However I would point out that if you are getting frequent deadlocks with a query, this is an indication that using NOLOCK is going to return incorrect results. There are some advantages to using NOLOCK, the main one being that it helps avoid deadlocks with other queries running against the same data. This is known as a Dirty Read, and in some cases it may cause a query to return the same row multiple times or even skip rows. SQL Sever then returns the data to the query even though those transactions may not have been committed. When using the NOLOCK hint in a SELECT statement, SQL Server will not put read locks in place, and any INSERT/UPDATE/DELETE statements that may also be occurring are allowed to modify the data you are trying to read. Values in the data can be changed and rows can appear or disappear in the data set before the end of the transaction.īasically, what this means is the data returned to the SELECT statement may or may not actually exist in the database. When this option is set, it is possible to read uncommitted modifications, which are called dirty reads. READ UNCOMMITTED transactions are also not blocked by exclusive locks that would prevent the current transaction from reading rows that have been modified but not committed by other transactions. Transactions running at the READ UNCOMMITTED level do not issue shared locks to prevent other transactions from modifying data read by the current transaction. Specifies that statements can read rows that have been modified by other transactions but not yet committed. I do not think it means what you think it means.” What does NOLOCK do?Īdding NOLOCK to your query is the same as setting the Transaction Isolation Level to READ UNCOMMITTED, which Microsoft defines as: To quote the great DBA sage Inigo Montoya, “ You keep using that query hint. Always putting WITH (NOLOCK) on every table in your SELECT query is a really bad practice. When I inquire about this, the typical response I get is usually along the lines of “I don’t want my query to create any locks.” or “It helps my query run faster.” Well…that’s not exactly what NOLOCK is for. You’ll want to change the session_id to match the session ID of the SSMS window where you’re running the offending script to query tempdb.When working with developers, I’ve noticed that some have a tendency to put WITH (NOLOCK) on every table in a SELECT query. The following script sets up Extended Events. If you’re feeling really sassy, you can even attempt to correlate those two events (left as an exercise for the reader). The easiest thing you can do is set up an Extended Events session to look for lock_acquired and lock_released events. Unfortunately, the Sch-S locks conflict with attempts to drop or modify temporary tables (those changes need an Sch-M lock). When we run this query, the locks are only released when the statement completes. This makes sense - we can’t have things disappearing out of the database while we’re reading them. Our first query to look at the tempdb metadata takes out a schema stability ( Sch-S) lock on each temporary object that it finds. While this is awful, it’s not terribly surprising. If you run the first query enough, you’ll end up with a deadlock. IF OBJECT_ID( '#lol') IS NOT NULL DROP TABLE #lol
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