In SQL Server, the isolation level is a property that determines how transactions are isolated from each other, and how they access and modify data in the database.
SQL Server supports four different isolation levels, which are:
- READ UNCOMMITTED: This is the lowest isolation level, and it allows transactions to read data that has been modified by other transactions but not yet committed. This can result in dirty reads, where a transaction reads data that is later rolled back by the other transaction.
- READ COMMITTED: This is a slightly higher isolation level, and it allows transactions to read only committed data. This prevents dirty reads, but it can still result in non-repeatable reads and phantom reads, where a transaction reads data that is later modified by another transaction.
- REPEATABLE READ: This is a higher isolation level, and it prevents non-repeatable reads and phantom reads by ensuring that data read by a transaction cannot be modified by other transactions until the transaction completes. However, it can still result in phantom reads if new rows are inserted into the database by other transactions.
- SERIALIZABLE: This is the highest isolation level, and it prevents all types of concurrent access anomalies by ensuring that transactions are executed in a completely serializable order. This guarantees full isolation and consistency, but it can reduce concurrency and performance.
Getting the SQL isolation level right can make all the difference in the world when it comes to peformance.
Here is a short video on SQL Isolation levels from our Podcast.
The isolation level can be set at the session level or the transaction level, and it can be changed using the SET TRANSACTION ISOLATION LEVEL statement. The appropriate isolation level to use depends on the specific requirements of the application and the trade-offs between isolation and concurrency.
More details at DatabaseHealth.com
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Steve, your resources are always so helpful. In this one, though, shouldn’t you have at least made some mention of SNAPSHOT as another isolation level? Granted, it’s not an *ANSI* isolation level, but SQL Server does support it: https://learn.microsoft.com/en-us/sql/t-sql/statements/set-transaction-isolation-level-transact-sql
Sure, snapshot isolation does have its pros and cons, and its related features (like RCSI) and even underlying implementation have changed since its introduction (especially SQL 2019 and ADR), but it has been a great solution for some problems so just seems worth at least a mention in a post listing supported SQL isolation levels. :-)
Or I’m sure many readers would be just as interested if you may want to share a bit on why you might still specifically prefer NOT to list it as an option here. As always, just trying to help.