table sales_nologging NOLOGGING as select * from sales;
Table created.
Elapsed: 00:00:06.59

For just over 900,000 rows, the time difference is around 18 seconds.


Let's suppose a table is created using the NOLOGGING option, regardless of how NOLOGGING is being invoked (in a CREATE statement using NOLOGGING, or in a tablespace with NOLOGGING set). What is the end result of creating a table, inserting data, committing, followed by a delete statement and a rollback statement? Does NOLOGGING mean the DML is not recorded and that you cannot rollback because there was nothing logged in the redo logs?

SQL> create table test (id number) nologging;
Table created.
SQL> insert into test values (1);
1 row created.
SQL> commit;
Commit complete.
SQL> delete from test;
1 row deleted.
SQL> rollback;
Rollback complete.
SQL> select * from test;
ID
----------
1

The answer is no, that is not what NOLOGGING means.

NOLOGGING Means

"The NOLOGGING clause also specifies that subsequent direct loads using SQL*Loader and direct load INSERT operations are not logged. Subsequent DML statements (UPDATE, DELETE, and conventional path insert) are unaffected by the NOLOGGING attribute of the table and generate redo."


*** As demonstrated, using the NOLOGGING option can be a time saver, but it can also put you at risk if you do not use it wisely. If you create a table with NOLOGGING, but cannot afford to lose the data, the first step after the data load is complete is to take a backup. If a good part of your loading data into a database work revolves around using SQL*Loader loading data into stage tables, make the tables (or tablespace) NOLOGGING and save yourself some time
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