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SQL Server Isolation Levels: A Deep Dive into Transaction Isolation and Concurrency Control: Question and Answer

Q.1. What are Isolation Levels?

Ans: The isolation behavior of a transaction can be adjusted to specific requirements by setting the isolation level. Isolation levels govern how concurrent transactions interact. They determine whether transactions obstruct each other, allow each other to proceed, or present a snapshot of a previously stable data state in case of conflicts.

Q.2. How to Set an Isolation Level?

Ans: The SET TRANSACTION ISOLATION LEVEL statement can be used to set isolation levels with the following syntax:

SET TRANSACTION ISOLATION LEVEL 
{ 
    READ UNCOMMITTED 
    | READ COMMITTED 
    | REPEATABLE READ 
    | SNAPSHOT 
    | SERIALIZABLE 
}

This statement is used with the BEGIN TRANSACTION statement in the following example:

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
BEGIN TRANSACTION 
SELECT TestColumn FROM TestTable
COMMIT

Q.3. What Isolation Levels are Supported by SQL Server?

Ans: SQL Server supports the following five isolation levels:

  1. Read Uncommitted
  2. Read Committed
  3. Repeatable Read
  4. Snapshot
  5. Serializable

Q.4. What is The Read Uncommitted Isolation Level?

Ans: The Read Uncommitted isolation level in a database allows you to access data without adhering to any locks, providing immediate access to the current state of the data. However, this approach comes with a risk of encountering a "dirty read," which means you might read data that has not been committed yet. If your application requires accurate and committed data, it is advisable to avoid using this isolation level. Transactions using Read Uncommitted may return logically incorrect data.

Are you curious to know if other transactions can read, delete, update, or insert new data within the range of data once a transaction has read it? To delve deeper into this topic, you can consider enrolling in an online SQL certification course.

Q.5. Implementation of The Read Uncommitted Isolation Level - An Example?

Ans: Let's examine the Read Uncommitted isolation level using an example.

CREATE DATABASE MyDB GO
USE MYDB
GO
CREATE TABLE TestTable
(
TestID INT IDENTITY PRIMARY KEY,
TestColumn INT
>
INSERT INTO TestTable(TestColumn) VALUES (100)

To follow along, perform the following steps:

  1. Execute the script to create the TestTable in the MyDB database using either SSMS or SSDT.

  2. Open two query windows, each connected to the MyDB database containing the TestTable created in step 1. These two instances will simulate two concurrent transactions by two users.

  3. In example 1, run the following code block to perform an UPDATE on a row of data:

BEGIN TRANSACTION UPDATE TestTable SET TestColumn = 200 WHERE TestId = 1
  1. In example 2, run the following query:
SELECT TestColumn FROM TestTable WHERE TestId = 1

You'll notice that your SELECT query is blocked because you are attempting to read the same data that instance 1 is working with. Until instance 1 issues a COMMIT or a ROLLBACK, your query will remain blocked or time out.

  1. To cancel the blocked SELECT query, press Alt+Break or click the Cancel button on the toolbar. Then, on the connection held by instance 2, execute the following statement to set the isolation level of your SELECT query to Read Uncommitted:
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
  1. Re-run the SELECT query as follows:
SELECT TestColumn FROM TestTable WHERE TestId = 1

You'll notice that the query isn't blocked, and it returns 200.

  1. Go back to instance 1 and perform a ROLLBACK.

  2. Return to instance 2 and run the same SELECT query again. This time, you should get 100.

    As you may have noticed, instance 2 returned different results for the same query at various points in time. The value 200 was never committed to the database, but because you explicitly requested a dirty read by specifying the READ UNCOMMITTED isolation level, you read data that was never intended to be final. As a result, you ended up reading logically incorrect data. On the plus side, your query was not blocked.

Q.6. What is The Read Committed Isolation Level?

Ans: The Read Committed isolation level is the default setting in SQL Server. This choice is because read committed offers a well-balanced compromise between data integrity and performance. With this isolation level, locks are respected, and dirty reads are prevented. In the previous example, the connection operated under the read committed isolation level until you explicitly changed it to read uncommitted. As a result, the second transaction (the autocommit mode transaction in the SELECT query) got blocked by the transaction executing the UPDATE query.

Q.7. When do Phantom Read and Nonrepeatable Read Occur in The Read Committed Isolation Level?

Ans: Although the read committed isolation level prevents dirty reads, it still allows for the occurrence of phantom reads and nonrepeatable reads. This is because read committed does not prevent one transaction from modifying the same data while another transaction is reading from it.

A phantom read can occur in the following scenarios:

  1. Transaction 1 begins.
  2. Transaction 1 reads a row.
  3. Transaction 2 begins.
  4. Transaction 2 deletes the row that was read by transaction 1.
  5. Transaction 2 is completed. Since the row no longer exists, Transaction 1 cannot repeat its initial read, resulting in a phantom row.

A nonrepeatable read can occur in the following circumstances:

  1. The first transaction begins.
  2. Transaction 1 reads a row of data.
  3. The second transaction begins.
  4. Transaction 2 modifies the value of the same row that transaction 1 read.
  5. Transaction 2 is completed.
  6. Transaction 1 reads the row once more. Transaction 1 contains inconsistent data because the row now contains values that differ from the previous read, all within the scope of transaction 1.

Q.8. What is The Repeatable Read Isolation Level?

Ans: As the name implies, the repeatable read isolation level is designed to prevent nonrepeatable reads. It achieves this by placing locks on the data used in a query within a transaction. However, it's important to note that this level comes with a higher cost as concurrent transactions may end up blocking each other. Therefore, it is recommended to use this isolation level only when necessary.

The advantage is that a concurrent transaction could add new data that matches the original transaction's WHERE clause. This is possible because the first transaction will only lock the rows it reads into its result set. In other words, a transaction using this isolation level acquires read locks on all retrieved data but not range locks.

To gain further understanding of these isolation levels, consider exploring various SQL certification courses available.

Q.9. How do Phantom Reads Occur in Repeatable Read Isolation Levels?

Ans: If you examine closely, you'll notice that while nonrepeatable reads are avoided when using the repeatable read isolation level, phantom reads can still occur. They may arise under the following conditions:

  1. The first transaction begins.
  2. Transaction 1 reads all rows with TestColumn = 100, for example.
  3. The second transaction begins.
  4. Transaction 2 creates a new row with the parameters TestID = 2, TestColumn = 100.
  5. Transaction 2 is completed.
  6. Transaction 1 executes an UPDATE query and modifies TestColumn for all rows with TestColumn equal to 100. This action also updates the row inserted by transaction 2.
  7. Transaction 1 is completed.

As shared locks are not released until the transaction is completed, concurrency is lower compared to utilizing the read committed isolation level. Therefore, exercising caution is essential to avoid unexpected results.

Q.10. What is The Serializable Isolation Level?

Ans: At the Serializable isolation level, transactions are not allowed to have dirty, phantom, or nonrepeatable reads. This strict isolation level imposes the most stringent locks on the data being read or modified, ensuring complete data integrity. It may seem like the perfect isolation level due to its robust data consistency.

However, there is a reason why you should rarely use it. This isolation level can lead to blocking of other running transactions, resulting in decreased concurrent performance and potentially causing deadlocks. Although it ensures data integrity, it significantly impacts system performance.

In most practical situations, a lower level of isolation is sufficient and more appropriate. Understanding these isolation levels can be achieved by taking online SQL courses and learning more about them.

Conclusion

Isolation levels play a crucial role in determining how concurrent transactions interact within a database system. These levels, including Read Uncommitted, Read Committed, Repeatable Read, Snapshot, and Serializable, provide varying degrees of data consistency and concurrency control.Setting the appropriate isolation level is a critical decision that requires careful consideration of the trade-offs between data integrity and system performance. While Read Uncommitted offers immediate access to data without locks, it carries the risk of dirty reads. Read Committed strikes a balance between performance and data integrity by preventing dirty reads, but it allows for phantom and nonrepeatable reads. Repeatable Read mitigates nonrepeatable reads but can lead to higher levels of blocking. Serializable ensures the highest data integrity but can cause significant performance issues due to increased locking.

Understanding the implications of each isolation level is essential for designing robust and efficient database systems. Organizations and developers should carefully assess their application's requirements and choose the isolation level that best aligns with their needs.To gain a deeper understanding of isolation levels and their impact, individuals can explore SQL certification courses or other educational resources in the field of database management. By doing so, they can make informed decisions when designing and implementing database solutions that balance data consistency and performance.

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