Isolation levels in SQL Server define the degree to which one transaction must be isolated from resource or data modifications made by other transactions. SQL Server supports several isolation levels, each providing a different level of consistency, concurrency, and isolation. The isolation levels in SQL Server are defined by the SQL standard and include the following:
- READ UNCOMMITTED:
- Description: Allows a transaction to read
data that is being modified by another transaction without waiting for
the other transaction to complete.
- Issues: Non-repeatable reads, dirty
reads, and phantom reads are possible.
- READ COMMITTED:
- Description: Ensures that a transaction
reads only committed data. It prevents dirty reads but still allows
non-repeatable reads and phantom reads.
- Issues: Non-repeatable reads and
phantom reads are possible.
- REPEATABLE READ:
- Description: Ensures that if a transaction
reads a value, it will get the same value if it reads it again within the
same transaction. It prevents dirty reads and non-repeatable reads but
allows phantom reads.
- Issues: Phantom reads are possible.
- SNAPSHOT:
- Description: Allows a transaction to read a
version of data as it existed at the start of the transaction. This
provides a consistent snapshot of the data for the duration of the
transaction.
- Issues: Avoids dirty reads,
non-repeatable reads, and phantom reads.
- SERIALIZABLE:
- Description: Provides the highest level of
isolation. It ensures that transactions are completely isolated from one
another. It prevents dirty reads, non-repeatable reads, and phantom
reads.
- Issues: Increased contention and
potential for slower performance due to locks.
The isolation level can
be set for a session using the SET TRANSACTION ISOLATION
LEVEL statement. For example:
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
it's
important to note that selecting a higher isolation level typically comes with
an increased risk of performance issues, such as blocking and decreased
concurrency. Developers and database administrators need to carefully choose
the appropriate isolation level based on the requirements of the application
and the specific trade-offs they are willing to make in terms of consistency
and performance.
