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SQL Server 2008 supports ve isolation levels that control the behavior of your read operations. Three of them are available only with pessimistic concurrency, one is available only with optimistic concurrency, and one is available with either. We look at these levels now, but a complete understanding of isolation levels also requires an understanding of locking and row versioning. In my descriptions of the isolation levels, I mention the locks or row versions that support that level, but keep in mind that locking and row versioning are discussed in detail later in the chapter.
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In Read Uncommitted isolation, all the behaviors described previously, except lost updates, are possible. Your queries can read uncommitted data, and both nonrepeatable reads and phantoms are possible. Read Uncommitted isolation is implemented by allowing your read operations to not take any locks, and because SQL Server isn t trying to acquire locks, it won t be blocked by con icting locks acquired by other processes. Your process is able to read data that another process has modi ed but not yet committed. In addition to reading individual values that are not yet committed, the Read Uncommitted isolation level introduces other undesirable behaviors. When using this isolation level and scanning an entire table, SQL Server can decide to do an allocation order scan (in page-number order), instead of a logical order scan (which would follow the page pointers). If there are concurrent operations by other processes that change data and move rows to a new location in the table, your allocation order scan can end up reading the same row twice. This can happen when you ve read a row before it is updated, and then the update moves the row to a higher page number than your scan encounters later. In addition, performing an
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allocation order scan under Read Uncommitted can cause you to miss a row completely. This can happen when a row on a high page number that hasn t been read yet is updated and moved to a lower page number that has already been read. Although this scenario isn t usually the ideal option, with Read Uncommitted, you can t get stuck waiting for a lock, and your read operations don t acquire any locks that might affect other processes that are reading or writing data. When using Read Uncommitted, you give up the assurance of strongly consistent data in favor of high concurrency in the system without users locking each other out. So when should you choose Read Uncommitted Clearly, you don t want to use it for nancial transactions in which every number must balance. But it might be ne for certain decision-support analyses for example, when you look at sales trends for which complete precision isn t necessary and the tradeoff in higher concurrency makes it worthwhile. Read Uncommitted isolation is a pessimistic solution to the problem of too much blocking activity because it just ignores the locks and does not provide you with transactional consistency.
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SQL Server 2008 supports two varieties of Read Committed isolation, which is the default isolation level. This isolation level can be either optimistic or pessimistic, depending on the database setting READ_COMMITTED_SNAPSHOT. Because the default for the database option is off, the default for this isolation level is to use pessimistic concurrency control. Unless indicated otherwise, when I refer to the Read Committed isolation level, I am referring to both variations of this isolation level. I refer to the pessimistic implementation as Read Committed (locking), and I refer to the optimistic implementation as Read Committed (snapshot). Read Committed isolation ensures that an operation never reads data that another application has changed but not yet committed. (That is, it never reads data that logically never existed.) With Read Committed (locking), if another transaction is updating data and consequently has exclusive locks on data rows, your transaction must wait for those locks to be released before you can use that data (whether you re reading or modifying). Also, your transaction must put share locks (at a minimum) on the data that are visited, which means that data might be unavailable to others to use. A share lock doesn t prevent others from reading the data, but it makes them wait to update the data. By default, share locks can be released after the data has been processed they don t have to be held for the duration of the transaction, or even for the duration of the statement. (That is, if shared row locks are acquired, each row lock can be released as soon as the row is processed, even though the statement might need to process many more rows.) Read Committed (snapshot) also ensures that an operation never reads uncommitted data, but not by forcing other processes to wait. In Read Committed (snapshot), every time a row is updated, SQL Server generates a version of the changed row with its previous committed values. The data being changed is still locked, but other processes can see the previous versions of the data as it was before the update operation began.
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