[MSDN] DBCC SHRINKDATABASE (Transact-SQL) [EN-US]
Shrinks the size of the data and log files in the specified database.
The following table describes the columns in the result set.
Column name |
Description |
|---|---|
DbId |
Database identification number of the file the Database Engine tried to shrink. |
FileId |
File identification number of the file the Database Engine tried to shrink. |
CurrentSize |
Number of 8-KB pages the file currently occupies. |
MinimumSize |
Number of 8-KB pages the file could occupy, at minimum. This corresponds to the minimum size or originally created size of a file. |
UsedPages |
Number of 8-KB pages currently used by the file. |
EstimatedPages |
Number of 8-KB pages that the Database Engine estimates the file could be shrunk down to. |
The Database Engine does not display rows for those files not shrunk. |
To shrink all data and log files for a specific database, execute the DBCC SHRINKDATABASE command. To shrink one data or log file at a time for a specific database, execute the DBCC SHRINKFILE command.
To view the current amount of free (unallocated) space in the database, run sp_spaceused.
DBCC SHRINKDATABASE operations can be stopped at any point in the process, and any completed work is retained.
The database cannot be made smaller than the minimum size of the database. The minimum size is the size specified when the database is originally created, or the last size explicitly set by using a file size changing operation such as DBCC SHIRNKFILE or ALTER DATABASE. For example, if a database is originally created with a size of 10 MB in size and grows to 100 MB, the smallest the database can be reduced to is 10 MB, even if all the data in the database has been deleted.
Running DBCC SHRINKDATABASE without specifying either the NOTRUNCATE option or the TRUNCATEONLY option is equivalent to running a DBCC SHRINKDATABASE operation with NOTRUNCATE followed by running a DBCC SHRINKDATABASE operation with TRUNCATEONLY.
The database being shrunk does not have to be in single user mode; other users can be working in the database when it is shrunk. This includes system databases.
You cannot shrink a database while the database is being backed up. Conversely, you cannot backup a database while a shrink operation on the database is in process.
How DBCC SHRINKDATABASE Works
DBCC SHRINKDATABASE shrinks data files on a per-file basis, but shrinks log files as if all the log files existed in one contiguous log pool. Files are always shrunk from the end.
Assume a database named mydb with a data file and two log files. The data and log files are 10 MB each and the data file contains 6 MB of data.
For each file, the Database Engine calculates a target size. This is the size to which the file is to be shrunk. When DBCC SHRINKDATABASE is specified with target_percent, the Database Engine calculates target size to be the target_percent amount of space free in the file after shrinking. For example, if you specify a target_percent of 25 for shrinking mydb, the Database Engine calculates the target size for the data file to be 8 MB (6 MB of data plus 2 MB of free space). Therefore, the Database Engine moves any data from the last 2 MB of the data file to any free space in the first 8 MB of the data file and then shrinks the file.
Assume the data file of mydb contains 7 MB of data. Specifying a target_percent of 30 allows for this data file to be shrunk to the free percentage of 30. However, specifying a target_percent of 40 does not shrink the data file because the Database Engine will not shrink a file to a size smaller than the data currently occupies. You can also think of this issue another way: 40 percent wanted free space + 70 percent full data file (7 MB out of 10 MB) is more than 100 percent. Because the percentage free that is wanted plus the current percentage that the data file occupies is over 100 percent (by 10 percent), any target_size greater than 30 will not shrink the data file.
For log files, the Database Engine uses target_percent to calculate the target size for the whole log; therefore, target_percent is the amount of free space in the log after the shrink operation. Target size for the whole log is then translated to a target size for each log file.
DBCC SHRINKDATABASE tries to shrink each physical log file to its target size immediately. If no part of the logical log resides in the virtual logs beyond the target size of the log file, the file is successfully truncated and DBCC SHRINKDATABASE finishes without any messages. However, if part of the logical log resides in the virtual logs beyond the target size, the Database Engine frees as much space as possible, and then issues an informational message. The message describes what actions are required to move the logical log out of the virtual logs at the end of the file. After the actions are performed, DBCC SHRINKDATABASE can be used to free the remaining space. For more information, see Shrinking the Transaction Log.
Because a log file can only be shrunk to a virtual log file boundary, shrinking a log file to a size smaller than the size of a virtual log file might not be possible, even if it is not being used. The size of the virtual log file is chosen dynamically by the Database Engine when log files are created or extended. For more information about virtual log files, see Transaction Log Physical Architecture.
Best Practices
Consider the following information when you plan to shrink a database:
A shrink operation is most effective after an operation that creates lots of unused space, such as a 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 space that was shrunk is required for regular operations. In these cases, repeatedly shrinking the database is a wasted operation.
A shrink operation does not 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, do not set the AUTO_SHRINK database option to ON.
Troubleshooting
It is possible for shrink operations to be blocked by a transaction that is running under a row versioning-based isolation level. For example, if a large delete operation running under a row versioning-based isolation level is in progress when a DBCC SHRINK DATABASE operation is executed, the shrink operation will wait for the delete operation to complete before shrinking the files. When this happens, DBCC SHRINKFILE and DBCC SHRINKDATABASE operations print out an informational message (5202 for SHRINKDATABASE and 5203 for SHRINKFILE) to the SQL Server error log every five minutes in the first hour and then every hour after that. For example, if the error log contains the following error message:
DBCC SHRINKDATABASE for database ID 9 is waiting for the snapshot transaction with timestamp 15 and other snapshot transactions linked to timestamp 15 or with timestamps older than 109 to finish.
This means that the shrink operation is blocked by snapshot transactions that have timestamps older than 109, which is the last transaction that the shrink operation completed. It also indicates that the transaction_sequence_num, or first_snapshot_sequence_num columns in the sys.dm_tran_active_snapshot_database_transactions (Transact-SQL) dynamic management view contains a value of 15. If either the transaction_sequence_num, or first_snapshot_sequence_num columns in the view contains a number that is less than the last transaction completed by a shrink operation (109), the shrink operation will wait for those transactions to finish.
To resolve the problem, you can do one of the following tasks:
Terminate the transaction that is blocking the shrink operation.
Terminate the shrink operation. Any completed work is retained.
Do nothing and allow the shrink operation to wait until the blocking transaction completes.
For more information about the SQL Server error log, see Viewing the SQL Server Error Log.
Requires membership in the sysadmin fixed server role or the db_owner fixed database role.
A. Shrinking a database and specifying a percentage of free space
The following example decreases the size of the data and log files in the UserDB user database to allow for 10 percent free space in the database.