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Shrink a database

  • Članak

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

This article describes how to shrink a database in SQL Server by using Object Explorer in SQL Server Management Studio or Transact-SQL.

Shrinking data files recovers space by moving pages of data from the end of the file to unoccupied space closer to the front of the file. When enough free space is created at the end of the file, data pages at end of the file can be deallocated and returned to the file system.

Limitations and restrictions

  • The database can't be made smaller than the minimum size of the database. The minimum size is the size specified when the database was originally created, or the last explicit size set by using a file-size-changing operation, such as DBCC SHRINKFILE. For example, if a database was originally created with a size of 10 MB and grew to 100 MB, the smallest size the database could be reduced to is 10 MB, even if all the data in the database has been deleted.

  • You can't shrink a database while the database is being backed up. Conversely, you can't back up a database while a shrink operation on the database is in process.

Recommendations

  • To view the current amount of free (unallocated) space in the database. For more information, see Display Data and Log Space Information for a Database

  • Consider the following information when you plan to shrink a database:

    • A shrink operation is most effective after an operation that creates a large amount of unused storage space, such as a large DELETE statement, truncate table, or a drop table operation.

    • Most databases require some free space to be available for regular day-to-day operations. If you shrink a database repeatedly and notice that the database size grows again, this indicates that the free space is required for regular operations. In these cases, repeatedly shrinking the database is a wasted operation. Autogrow events necessary to grow the database file(s) hinder performance.

    • A shrink operation doesn't preserve the fragmentation state of indexes in the database, and generally increases fragmentation to a degree. This is another reason not to repeatedly shrink the database.

    • Unless you have a specific requirement, don't set the AUTO_SHRINK database option to ON.

Permissions

Requires membership in the sysadmin fixed server role or the db_owner fixed database role.

Remarks

Shrink operations in progress can block other queries on the database, and can be blocked by queries already in progress. Introduced in SQL Server 2022 (16.x), shrink database operations have a WAIT_AT_LOW_PRIORITY option. This feature is a new additional option for DBCC SHRINKDATABASE and DBCC SHRINKFILE. If a new shrink operation in WAIT_AT_LOW_PRIORITY mode can't obtain the necessary locks due to a long-running query already in progress, the shrink operation will eventually time out after one minute and silently exit, preventing other queries from being blocked. For more information, see DBCC SHRINKDATABASE.

For information on file management and shrink operations specific to Azure SQL Database, see Manage file space for databases in Azure SQL Database.

Use SQL Server Management Studio

Applies to: SQL Server, Azure SQL Managed Instance

Shrink a database

  1. In Object Explorer, connect to an instance of the SQL Server Database Engine, and then expand that instance.

  2. Expand Databases, and then right-click the database that you want to shrink.

  3. Point to Tasks, point to Shrink, and then select Database.

    • Database

      Displays the name of the selected database.

    • Current allocated space

      Displays the total used and unused space for the selected database.

    • Available free space

      Displays the sum of free space in the log and data files of the selected database.

    • Reorganize files before releasing unused space

      Selecting this option is equivalent to executing DBCC SHRINKDATABASE specifying a target percent option. Clearing this option is equivalent to executing DBCC SHRINKDATABASE with TRUNCATEONLY option. By default, this option isn't selected when the dialog is opened. If this option is selected, the user must specify a target percent option.

    • Maximum free space in files after shrinking

      Enter the maximum percentage of free space to be left in the database files after the database has been shrunk. Permissible values are between 0 and 99.

  4. Select OK.

Use Transact-SQL

Shrink a database

  1. Connect to the Database Engine.

  2. From the Standard bar, select New Query.

  3. Copy and paste the following example into the query window and select Execute. This example uses DBCC SHRINKDATABASE to decrease the size of the data and log files in the UserDB database, and to allow for 10 percent free space in the database.

DBCC SHRINKDATABASE (UserDB, 10);
GO

After you shrink a database

Data that is moved to shrink a file can be scattered to any available location in the file. This causes index fragmentation and can slow the performance of queries that search a range of the index. To eliminate the fragmentation, consider rebuilding the indexes on the file after shrinking. For more information, see Rebuild an index.

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