What is "with (nolock)" in SQL Server?
Can someone explain the implications of using with (nolock) on queries, when you should/shouldn't use it?
For example, if you have a banking application with high transaction rates and a lot of data in certain tables, in what types of queries would nolock be okay? Are there cases when you should always use it/never use it?
WITH (NOLOCK) is the equivalent of using READ UNCOMMITED as a transaction isolation level. So, you stand the risk of reading an uncommitted row that is subsequently rolled back, ie data that never made it into the database. So, while it can prevent reads being deadlocked by other operations, it comes with a risk. In a banking application with high transaction rates, it's probably not going to be the right solution to whatever problem you're trying to solve with it IMHO.
The question is what is worse:
For financial databases, deadlocks are far worse than wrong values. I know that sounds backwards, but hear me out. The traditional example of DB transactions is you update two rows, subtracting from one and adding to another. That is wrong.
In a financial database you use business transactions. That means adding one row to each account. It is of utmost importance that these transactions complete and the rows are successfully written.
Getting the account balance temporarily wrong isn't a big deal, that is what the end of day reconciliation is for. And an overdraft from an account is far more likely to occur because two ATMs are being used at once than because of a uncommitted read from a database.
That said, SQL Server 2005 fixed most of the bugs that made NOLOCK necessary. So unless you are using SQL Server 2000 or earlier, you shouldn't need it.
Further Reading
Row-Level Versioning
Unfortunately it's not just about reading uncommitted data. In the background you may end up reading pages twice (in the case of a page split), or you may miss the pages altogether. So your results may be grossly skewed.
Check out Itzik Ben-Gan's article. Here's an excerpt:
" With the NOLOCK hint (or setting the isolation level of the session to READ UNCOMMITTED) you tell SQL Server that you don't expect consistency, so there are no guarantees. Bear in mind though that "inconsistent data" does not only mean that you might see uncommitted changes that were later rolled back, or data changes in an intermediate state of the transaction. It also means that in a simple query that scans all table/index data SQL Server may lose the scan position, or you might end up getting the same row twice . "
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