The tempdb system database is a global resource that is available to all users connected to the instance of SQL Server and is used to hold the following:
Temporary user objects that are explicitly created, such as: global or local temporary tables, temporary stored procedures, table variables, or cursors.
Internal objects that are created by the SQL Server Database Engine, for example, work tables to store intermediate results for spools or sorting.
Row versions that are generated by data modification transactions in a database that uses read-committed using row versioning isolation or snapshot isolation transactions.
Row versions that are generated by data modification transactions for features, such as: online index operations, Multiple Active Result Sets (MARS), and AFTER triggers.
Operations within tempdb are minimally logged. This enables transactions to be rolled back. tempdb is re-created every time SQL Server is started so that the system always starts with a clean copy of the database. Temporary tables and stored procedures are dropped automatically on disconnect, and no connections are active when the system is shut down. Therefore, there is never anything in tempdb to be saved from one session of SQL Server to another. Backup and restore operations are not allowed on tempdb.
Physical Properties of tempdb
The following table lists the initial configuration values of the tempdb data and log files. The sizes of these files may vary slightly for different editions of SQL Server.
|File||Logical name||Physical name||Initial size||File growth|
|Primary data||tempdev||tempdb.mdf||8 megabytes||Autogrow by 64 MB until the disk is full|
|Secondary data files*||temp#||tempdb_mssql_#.ndf||8 megabytes||Autogrow by 64 MB until the disk is full|
|Log||templog||templog.ldf||8 megabytes||Autogrow by 64 megabytes to a maximum of 2 terabytes|
* The number of files depends on the number of (logical) cores on the machine. The value will be the number of cores or 8, whichever is lower.
The default value for the number of data files is based on the general guidelines in KB 2154845.
Performance Improvements in tempdb
In SQL Server, tempdb performance is improved in the following ways:
Temporary tables and table variables may be cached. Caching allows operations that drop and create the temporary objects to execute very quickly and reduces page allocation contention.
Allocation page latching protocol is improved. This reduces the number of UP (update) latches that are used.
Logging overhead for tempdb is reduced. This reduces disk I/O bandwidth consumption on the tempdb log file.
Setup adds multiple tempdb data files during a new instance installation. This task can be accomplished with the new UI input control on the Database Engine Configuration section and a command line parameter /SQLTEMPDBFILECOUNT. By default, setup will add as many tempdb files as the CPU count or 8, whichever is lower.
When there are multiple tempdb data files, all files will autogrow at same time and by the same amount depending on growth settings. Trace flag 1117 is no longer required.
All allocations in tempdb use uniform extents. Trace flag 1118 is no longer required.
For the primary filegroup, the AUTOGROW_ALL_FILES property is turned on and the property cannot be modified.
Moving the tempdb Data and Log Files
To move the tempdb data and log files, see Move System Databases.
The following table lists the default value for each database option in the tempdb database and whether the option can be modified. To view the current settings for these options, use the sys.databases catalog view.
|Database option||Default value||Can be modified|
|Database Availability Options||ONLINE
|PAGE_VERIFY||CHECKSUM for new installations of SQL Server.
NONE for upgrades of SQL Server.
|Service Broker Options||ENABLE_BROKER||Yes|
For a description of these database options, see ALTER DATABASE SET Options (Transact-SQL).
The following operations cannot be performed on the tempdb database:
Backing up or restoring the database.
Changing collation. The default collation is the server collation.
Changing the database owner. tempdb is owned by sa.
Creating a database snapshot.
Dropping the database.
Dropping the guest user from the database.
Enabling change data capture.
Participating in database mirroring.
Removing the primary filegroup, primary data file, or log file.
Renaming the database or primary filegroup.
Running DBCC CHECKALLOC.
Running DBCC CHECKCATALOG.
Setting the database to OFFLINE.
Setting the database or primary filegroup to READ_ONLY.
Any user can create temporary objects in tempdb. Users can only access their own objects, unless they receive additional permissions. It is possible to revoke the connect permission to tempdb to prevent a user from using tempdb, but this is not recommended as some routine operations require the use of tempdb.