Isolation levels in MySQL

Introduction

Isolation levels control how much one transaction is separated from others. This helps keep data correct when many people use the database at the same time.

When you want to avoid errors caused by multiple people changing data at once.

When you need to balance speed and accuracy in your database.

When you want to prevent reading data that is not yet final.

When you want to control how transactions see changes made by others.

When you want to avoid problems like dirty reads or lost updates.

Syntax

MySQL

SET TRANSACTION ISOLATION LEVEL level_name;

Replace level_name with one of the standard levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE.

This command sets the isolation level for the current session or next transaction.

Examples

This level allows reading uncommitted changes from other transactions (fast but risky).

MySQL

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;

This level only reads data that has been committed by others (common default).

MySQL

SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

This level ensures that if you read data twice in the same transaction, it stays the same.

MySQL

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;

This is the strictest level, making transactions appear one after another to avoid conflicts.

MySQL

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Sample Program

This example starts a transaction, sets the isolation level to REPEATABLE READ, checks the current isolation level, then commits.

MySQL

START TRANSACTION;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SELECT @@transaction_isolation;
COMMIT;

Important Notes

Higher isolation levels reduce errors but can slow down the database.

MySQL default isolation level is REPEATABLE READ.

Changing isolation levels affects only the current session or transaction unless set globally.

Summary

Isolation levels control how transactions see and affect data.

Four main levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE.

Choose the level based on your need for speed versus data accuracy.

Practice

1. Which isolation level allows a transaction to read data that has been modified by other transactions but not yet committed?

easy

A. REPEATABLE READ

B. READ COMMITTED

C. READ UNCOMMITTED

D. SERIALIZABLE

Solution

1. Step 1: Understand isolation levels and dirty reads

   READ UNCOMMITTED allows reading uncommitted changes, causing dirty reads.

2. Step 2: Compare with other levels

   Other levels prevent dirty reads by requiring commits before reading.

3. Final Answer:

   READ UNCOMMITTED -> Option C

4. Quick Check:

   Dirty reads allowed = READ UNCOMMITTED [OK]

Hint: Dirty reads happen only in READ UNCOMMITTED [OK]

Common Mistakes:

• Confusing READ COMMITTED with allowing dirty reads
• Thinking SERIALIZABLE allows dirty reads
• Assuming REPEATABLE READ reads uncommitted data

2. Which of the following is the correct syntax to set the transaction isolation level to SERIALIZABLE in MySQL?

easy

A. SET ISOLATION SERIALIZABLE;

B. SET ISOLATION LEVEL = SERIALIZABLE;

C. SET TRANSACTION LEVEL SERIALIZABLE;

D. SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Solution

1. Step 1: Recall correct syntax for setting isolation level

   The correct syntax is: SET TRANSACTION ISOLATION LEVEL followed by the level.

2. Step 2: Check each option

   Only SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; matches the correct syntax exactly.

3. Final Answer:

   SET TRANSACTION ISOLATION LEVEL SERIALIZABLE; -> Option D

4. Quick Check:

   Correct syntax starts with SET TRANSACTION ISOLATION LEVEL [OK]

Hint: Use full phrase: SET TRANSACTION ISOLATION LEVEL [OK]

Common Mistakes:

• Omitting TRANSACTION keyword
• Using = sign incorrectly
• Shortening syntax incorrectly

3. Consider two transactions T1 and T2 running concurrently. T1 reads a row, then T2 updates and commits that row. If the isolation level is REPEATABLE READ, what will T1 see when it reads the same row again?

medium

A. The updated value from T2

B. The original value before T2's update

C. An error due to conflict

D. A partially updated value

Solution

1. Step 1: Understand REPEATABLE READ behavior

   REPEATABLE READ ensures that repeated reads in the same transaction see the same data snapshot.

2. Step 2: Apply to scenario

   T1 will see the original value it read first, ignoring T2's committed update during T1's transaction.

3. Final Answer:

   The original value before T2's update -> Option B

4. Quick Check:

   REPEATABLE READ = consistent snapshot reads [OK]

Hint: REPEATABLE READ keeps data snapshot stable during transaction [OK]

Common Mistakes:

• Assuming T1 sees T2's committed update immediately
• Confusing with READ COMMITTED behavior
• Thinking it causes errors on concurrent updates

4. You set the isolation level to READ COMMITTED but notice phantom reads occurring. What is the likely cause?

medium

A. READ COMMITTED does not prevent phantom reads

B. The isolation level was not set correctly

C. Phantom reads only occur in READ UNCOMMITTED

D. The database engine does not support transactions

Solution

1. Step 1: Recall what phantom reads are

   Phantom reads happen when new rows appear in repeated queries within a transaction.

2. Step 2: Understand READ COMMITTED limits

   READ COMMITTED prevents dirty reads but does not prevent phantom reads.

3. Final Answer:

   READ COMMITTED does not prevent phantom reads -> Option A

4. Quick Check:

   Phantom reads allowed in READ COMMITTED [OK]

Hint: Phantom reads happen above READ COMMITTED level [OK]

Common Mistakes:

• Assuming READ COMMITTED blocks phantom reads
• Blaming incorrect isolation level setting
• Ignoring database engine capabilities

5. You want to ensure no dirty reads, no non-repeatable reads, and no phantom reads in a high-concurrency environment. Which isolation level should you choose and why?

hard

A. SERIALIZABLE, because it prevents all concurrency anomalies

B. READ UNCOMMITTED, because it allows fastest access

C. READ COMMITTED, because it balances speed and accuracy

D. REPEATABLE READ, because it prevents dirty and phantom reads

Solution

1. Step 1: Identify isolation level requirements

   No dirty reads, no non-repeatable reads, and no phantom reads means highest isolation.

2. Step 2: Match requirements to isolation levels

   SERIALIZABLE is the strictest level preventing all these anomalies by serializing transactions.

3. Step 3: Compare with other levels

   READ COMMITTED and REPEATABLE READ allow some anomalies; READ UNCOMMITTED allows dirty reads.

4. Final Answer:

   SERIALIZABLE, because it prevents all concurrency anomalies -> Option A

5. Quick Check:

   SERIALIZABLE = strictest isolation, no anomalies [OK]

Hint: Choose SERIALIZABLE for full isolation and no anomalies [OK]

Common Mistakes:

• Choosing REPEATABLE READ and ignoring phantom reads
• Picking READ COMMITTED for full isolation
• Confusing speed with isolation guarantees