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, i.e. 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. "
The text book example for legitimate usage of the nolock hint is report sampling against a high update OLTP database.
To take a topical example. If a large US high street bank wanted to run an hourly report looking for the first signs of a city level run on the bank, a nolock query could scan transaction tables summing cash deposits and cash withdrawals per city. For such a report the tiny percentage of error caused by rolled back update transactions would not reduce the value of the report.
Not sure why you are not wrapping financial transactions in database transactions (as when you transfer funds from one account to another - you don't commit one side of the transaction at-a-time - this is why explicit transactions exist). Even if your code is braindead to business transactions as it sounds like it is, all transactional databases have the potential to do implicit rollbacks in the event of errors or failure. I think this discussion is way over your head.
If you are having locking problems, implement versioning and clean up your code.
No lock not only returns wrong values it returns phantom records and duplicates.
It is a common misconception that it always makes queries run faster. If there are no write locks on a table, it does not make any difference. If there are locks on the table, it may make the query faster, but there is a reason locks were invented in the first place.
In fairness, here are two special scenarios where a nolock hint may provide utility
1) Pre-2005 sql server database that needs to run long query against live OLTP database this may be the only way
2) Poorly written application that locks records and returns control to the UI and readers are indefinitely blocked. Nolock can be helpful here if application cannot be fixed (third party etc) and database is either pre-2005 or versioning cannot be turned on.
You can use it when you're only reading data, and you don't really care about whether or not you might be getting back data that is not committed yet.
It can be faster on a read operation, but I cannot really say by how much.
In general, I recommend against using it - reading uncommitted data can be a bit confusing at best.
NOLOCK is equivalent to READ UNCOMMITTED, however Microsoft says you should not use it for UPDATE or DELETE statements:
For UPDATE or DELETE statements: This feature will be removed in a future version of Microsoft SQL Server. Avoid using this feature in new development work, and plan to modify applications that currently use this feature.
http://msdn.microsoft.com/en-us/library/ms187373.aspx
This article applies to SQL Server 2005, so the support for NOLOCK exists if you are using that version. In order to future-proof you code (assuming you've decided to use dirty reads) you could use this in your stored procedures:
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
Another case where it's usually okay is in a reporting database, where data is perhaps already aged and writes just don't happen. In this case, though, the option should be set at the database or table level by the administrator by changing the default isolation level.
In the general case: you can use it when you are very sure that it's okay to read old data. The important thing to remember is that its very easy to get that wrong. For example, even if it's okay at the time you write the query, are you sure something won't change in the database in the future to make these updates more important?
I'll also 2nd the notion that it's probably not a good idea in banking app. Or inventory app. Or anywhere you're thinking about transactions.
Short answer:
Only use WITH (NOLOCK) in SELECT statement on tables that have a clustered index.
Long answer:
WITH(NOLOCK) is often exploited as a magic way to speed up database reads.
The result set can contain rows that have not yet been committed, that are often later rolled back.
If WITH(NOLOCK) is applied to a table that has a non-clustered index then row-indexes can be changed by other transactions as the row data is being streamed into the result-table. This means that the result-set can be missing rows or display the same row multiple times.
READ COMMITTED adds an additional issue where data is corrupted within a single column where multiple users change the same cell simultaneously.
Simple answer - whenever your SQL is not altering data, and you have a query that might interfere with other activity (via locking).
It's worth considering for any queries used for reports, especially if the query takes more than, say, 1 second.
It's especially useful if you have OLAP-type reports you're running against an OLTP database.
The first question to ask, though, is "why am I worrying about this?" ln my experience, fudging the default locking behavior often takes place when someone is in "try anything" mode and this is one case where unexpected consequences are not unlikely. Too often it's a case of premature optimization and can too easily get left embedded in an application "just in case." It's important to understand why you're doing it, what problem it solves, and whether you actually have the problem.
My 2 cents - it makes sense to use WITH (NOLOCK) when you need to generate reports. At this point, the data wouldn't change much & you wouldn't want to lock those records.
If you are handling finance transactions then you will never want to use nolock. nolock is best used to select from large tables that have lots updates and you don't care if the record you get could possibly be out of date.
For financial records (and almost all other records in most applications) nolock would wreak havoc as you could potentially read data back from a record that was being written to and not get the correct data.
I've used to retrieve a "next batch" for things to do. It doesn't matter in this case which exact item, and I have a lot of users running this same query.
Use nolock when you are okay with the "dirty" data. Which means nolock can also read data which is in the process of being modified and/or uncommitted data.
It's generally not a good idea to use it in high transaction environment and that is why it is not a default option on query.
I use with (nolock) hint particularly in SQLServer 2000 databases with high activity. I am not certain that it is needed in SQL Server 2005 however. I recently added that hint in a SQL Server 2000 at the request of the client's DBA, because he was noticing a lot of SPID record locks.
All I can say is that using the hint has NOT hurt us and appears to have made the locking problem solve itself. The DBA at that particular client basically insisted that we use the hint.
By the way, the databases I deal with are back-ends to enterprise medical claims systems, so we are talking about millions of records and 20+ tables in many joins. I typically add a WITH (nolock) hint for each table in the join (unless it is a derived table, in which case you can't use that particular hint)
The simplest answer is a simple question - do you need your results to be repeatable? If yes then NOLOCKS is not appropriate under any circumstances
If you don't need repeatability then nolocks may be useful, especially if you don't have control over all processes connecting to the target database.
