[sql-server] Why use a READ UNCOMMITTED isolation level?

In plain English, what are the disadvantages and advantages of using

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED

in a query for .NET applications and reporting services applications?

When is it ok to use READ UNCOMMITTED?

Rule of thumb

Good: Big aggregate reports showing constantly changing totals.

Risky: Nearly everything else.

The good news is that the majority of read-only reports fall in that Good category.

More detail...

Ok to use it:

• Nearly all user-facing aggregate reports for current, non-static data e.g. Year to date sales. It risks a margin of error (maybe < 0.1%) which is much lower than other uncertainty factors such as inputting error or just the randomness of when exactly data gets recorded minute to minute.

That covers probably the majority of what an Business Intelligence department would do in, say, SSRS. The exception of course, is anything with $ signs in front of it. Many people account for money with much more zeal than applied to the related core metrics required to service the customer and generate that money. (I blame accountants).

When risky

• Any report that goes down to the detail level. If that detail is required it usually implies that every row will be relevant to a decision. In fact, if you can't pull a small subset without blocking it might be for the good reason that it's being currently edited.

• Historical data. It rarely makes a practical difference but whereas users understand constantly changing data can't be perfect, they don't feel the same about static data. Dirty reads won't hurt here but double reads can occasionally be. Seeing as you shouldn't have blocks on static data anyway, why risk it?

• Nearly anything that feeds an application which also has write capabilities.

When even the OK scenario is not OK.

• Are any applications or update processes making use of big single transactions? Ones which remove then re-insert a lot of records you're reporting on? In that case you really can't use NOLOCK on those tables for anything.

Regarding reporting, we use it on all of our reporting queries to prevent a query from bogging down databases. We can do that because we're pulling historical data, not up-to-the-microsecond data.

My favorite use case for read uncommited is to debug something that is happening inside a transaction.

Start your software under a debugger, while you are stepping through the lines of code, it opens a transaction and modifies your database. While the code is stopped, you can open a query analyzer, set on the read uncommited isolation level and make queries to see what is going on.

You also can use it to see if long running procedures are stuck or correctly updating your database.

It is great if your company loves to make overly complex stored procedures.

This can be useful to see the progress of long insert queries, make any rough estimates (like COUNT(*) or rough SUM(*)) etc.

In other words, the results the dirty read queries return are fine as long as you treat them as estimates and don't make any critical decisions based upon them.

This will give you dirty reads, and show you transactions that's not committed yet. That is the most obvious answer. I don't think its a good idea to use this just to speed up your reads. There is other ways of doing that if you use a good database design.

Its also interesting to note whats not happening. READ UNCOMMITTED does not only ignore other table locks. It's also not causing any locks in its own.

Consider you are generating a large report, or you are migrating data out of your database using a large and possibly complex SELECT statement. This will cause a shared lock that's may be escalated to a shared table lock for the duration of your transaction. Other transactions may read from the table, but updates are impossible. This may be a bad idea if its a production database since the production may stop completely.

If you are using READ UNCOMMITTED you will not set a shared lock on the table. You may get the result from some new transactions or you may not depending where it the table the data were inserted and how long your SELECT transaction have read. You may also get the same data twice if for example a page split occurs (the data will be copied to another location in the data file).

So, if its very important for you that data can be inserted while doing your SELECT, READ UNCOMMITTED may make sense. You have to consider that your report may contain some errors, but if its based on millions of rows and only a few of them are updated while selecting the result this may be "good enough". Your transaction may also fail all together since the uniqueness of a row may not be guaranteed.

A better way altogether may be to use SNAPSHOT ISOLATION LEVEL but your applications may need some adjustments to use this. One example of this is if your application takes an exclusive lock on a row to prevent others from reading it and go into edit mode in the UI. SNAPSHOT ISOLATION LEVEL does also come with a considerable performance penalty (especially on disk). But you may overcome that by throwing hardware on the problem. :)

You may also consider restoring a backup of the database to use for reporting or loading data into a data warehouse.

Use READ_UNCOMMITTED in situation where source is highly unlikely to change.

• When reading historical data. e.g some deployment logs that happened two days ago.
• When reading metadata again. e.g. metadata based application.

Don't use READ_UNCOMMITTED when you know souce may change during fetch operation.

The advantage is that it can be faster in some situations. The disadvantage is the result can be wrong (data which hasn't been committed yet could be returned) and there is no guarantee that the result is repeatable.

If you care about accuracy, don't use this.

More information is on MSDN:

Implements dirty read, or isolation level 0 locking, which means that no shared locks are issued and no exclusive locks are honored. When this option is set, it is possible to read uncommitted or dirty data; values in the data can be changed and rows can appear or disappear in the data set before the end of the transaction. This option has the same effect as setting NOLOCK on all tables in all SELECT statements in a transaction. This is the least restrictive of the four isolation levels.

It can be used for a simple table, for example in an insert-only audit table, where there is no update to existing row, and no fk to other table. The insert is a simple insert, which has no or little chance of rollback.

I always use READ UNCOMMITTED now. It's fast with the least issues. When using other isolations you will almost always come across some Blocking issues.

As long as you use Auto Increment fields and pay a little more attention to inserts then your fine, and you can say goodbye to blocking issues.

You can make errors with READ UNCOMMITED but to be honest, it is very easy make sure your inserts are full proof. Inserts/Updates which use the results from a select are only thing you need to watch out for. (Use READ COMMITTED here, or ensure that dirty reads aren't going to cause a problem)

So go the Dirty Reads (Specially for big reports), your software will run smoother...