Different Isolation levels in sql server

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Introduction:

In my previous article I have explained about  how to Get system name using sql or c#.net. In  this article I will explain about different transaction levels in sql server. And which one we should use for better performance.

Explanation:

Transaction isolation level Controls the default transaction locking behavior for all Microsoft SQL Server SELECT statements issued by a connection.
Different transaction isolation levels are
·         READ COMMITTED
·         READ UNCOMMITTED
·         REPEATABLE READ
·         SERIALIZABLE
·         SNAPSHOT



READ COMMITTED


Specifies that shared locks are held while the data is being read to avoid dirty reads, but the data can be changed before the end of the transaction, resulting in nonrepeatable reads or phantom data. This option is the SQL Server default.

READ UNCOMMITTED


Implements dirty read, or isolation level 0 locking, which means that no shared locks are issued and no exclusive locks are honored. When this option is set, it is possible to read uncommitted or dirty data; values in the data can be changed and rows can appear or disappear in the data set before the end of the transaction. This option has the same effect as setting NOLOCK on all tables in all SELECT statements in a transaction. This is the least restrictive of the four isolation levels.

REPEATABLE READ


Locks are placed on all data that is used in a query, preventing other users from updating the data, but new phantom rows can be inserted into the data set by another user and are included in later reads in the current transaction. Because concurrency is lower than the default isolation level, use this option only when necessary.

SERIALIZABLE


Places a range lock on the data set, preventing other users from updating or inserting rows into the data set until the transaction is complete. This is the most restrictive of the four isolation levels. Because concurrency is lower, use this option only when necessary. This option has the same effect as setting HOLDLOCK on all tables in all SELECT statements in a transaction.

SNAPSHOT

Specifies that data read by any statement in a transaction will be the transactionally consistent version of the data that existed at the start of the transaction. The transaction can only recognize data modifications that were committed before the start of the transaction. Data modifications made by other transactions after the start of the current transaction are not visible to statements executing in the current transaction. The effect is as if the statements in a transaction get a snapshot of the committed data as it existed at the start of the transaction.
These isolation levels allow different types of behavior.

Isolation level
Dirty read
Nonrepeatable read
Phantom
Read uncommitted
Yes
Yes
Yes
Read committed
No
Yes
Yes
Repeatable read
No
No
Yes
Serializable
No
No
No

Transactions must be run at an isolation level of repeatable read or higher to prevent lost updates that can occur when two transactions each retrieve the same row, and then later update the row based on the originally retrieved values. If the two transactions update rows using a single UPDATE statement and do not base the update on the previously retrieved values, lost updates cannot occur at the default isolation level of read committed.
Remarks
Only one of the options can be set at a time, and it remains set for that connection until it is explicitly changed. This becomes the default behavior unless an optimization option is specified at the table level in the FROM clause of the statement.
The setting of SET TRANSACTION ISOLATION LEVEL is set at execute or run time and not at parse time.
Syntax

SET TRANSACTION ISOLATION LEVEL
    { READ COMMITTED
        | READ UNCOMMITTED
        | REPEATABLE READ
        | SERIALIZABLE
    }
   
Examples
This example sets the TRANSACTION ISOLATION LEVEL for the session. For each Transact-SQL statement that follows, SQL Server holds all of the shared locks until the end of the transaction.
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
GO
BEGIN TRANSACTION
SELECT * FROM publishers
SELECT * FROM authors
...
COMMIT TRANSACTION

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