Oracle TEMPORARY Tablespaces and TEMPFILES¹

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Revision as of 00:22, 12 April 2006

What are Temporary Tablespaces?

Temporary tablespaces are used to manage space for database sort operations and for storing global temporary tables. For example, if you join two large tables, and Oracle cannot do the sort in memory (see SORT_AREA_SIZE initialisation parameter), space will be allocated in a temporary tablespace for doing the sort operation. Other SQL operations that might require disk sorting are: CREATE INDEX, ANALYZE, Select DISTINCT, ORDER BY, GROUP BY, UNION, INTERSECT, MINUS, Sort-Merge joins, etc.

The DBA should assign a temporary tablespace to each user in the database to prevent them from allocating sort space in the SYSTEM tablespace. This can be done with one of the following commands:

SQL> CREATE USER scott DEFAULT TABLESPACE data TEMPORARY TABLESPACE temp; SQL> ALTER USER scott TEMPORARY TABLESPACE temp;
NOTE: a temporary tablespace cannot contain permanent objects and therefore doesn't need to be backed up.
NOTE: temporary tablespaces are excluded from RMAN backups. This reqires them to be re-added at recovery time.

What are TEMPFILES?

Unlike normal data files, TEMPFILE's are not fully initialised (sparse). When you create a TEMPFILE, Oracle only writes to the header and last block of the file. This is why it is much quicker to create a TEMPFILE than to create a normal database file.

TEMPFILE's are not recorded in the database's control file. This implies that one can just recreate them whenever you restore the database, or after deleting them by accident. This opens interesting possibilities like having different TEMPFILE configurations between permanent and standby databases, or configure TEMPFILE's to be local instead of shared in a RAC environment.

You cannot remove datafiles from a tablespace until you drop the entire tablespace. However, you can remove a TEMPFILE from a database. Look at this example:

SQL> ALTER DATABASE TEMPFILE '/oradata/temp02.dbf' DROP INCLUDING DATAFILES;
If you remove all tempfiles from a temporary tablespace, you may encounter error: ORA-25153: Temporary Tablespace is Empty. Use the following statement to add a TEMPFILE to a temporary tablespace:

SQL> ALTER TABLESPACE temp ADD TEMPFILE '/oradata/temp03.dbf' SIZE 100M;
Except for adding a tempfile, as illustrated in the above example, you cannot use the ALTER TABLESPACE statement for a locally managed temporary tablespace (operations like rename, set to read only, recover, etc. will fail).

How do I create Temporary Tablespaces?

Oracle provides various ways of creating TEMPORARY tablespaces (mainly to provide backward compatibility). You should use the most recent method available:

• Prior to Oracle 7.3 - CREATE TABLESPACE temp DATAFILE ...;
• Oracle 7.3 & 8.0 - CREATE TABLESPACE temp DATAFILE ... TEMPORARY;
• Oracle 8i and above - CREATE TEMPORARY TABLESPACE temp TEMPFILE ...;

Oracle 8i and 9i example:

SQL> CREATE TEMPORARY TABLESPACE temp TEMPFILE '/oradata/mytemp_01.tmp' SIZE 20M EXTENT MANAGEMENT LOCAL UNIFORM SIZE 16M;
For best performance, the UNIFORM SIZE must be a multiple of the SORT_AREA_SIZE parameter.

Oracle 9i example using OMF (Oracle Managed Files):

SQL> CREATE TEMPORARY TABLESPACE temp;

Default Temporary Tablespaces

In Oracle 9i and above, you can define a Default Temporary Tablespace at database creation time, or by issuing an "ALTER DATABASE" statement:

SQL> ALTER DATABASE DEFAULT TEMPORARY TABLESPACE temp;
The default Default Temporary Tablespace is SYSTEM. Each database can be assigned one and only one Default Temporary Tablespace. Using this feature, a Temporary Tablespace is automatically assigned to users. The following restrictions apply to default temporary tablespaces:

• The Default Temporary Tablespace must be of type TEMPORARY
• The DEFAULT TEMPORARY TABLESPACE cannot be taken off-line
• The DEFAULT TEMPORARY TABLESPACE cannot be dropped until you create another one.

To see the default temporary tablespace for a database, execute the following query:

SQL> SELECT * FROM DATABASE_PROPERTIES where PROPERTY_NAME='DEFAULT_TEMP_TABLESPACE';
All new users that are not explicitly assigned a TEMPORARY TABLESPACE, will get the Default Temporary Tablespace as its TEMPORARY TABLESPACE. Also, when you assign a TEMPORARY tablespace to a user, Oracle will not change this value next time you change the Default Temporary Tablespace for the database.

Other Considerations

Some performance considerations for temporary tablespaces:

• Always use temporary tablespaces instead of permanent content tablespaces for sorting (no logging and uses one large sort segment to reduce recursive SQL and ST space management enqueue contention).
• Ensure that you create your temporary tablespaces as locally managed instead of dictionary managed (Use sort space bitmap instead of sys.fet$ and sys.uet$ for allocating space).
• Always use TEMPFILE's instead of DATAFILE's (reduce backup and recovery time + other advantages as described above).
• Stripe your temporary tablespaces over multiple disks to alleviate possible disk contention and to speed-up sorting operations (user processes can read/write to it directly).

How do I find out the parameters needed for re-adding TEMPFILE's?

SQL> ALTER DATABASE BACKUP CONTROLFILE TO TRACE;

This will create a "create controlfile" script in the udump directory. You will see at the end of the script instructions to recreate your TEMPFILE's. On recovery the controlfile still has the entry for the tablespace, but you must recreate the tempfile.

¹ This document is based on an article from The Oracle FAQ