In general, using explicit table hints frequently is considered as a bad practice that you should generally avoid. One benefit of using the WITH keyword is that you can specify multiple table hints using the WITH keyword against the same table. With that said, it is better to include the WITH keyword when specifying the table hints. The transaction isolation level can be set globally at the connection level using the SET TRANSACTION ISOLATION LEVEL T-SQL command, as will see later in this article.Īlthough the NOLOCK table hint, similar to all other table hints, can be used without using the WITH keyword, Microsoft announced that omitting the WITH keyword is a deprecated feature and will be removed from future Microsoft SQL Server versions. This is similar to the READ UNCOMMITTED transaction isolation level, that allows the query to see the data changes before committing the transaction that is changing it. In other words, the WITH (NOLOCK) table hint retrieves the rows without waiting for the other queries, that are reading or modifying the same data, to finish its processing. In addition to that, no deadlock will occur against the queries, that are requesting the same data from that table, allowing a higher level of concurrency due to a lower footprint. In this way, the query will consume less memory in holding locks against that data. The WITH (NOLOCK) table hint is used to override the default transaction isolation level of the table or the tables within the view in a specific query, by allowing the user to retrieve the data without being affected by the locks, on the requested data, due to another process that is changing it. The default transaction isolation level in SQL Server is the READ COMMITTED isolation level, in which retrieving the changing data will be blocked until these changes are committed.
One of the more heavily used table hints in the SELECT T-SQL statements is the WITH (NOLOCK) hint. The table hints can be added to the FROM clause of the T-SQL query, affecting the table or the view that is referenced in the FROM clause only. SQL Server table hints are a special type of explicit command that is used to override the default behavior of the SQL Server query optimizer during the T-SQL query execution This is accomplished by enforcing a specific locking method, a specific index or query processing operation, such index seek or table scan, to be used by the SQL Server query optimizer to build the query execution plan.
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