TRUNCATE TABLE (Transact-SQL)
Removes all rows from a table without logging the individual row deletions. TRUNCATE TABLE is similar to the DELETE statement with no WHERE clause; however, TRUNCATE TABLE is faster and uses fewer system and transaction log resources.
Compared to the DELETE statement, TRUNCATE TABLE has the following advantages:
Less transaction log space is used.
The DELETE statement removes rows one at a time and records an entry in the transaction log for each deleted row. TRUNCATE TABLE removes the data by deallocating the data pages used to store the table data and records only the page deallocations in the transaction log.
Fewer locks are typically used.
When the DELETE statement is executed using a row lock, each row in the table is locked for deletion. TRUNCATE TABLE always locks the table and page but not each row.
Without exception, zero pages are left in the table.
After a DELETE statement is executed, the table can still contain empty pages. For example, empty pages in a heap cannot be deallocated without at least an exclusive (LCK_M_X) table lock. If the delete operation does not use a table lock, the table (heap) will contain many empty pages. For indexes, the delete operation can leave empty pages behind, although these pages will be deallocated quickly by a background cleanup process.
TRUNCATE TABLE removes all rows from a table, but the table structure and its columns, constraints, indexes, and so on remain. To remove the table definition in addition to its data, use the DROP TABLE statement.
If the table contains an identity column, the counter for that column is reset to the seed value defined for the column. If no seed was defined, the default value 1 is used. To retain the identity counter, use DELETE instead.
You cannot use TRUNCATE TABLE on tables that:
Are referenced by a FOREIGN KEY constraint. (You can truncate a table that has a foreign key that references itself.)
Participate in an indexed view.
Are published by using transactional replication or merge replication.
For tables with one or more of these characteristics, use the DELETE statement instead.
TRUNCATE TABLE cannot activate a trigger because the operation does not log individual row deletions. For more information, see CREATE TRIGGER (Transact-SQL).
Truncating Large Tables
Microsoft SQL Server has the ability to drop or truncate tables that have more than 128 extents without holding simultaneous locks on all the extents required for the drop. For more information, see Dropping and Rebuilding Large Objects.
The minimum permission required is ALTER on table_name. TRUNCATE TABLE permissions default to the table owner, members of the sysadmin fixed server role, and the db_owner and db_ddladmin fixed database roles, and are not transferable. However, you can incorporate the TRUNCATE TABLE statement within a module, such as a stored procedure, and grant appropriate permissions to the module using the EXECUTE AS clause. For more information, see Using EXECUTE AS to Create Custom Permission Sets.
The following example removes all data from the JobCandidate table. SELECT statements are included before and after the TRUNCATE TABLE statement to compare results.
USE AdventureWorks2008R2; GO SELECT COUNT(*) AS BeforeTruncateCount FROM HumanResources.JobCandidate; GO TRUNCATE TABLE HumanResources.JobCandidate; GO SELECT COUNT(*) AS AfterTruncateCount FROM HumanResources.JobCandidate; GO