Wednesday, April 6, 2011

Isolation Levels in SQL Server 2005

The ISO standard defines the following isolation levels, all of which are supported by the SQL Server Database Engine:

1.    Read uncommitted
2.    Read committed
3.    Repeatable read
4.    Searializable Read
5.    Read committed snapshot
6.    Snapshot

1. Read uncommitted is the lowest level where transactions are isolated only enough to ensure that physically corrupt data is not read.

2. Read committed is database Engine default level, this allows reading only committed data but it can’t obtain and hold lock hence causes issues like Lost Updates, Non-repeatable reads and Phantom reads.

3. Repeatable read: This allows reading only committed date and also it can obtain and hold locks till the transactions are completed. But it cannot avoid phantom reads as it cannot manage range locks.

4. Serializable Snapshot is the highest level, where transactions are completely isolated from one another.

5. Snapshot: It works using ROW Versioning. So when any-row update happening a version of already committed row is kept in tempdb and in between the update if any select comand fires for the same row the data will-be shown of the row version in temdb. Until the update transaction is done sql will-use the row version already in tempdb. This causes performance problems when situations raise like too-many row versions are stored in tempdb as too many updates are happening on same row. So this isolation can be used on less frequently updated tables.
 
6. Read Committed Snapshot: It also works on ROW Versioning but it also avoids performance problems raise in snapshot isolation. This takes snapshot of the row version for every statement execution in a transaction but snapshot isolation takes snapshot at transaction level means it maintains the older row version until the entire transaction is completed.


The following statements are not allowed within a transaction that is running under snpshot isolation.
1.    CREATE INDEX
2.    CREATE XML INDEX
3.    ALTER INDEX
4.    ALTER TABLE
5.    DBCC DBREINDEX
6.    ALTER PARTITION FUNCTION
7.    ALTER PARTITION SCHEME
8.    DROP INDEX
9.    Common language runtime (CLR) DDL



USE Adventureworks;
GO
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
GO
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