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What is SQL Server NOLOCK hint?

Did you know that SQL Server hints are a specific type of explicit command used to override the SQL Server query optimizer’s default behavior during the T-SQL query execution?

According to our experts, this is done by using a particular locking method, a particular index, or query processing operation, like table scan or index seek, which is used by the SQL Server query optimizer in order to build the query execution plan.

In addition, table hints can be added to the FROM clause.

The NOLOCK hint is a query hint that can be used in SQL Server to specify that the query should not acquire a lock on the tables it is accessing when it is executed. This can improve query performance, but it can also cause the query to return dirty data and may lead to data inconsistencies in your database.

Interestingly, one of the most used table hints is the NOLOCK hint. Here, the default transaction isolation level is READ COMMITTED. In other words, retrieving the changing data is blocked until the changes are committed.

The NOLOCK table hint overrides the default transaction isolation level of the table or the tables within the view in a particular query. This permits users to access the data without being blocked by the locks.

This way, the query consumes less memory while holding locks. Furthermore, there won’t be any deadlock against the queries currently trying to access the same data from that table.

In fact, the NOLOCK table hint retrieves the required rows without having to wait around for other queries to complete their tasks.

Unfortunately, SQL Server NOLOCK hint increases the chances of dirty reads. Hence, our experts advise against using explicit table hints frequently

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Conclusion

To wrap up, our Support Engineers introduced us to SQL Server NOLOCK hint.