Deleting All Rows by Using TRUNCATE TABLE
The TRUNCATE TABLE statement is a fast, nonlogged method of deleting all rows in a table. TRUNCATE TABLE is functionally the same to the DELETE statement without a 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.
As with DELETE, the definition of a table emptied using TRUNCATE TABLE remains in the database, together with its indexes and other associated objects.
Truncating Large Tables
Microsoft SQL Server 2005 introduces 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 following example removes all data from the
SELECT statements are included before and after the
TRUNCATE TABLE statement to compare results.
USE AdventureWorks; GO SELECT COUNT(*) AS BeforeTruncateCount FROM HumanResources.JobCandidate; GO TRUNCATE TABLE HumanResources.JobCandidate; GO SELECT COUNT(*) AS AfterTruncateCount FROM HumanResources.JobCandidate; GO