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.
