Tuesday, August 26, 2008


NOLOGGING tablespaces
For some database operations, you can control whether the database generates redo records. Without redo, no media recovery is possible. However, suppressing redo
generation can improve performance, and may be appropriate for easily recoverable operations.

An example of such an operation is a CREATE TABLE...AS SELECT statement, which can be repeated in case of database or instance failure. Specify the NOLOGGING clause in the CREATE TABLESPACE statement if you wish to suppress redo when these operations are performed for objects within the tablespace.

If you do not include this clause, or if you specify LOGGING instead, then the database generates redo when changes are made to objects in the tablespace. Redo is never generated for temporary segments or in temporary tablespaces, regardless of the logging attribute. The logging attribute specified at the tablespace level is the default attribute for objects created within the tablespace. You can override this default logging attribute by specifying LOGGING or NOLOGGING at the schema object level--for example, in a CREATE TABLE statement.

If you have a standby database, NOLOGGING mode causes problems with the availability and accuracy of the standby database. To overcome this problem, you can specify FORCE LOGGING mode. When you include the FORCE LOGGING clause in the CREATE TABLESPACE statement, you force the generation of redo records for all operations that make changes to objects in a tablespace. This overrides any specification made at the object level.

If you transport a tablespace that is in FORCE LOGGING mode to another database, the new tablespace will not maintain the FORCE LOGGING mode.

Consider Using NOLOGGING When Creating Tables
To create a table most efficiently use the NOLOGGING clause in the CREATE TABLE...AS SELECT statement. The NOLOGGING clause causes minimal redo information to be generated during the table creation. This has the following benefits:

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

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.

If you cannot afford to lose the table after you have created it (for example, you will no longer have access to the data used to create the table) you should take a backup immediately after the table is created. In some situations, such as for tables that are created for temporary use, this precaution may not be necessary.

In general, the relative performance improvement of specifying NOLOGGING is greater for larger tables than for smaller tables. For small tables, NOLOGGING has little effect on the time it takes to create a table. However, for larger tables the performance improvement can be significant, especially when you are also parallelizing the table creation.