SQL Server NOLOCK, HOLDLOCK, UPDLOCK, TABLOCK, Tablockx

SQL Server NOLOCK, HOLDLOCK, UPDLOCK, TABLOCK, Tablockx

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NOLOCK (without lock)

When this option is selected, SQL Server does not add any locks when reading or modifying data. In this case, it is possible for the user to read the data in the incomplete transaction (uncommited Transaction) or rollback (roll back), known as "dirty data."

HOLDLOCK (Hold Lock)

When this option is selected, SQL Server will persist this shared lock to the end of the entire transaction and will not be released on the way.

UPDLOCK (Modify Lock)

When this option is selected, SQL Server uses a modify lock instead of a shared lock when reading data, and holds the lock to the entire transaction or to the end of the command. Using this option ensures that multiple processes can read data at the same time, but only that process can modify the data.

TABLOCK (table Lock)

When this option is selected, SQL Server will place a shared lock on the entire table until the command ends. This option ensures that other processes can read only and cannot modify the data.

Paglock (page lock)

This option is the default option when SQL Server uses a shared page lock when it is selected.

Tablockx (Row it table lock)

When this option is selected, SQL Server will place an exclusive lock on the entire table until the command or transaction ends. This prevents other processes from reading or modifying the data in the table.

HOLDLOCK holds a shared lock until the entire transaction is complete and should be released immediately when the locked object is not needed, equal to the serializable transaction isolation level

　　 NOLOCK statement execution does not emit a shared lock, allowing dirty reads, equal to the READ UNCOMMITTED transaction isolation LEVEL

Paglock with multiple page locks where a table lock is used

READPAST let SQL Server skip any locking lines, perform transactions, apply to READ UNCOMMITTED transaction isolation LEVEL only skip RID lock, skip page, zone and table lock

Rowlock forcing the use of row locks

　　 Tablockx enforces exclusive table-level locks, which prevent any other transactions from using this table during a transaction

Uplock forcing updates to be used when reading tables without sharing locks

Note: The difference between a table that locks a database

SELECT * from table with (HOLDLOCK) Other transactions can read the table but cannot update the delete

SELECT * from table with (Tablockx) Other transactions cannot read tables, update and delete

SQL Server NOLOCK, HOLDLOCK, UPDLOCK, TABLOCK, Tablockx