A range of table-level locking hints can be specified using the SELECT, INSERT, UPDATE, and DELETE statements to direct Microsoft® SQL Server™ to the type of locks to be used. Table-level locking hints can be used when a finer control of the types of locks acquired on an object is required. These locking hints override the current transaction isolation level for the session.
Note The SQL Server query optimizer automatically makes the correct determination. It is recommended that table-level locking hints be used to change the default locking behavior only when absolutely necessary. Disallowing a locking level can adversely affect concurrency.
| Locking hint | Description |
|---|---|
| HOLDLOCK | Hold a shared lock until completion of the transaction instead of releasing the lock as soon as the required table, row, or data page is no longer required. HOLDLOCK is equivalent to SERIALIZABLE. |
| NOLOCK | Do not issue shared locks and do not honor exclusive locks. When this option is in effect, it is possible to read an uncommitted transaction or a set of pages that are rolled back in the middle of a read. Dirty reads are possible. Only applies to the SELECT statement. |
| PAGLOCK | Use page locks where a single table lock would usually be taken. |
| READCOMMITTED | Perform a scan with the same locking semantics as a transaction running at the READ COMMITTED isolation level. By default, SQL Server operates at this isolation level. |
| READPAST | Skip locked rows. This option causes a transaction to skip over rows locked by other transactions that would ordinarily appear in the result set, rather than block the transaction waiting for the other transactions to release their locks on these rows. The READPAST lock hint applies only to transactions operating at READ COMMITTED isolation and will read only past row-level locks. Applies only to the SELECT statement. |
| READUNCOMMITTED | Equivalent to NOLOCK. |
| REPEATABLEREAD | Perform a scan with the same locking semantics as a transaction running at the REPEATABLE READ isolation level. |
| ROWLOCK | Use row-level locks rather than use the coarser-grained page- and table-level locks. |
| SERIALIZABLE | Perform a scan with the same locking semantics as a transaction running at the SERIALIZABLE isolation level. Equivalent to HOLDLOCK. |
| TABLOCK | Use a table lock rather than using finer-grained row- or page-level locks. SQL Server holds this lock until the end of the statement. However, if you also specify HOLDLOCK, the lock is held until the end of the transaction. |
| TABLOCKX | Use an exclusive lock on a table. This lock prevents others from reading or updating the table and is held until the end of the statement or transaction. |
| UPDLOCK | Use update locks instead of shared locks while reading a table, and hold locks until the end of the statement or transaction. UPDLOCK has the advantage of allowing you to read data (without blocking other readers) and update it later with the assurance that the data has not changed since you last read it. |
For example, if the transaction isolation level is set to SERIALIZABLE, and the table-level locking hint NOLOCK is used with the SELECT statement, then key-range locks usually used to maintain serializable transactions are not taken.
USE pubs
GO
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
GO
BEGIN TRANSACTION
SELECT au_lname FROM authors WITH (NOLOCK)
GO
The locks generated are:
EXEC sp_lock
GO
spid |
dbid |
ObjId |
IndId |
Type |
Resource |
Mode |
Status |
1 |
1 |
0 |
0 |
DB |
S |
GRANT |
|
6 |
1 |
0 |
0 |
DB |
S |
GRANT |
|
7 |
1 |
0 |
0 |
DB |
S |
GRANT |
|
8 |
4 |
0 |
0 |
DB |
S |
GRANT |
|
8 |
4 |
0 |
0 |
DB |
S |
GRANT |
|
8 |
4 |
117575457 |
0 |
TAB |
Sch-S |
GRANT |
|
9 |
4 |
0 |
0 |
DB |
S |
GRANT |
|
9 |
1 |
21575115 |
0 |
TAB |
IS |
GRANT |
SELECT object_name(117575457)
GO
-----------------------------
authors
The only lock taken that references authors is a schema stability (Sch-S) lock. In this case, serializability is no longer guaranteed.
| DELETE | INSERT |
| SELECT | SET TRANSACTION ISOLATION LEVEL |
| UPDATE |