tempdb database

Applies to: yesSQL Server (all supported versions) YesAzure SQL Database

The tempdb system database is a global resource that's available to all users connected to the instance of SQL Server or connected to Azure SQL Database. tempdb holds:

  • Temporary user objects that are explicitly created. They include global or local temporary tables and indexes, temporary stored procedures, table variables, tables returned in table-valued functions, and cursors.

  • Internal objects that the database engine creates. They include:

    • Work tables to store intermediate results for spools, cursors, sorts, and temporary large object (LOB) storage.
    • Work files for hash join or hash aggregate operations.
    • Intermediate sort results for operations such as creating or rebuilding indexes (if SORT_IN_TEMPDB is specified), or certain GROUP BY, ORDER BY, or UNION queries.

    Each internal object uses a minimum of nine pages: an IAM page and an eight-page extent. For more information about pages and extents, see Pages and extents.

    Important

    Azure SQL Database single databases and elastic pools support global temporary tables and global temporary stored procedures that are stored in tempdb and are scoped to the database level.

    Global temporary tables and global temporary stored procedures are shared for all users' sessions within the same SQL database. User sessions from other SQL databases can't access global temporary tables. For more information, see Database scoped global temporary tables (Azure SQL Database). Azure SQL Managed Instance supports the same temporary objects as does SQL Server.

    For Azure SQL Database single databases and elastic pools, only the master database and tempdb database apply. For more information, see What is an Azure SQL Database server?. For a discussion of tempdb in the context of Azure SQL Database single databases and elastic pools, see tempdb database in Azure SQL Database single databases and elastic pools.

    For Azure SQL Managed Instance, all system databases apply.

  • Version stores, which are collections of data pages that hold the data rows that support features for row versioning. There are two types: a common version store and an online-index-build version store. The version stores contain:

    • Row versions that are generated by data modification transactions in a database that uses READ COMMITTED through 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 so that transactions can 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.

tempdb never has anything to be saved from one session of SQL Server to another. Backup and restore operations are not allowed on tempdb.

Physical properties of tempdb in SQL Server

The following table lists the initial configuration values of the tempdb data and log files in SQL Server. The values are based on the defaults for the model database. The sizes of these files might 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 secondary data files depends on the number of (logical) processors on the machine. As a general rule, if the number of logical processors is less than or equal to eight, use the same number of data files as logical processors. If the number of logical processors is greater than eight, use eight data files. Then if contention continues, increase the number of data files by multiples of four until the contention decreases to acceptable levels, or make changes to the workload/code.

Note

The default value for the number of data files is based on the general guidelines in KB 2154845.

Moving the tempdb data and log files in SQL Server

To move the tempdb data and log files, see Move system databases.

Database options for tempdb in SQL Server

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
ALLOW_SNAPSHOT_ISOLATION OFF Yes
ANSI_NULL_DEFAULT OFF Yes
ANSI_NULLS OFF Yes
ANSI_PADDING OFF Yes
ANSI_WARNINGS OFF Yes
ARITHABORT OFF Yes
AUTO_CLOSE OFF No
AUTO_CREATE_STATISTICS ON Yes
AUTO_SHRINK OFF No
AUTO_UPDATE_STATISTICS ON Yes
AUTO_UPDATE_STATISTICS_ASYNC OFF Yes
CHANGE_TRACKING OFF No
CONCAT_NULL_YIELDS_NULL OFF Yes
CURSOR_CLOSE_ON_COMMIT OFF Yes
CURSOR_DEFAULT GLOBAL Yes
Database Availability Options ONLINE

MULTI_USER

READ_WRITE
No

No

No
DATE_CORRELATION_OPTIMIZATION OFF Yes
DB_CHAINING ON No
ENCRYPTION OFF No
MIXED_PAGE_ALLOCATION OFF No
NUMERIC_ROUNDABORT OFF Yes
PAGE_VERIFY CHECKSUM for new installations of SQL Server

NONE for upgrades of SQL Server
Yes
PARAMETERIZATION SIMPLE Yes
QUOTED_IDENTIFIER OFF Yes
READ_COMMITTED_SNAPSHOT OFF No
RECOVERY SIMPLE No
RECURSIVE_TRIGGERS OFF Yes
Service Broker Options ENABLE_BROKER Yes
TRUSTWORTHY OFF No

For a description of these database options, see ALTER DATABASE SET Options (Transact-SQL).

tempdb database in SQL Database

tempdb sizes for DTU-based service tiers

Service-level objective Maximum tempdb data file size (GB) Number of tempdb data files Maximum tempdb data size (GB)
Basic 13.9 1 13.9
S0 13.9 1 13.9
S1 13.9 1 13.9
S2 13.9 1 13.9
S3 32 1 32
S4 32 2 64
S6 32 3 96
S7 32 6 192
S9 32 12 384
S12 32 12 384
P1 13.9 12 166.7
P2 13.9 12 166.7
P4 13.9 12 166.7
P6 13.9 12 166.7
P11 13.9 12 166.7
P15 13.9 12 166.7
Basic Elastic Pools (all DTU configurations) 13.9 12 166.7
Standard Elastic Pools (50 eDTU) 13.9 12 166.7
Standard Elastic Pools (100 eDTU) 32 1 32
Standard Elastic Pools (200 eDTU) 32 2 64
Standard Elastic Pools (300 eDTU) 32 3 96
Standard Elastic Pools (400 eDTU) 32 3 96
Standard Elastic Pools (800 eDTU) 32 6 192
Standard Elastic Pools (1200 eDTU) 32 10 320
Standard Elastic Pools (1600-3000 eDTU) 32 12 384
Premium Elastic Pools (all DTU configurations) 13.9 12 166.7

tempdb sizes for vCore-based service tiers

See vCore-based resource limits.

Restrictions

The following operations can't be performed on the tempdb database:

  • Adding filegroups.
  • 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.

Permissions

Any user can create temporary objects in tempdb. Users can access only their own objects, unless they receive additional permissions. It's possible to revoke the connect permission to tempdb to prevent a user from using tempdb. We don't recommend it because some routine operations require the use of tempdb.

Optimizing tempdb performance in SQL Server

The size and physical placement of the tempdb database can affect the performance of a system. For example, if the size that's defined for tempdb is too small, part of the system-processing load might be taken up with autogrowing tempdb to the size required to support the workload every time you restart the instance of SQL Server.

If possible, use instant file initialization to improve the performance of growth operations for data files.

Preallocate space for all tempdb files by setting the file size to a value large enough to accommodate the typical workload in the environment. Preallocation prevents tempdb from expanding too often, which affects performance. The tempdb database should be set to autogrow to increase disk space for unplanned exceptions.

Data files should be of equal size within each filegroup, because SQL Server uses a proportional-fill algorithm that favors allocations in files with more free space. Dividing tempdb into multiple data files of equal size provides a high degree of parallel efficiency in operations that use tempdb.

Set the file growth increment to a reasonable size to prevent the tempdb database files from growing by too small a value. If the file growth is too small compared to the amount of data that's being written to tempdb, tempdb might have to constantly expand. That will affect performance.

To check current size and growth parameters for tempdb, use the following query:

 SELECT name AS FileName,
    size*1.0/128 AS FileSizeInMB,
    CASE max_size
        WHEN 0 THEN 'Autogrowth is off.'
        WHEN -1 THEN 'Autogrowth is on.'
        ELSE 'Log file grows to a maximum size of 2 TB.'
    END,
    growth AS 'GrowthValue',
    'GrowthIncrement' =
        CASE
            WHEN growth = 0 THEN 'Size is fixed.'
            WHEN growth > 0 AND is_percent_growth = 0
                THEN 'Growth value is in 8-KB pages.'
            ELSE 'Growth value is a percentage.'
        END
FROM tempdb.sys.database_files;
GO

Put the tempdb database on a fast I/O subsystem. Use disk striping if there are many directly attached disks. Individual or groups of tempdb data files don't necessarily need to be on different disks or spindles unless you're also encountering I/O bottlenecks.

Put the tempdb database on disks that differ from the disks that user databases use.

Performance improvements in tempdb for SQL Server

Starting with SQL Server 2016 (13.x), tempdb performance is further optimized in the following ways:

  • Temporary tables and table variables are cached. Caching allows operations that drop and create the temporary objects to run very quickly. Caching also reduces page allocation and metadata contention.
  • The allocation page latching protocol is improved to reduce the number of UP (update) latches that are used.
  • Logging overhead for tempdb is reduced to reduce disk I/O bandwidth consumption on the tempdb log file.
  • Setup adds multiple tempdb data files during a new instance installation. You can accomplish this task by using the new UI input control in the Database Engine Configuration section and the command-line parameter /SQLTEMPDBFILECOUNT. By default, setup adds as many tempdb data files as the logical processor count or eight, whichever is lower.
  • When there are multiple tempdb data files, all files autogrow at the 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 can't be modified.

For more information on performance improvements in tempdb, see the blog article TEMPDB - Files and Trace Flags and Updates, Oh My!.

Memory-optimized tempdb metadata

Metadata contention in tempdb has historically been a bottleneck to scalability for many workloads running on SQL Server. SQL Server 2019 (15.x) introduces a new feature that's part of the in-memory database feature family: memory-optimized tempdb metadata.

This feature effectively removes this bottleneck and unlocks a new level of scalability for tempdb-heavy workloads. In SQL Server 2019 (15.x), the system tables involved in managing temporary table metadata can be moved into latch-free, non-durable, memory-optimized tables.

Watch this seven-minute video for an overview of how and when to use memory-optimized tempdb metadata:

Configuring and using memory-optimized tempdb metadata

To opt in to this new feature, use the following script:

ALTER SERVER CONFIGURATION SET MEMORY_OPTIMIZED TEMPDB_METADATA = ON;

This configuration change requires a restart of the service to take effect.

You can verify whether or not tempdb is memory-optimized by using the following T-SQL command:

SELECT SERVERPROPERTY('IsTempdbMetadataMemoryOptimized');

If the server fails to start for any reason after you enable memory-optimized tempdb metadata, you can bypass the feature by starting the SQL Server instance with minimal configuration through the -f startup option. You can then disable the feature and restart SQL Server in normal mode.

To protect the server from potential out-of-memory conditions, you can bind tempdb to a resource pool. This is done through the ALTER SERVER command rather than the steps you would normally follow to bind a resource pool to a database.

ALTER SERVER CONFIGURATION SET MEMORY_OPTIMIZED TEMPDB_METADATA = ON (RESOURCE_POOL = 'pool_name');

This change also requires a restart to take effect, even if memory-optimized tempdb metadata is already enabled.

Memory-optimized tempdb limitations

  • Toggling the feature on and off is not dynamic. Because of the intrinsic changes that need to be made to the structure of tempdb, a restart is required to either enable or disable the feature.

  • A single transaction is not allowed to access memory-optimized tables in more than one database. Any transactions that involve a memory-optimized table in a user database won't be able to access tempdb system views in the same transaction. If you try to access tempdb system views in the same transaction as a memory-optimized table in a user database, you'll receive the following error:

    A user transaction that accesses memory optimized tables or natively compiled modules cannot access more than one user database or databases model and msdb, and it cannot write to master.
    

    Example:

    BEGIN TRAN;
    
    SELECT *
    FROM tempdb.sys.tables;  -----> Creates a user in-memory OLTP transaction in tempdb
    
    INSERT INTO <user database>.<schema>.<mem-optimized table>
    VALUES (1); ----> Tries to create a user in-memory OLTP transaction in the user database but will fail
    
    COMMIT TRAN;
    
  • Queries against memory-optimized tables don't support locking and isolation hints, so queries against memory-optimized tempdb catalog views won't honor locking and isolation hints. As with other system catalog views in SQL Server, all transactions against system views will be in READ COMMITTED (or in this case, READ COMMITTED SNAPSHOT) isolation.

  • Columnstore indexes can't be created on temporary tables when memory-optimized tempdb metadata is enabled.

  • Due to the limitation on columnstore indexes, use of the sp_estimate_data_compression_savings system stored procedure with the COLUMNSTORE or COLUMNSTORE_ARCHIVE data compression parameter is not supported when memory-optimized tempdb metadata is enabled.

Note

These limitations apply only when you're referencing tempdb system views. You can create a temporary table in the same transaction as you access a memory-optimized table in a user database, if desired.

Capacity planning for tempdb in SQL Server

Determining the appropriate size for tempdb in a SQL Server production environment depends on many factors. As described earlier, these factors include the existing workload and the SQL Server features that are used. We recommend that you analyze the existing workload by performing the following tasks in a SQL Server test environment:

  • Set autogrow on for tempdb.
  • Run individual queries or workload trace files and monitor tempdb space use.
  • Execute index maintenance operations such as rebuilding indexes, and monitor tempdb space.
  • Use the space-use values from the previous steps to predict your total workload usage. Adjust this value for projected concurrent activity, and then set the size of tempdb accordingly.

Monitoring tempdb use

Running out of disk space in tempdb can cause significant disruptions in the SQL Server production environment. It can also prevent applications that are running from completing operations. You can use the sys.dm_db_file_space_usage dynamic management view to monitor the disk space that's used in the tempdb files:

 -- Determining the amount of free space in tempdb
SELECT SUM(unallocated_extent_page_count) AS [free pages],
  (SUM(unallocated_extent_page_count)*1.0/128) AS [free space in MB]
FROM sys.dm_db_file_space_usage;

-- Determining the amount of space used by the version store
SELECT SUM(version_store_reserved_page_count) AS [version store pages used],
  (SUM(version_store_reserved_page_count)*1.0/128) AS [version store space in MB]
FROM sys.dm_db_file_space_usage;

-- Determining the amount of space used by internal objects
SELECT SUM(internal_object_reserved_page_count) AS [internal object pages used],
  (SUM(internal_object_reserved_page_count)*1.0/128) AS [internal object space in MB]
FROM sys.dm_db_file_space_usage;

-- Determining the amount of space used by user objects
SELECT SUM(user_object_reserved_page_count) AS [user object pages used],
  (SUM(user_object_reserved_page_count)*1.0/128) AS [user object space in MB]
FROM sys.dm_db_file_space_usage;

To monitor the page allocation or deallocation activity in tempdb at the session or task level, you can use the sys.dm_db_session_space_usage and sys.dm_db_task_space_usage dynamic management views. These views can help you identify large queries, temporary tables, or table variables that are using lots of tempdb disk space. You can also use several counters to monitor the free space that's available in tempdb and the resources that are using tempdb.

-- Obtaining the space consumed by internal objects in all currently running tasks in each session
SELECT session_id,
  SUM(internal_objects_alloc_page_count) AS task_internal_objects_alloc_page_count,
  SUM(internal_objects_dealloc_page_count) AS task_internal_objects_dealloc_page_count
FROM sys.dm_db_task_space_usage
GROUP BY session_id;

-- Obtaining the space consumed by internal objects in the current session for both running and completed tasks
SELECT R2.session_id,
  R1.internal_objects_alloc_page_count
  + SUM(R2.internal_objects_alloc_page_count) AS session_internal_objects_alloc_page_count,
  R1.internal_objects_dealloc_page_count
  + SUM(R2.internal_objects_dealloc_page_count) AS session_internal_objects_dealloc_page_count
FROM sys.dm_db_session_space_usage AS R1
INNER JOIN sys.dm_db_task_space_usage AS R2 ON R1.session_id = R2.session_id
GROUP BY R2.session_id, R1.internal_objects_alloc_page_count,
  R1.internal_objects_dealloc_page_count;;

SORT_IN_TEMPDB option for indexes
System databases
sys.databases
sys.master_files
Move database files