Transaction Isolation Levels in MySQL0%

Transaction Isolation Levels in MySQL

Intermediate14 min read•Updated: 2026-09-12

Transaction Isolation Levels in MySQL

When thousands of concurrent transactions interact with the same database tables simultaneously, anomalies can occur. The SQL standard defines four Transaction Isolation Levels that balance data consistency against concurrency performance.

Understanding these levels and the concurrency phenomena they prevent is essential for database architects.

The Three Classic Concurrency Read Anomalies

1
Dirty Read: Transaction A modifies a row but has not committed yet. Transaction B reads this uncommitted row. If Transaction A rolls back, Transaction B acted on data that never officially existed!
2
Non-Repeatable Read (Fuzzy Read): Transaction A reads a row. Transaction B then updates or deletes that row and commits. Transaction A re-reads the same row and discovers different values.
3
Phantom Read: Transaction A queries a range of rows (e.g., WHERE salary > 50000). Transaction B inserts a new row matching that condition and commits. Transaction A re-runs the range query and discovers a new "phantom" row that wasn't there before.

The Four SQL Standard Isolation Levels

Isolation Level	Dirty Reads?	Non-Repeatable Reads?	Phantom Reads?	Performance
READ UNCOMMITTED	Permitted	Permitted	Permitted	Maximum
READ COMMITTED	Prevented	Permitted	Permitted	High
REPEATABLE READ (MySQL Default)	Prevented	Prevented	Prevented*	Moderate
SERIALIZABLE	Prevented	Prevented	Prevented	Slowest

*Note: In MySQL InnoDB, REPEATABLE READ also prevents phantom reads in consistent non-locking reads via MVCC (Multi-Version Concurrency Control) and Next-Key Locking!

Deep Dive into MySQL Defaults: REPEATABLE READ

In MySQL, REPEATABLE READ is the default isolation level.

Under REPEATABLE READ:

When Transaction A issues its first SELECT, InnoDB creates a Read View (a snapshot of the database at that exact moment in time).
Even if Transaction B modifies or inserts 10,000 rows and commits them, Transaction A continues seeing the exact snapshot established on its first query.
Consistency is guaranteed throughout Transaction A's entire lifecycle without locking read rows!

Inspecting and Setting Isolation Levels

1-- Check current global and session isolation levels in MySQL 8.0
2SELECT @@GLOBAL.transaction_isolation, @@SESSION.transaction_isolation;
3 
4-- Set isolation level for the next transaction in current session
5SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
6 
7-- Set isolation level for all future transactions in current session
8SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;
9 
10-- Set global isolation level for all future client connections
11SET GLOBAL TRANSACTION ISOLATION LEVEL REPEATABLE READ;

When to Change Isolation Levels?

Use READ COMMITTED: Common in high-throughput OLTP web applications (e.g., social feeds or analytics trackers) where non-repeatable reads are harmless and reducing lock contention is paramount. Many frameworks (like Ruby on Rails or Django with PostgreSQL) default to Read Committed.
Use REPEATABLE READ: Best for financial ledgers, billing calculations, and e-commerce inventory management where snapshot consistency across multiple queries is essential.
Use SERIALIZABLE: Used in ultra-strict compliance operations. It forces all plain SELECT queries to execute as SELECT ... FOR SHARE, heavily locking rows and serializing transactions sequentially at the cost of high deadlock probability.

Interactive Quiz• Knowledge Check

Multiple Choice Questions

Question 1

What is the default transaction isolation level in MySQL InnoDB?

Question 2

What is a "Dirty Read"?

Question 3

Which isolation level completely eliminates all read anomalies by converting standard SELECTs into locking reads?

Question 4

Which anomaly occurs when re-executing a range query returns newly inserted rows committed by another transaction?

Question 5

Which SQL command configures the isolation level for the current session?

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Creating & Managing MySQL Users

Transaction Isolation Levels in MySQL

Intermediate14 min read•Updated: 2026-09-12

Study Materials Cheatsheet

Transaction Isolation Levels in MySQL

Understanding these levels and the concurrency phenomena they prevent is essential for database architects.

The Three Classic Concurrency Read Anomalies

1
Dirty Read: Transaction A modifies a row but has not committed yet. Transaction B reads this uncommitted row. If Transaction A rolls back, Transaction B acted on data that never officially existed!
2
Non-Repeatable Read (Fuzzy Read): Transaction A reads a row. Transaction B then updates or deletes that row and commits. Transaction A re-reads the same row and discovers different values.
3
Phantom Read: Transaction A queries a range of rows (e.g., WHERE salary > 50000). Transaction B inserts a new row matching that condition and commits. Transaction A re-runs the range query and discovers a new "phantom" row that wasn't there before.

The Four SQL Standard Isolation Levels

Isolation Level	Dirty Reads?	Non-Repeatable Reads?	Phantom Reads?	Performance
READ UNCOMMITTED	Permitted	Permitted	Permitted	Maximum
READ COMMITTED	Prevented	Permitted	Permitted	High
REPEATABLE READ (MySQL Default)	Prevented	Prevented	Prevented*	Moderate
SERIALIZABLE	Prevented	Prevented	Prevented	Slowest

*Note: In MySQL InnoDB, REPEATABLE READ also prevents phantom reads in consistent non-locking reads via MVCC (Multi-Version Concurrency Control) and Next-Key Locking!

Deep Dive into MySQL Defaults: REPEATABLE READ

In MySQL, REPEATABLE READ is the default isolation level.

Under REPEATABLE READ:

When Transaction A issues its first SELECT, InnoDB creates a Read View (a snapshot of the database at that exact moment in time).
Even if Transaction B modifies or inserts 10,000 rows and commits them, Transaction A continues seeing the exact snapshot established on its first query.
Consistency is guaranteed throughout Transaction A's entire lifecycle without locking read rows!

Inspecting and Setting Isolation Levels

1-- Check current global and session isolation levels in MySQL 8.0
2SELECT @@GLOBAL.transaction_isolation, @@SESSION.transaction_isolation;
3 
4-- Set isolation level for the next transaction in current session
5SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
6 
7-- Set isolation level for all future transactions in current session
8SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;
9 
10-- Set global isolation level for all future client connections
11SET GLOBAL TRANSACTION ISOLATION LEVEL REPEATABLE READ;

When to Change Isolation Levels?

Use READ COMMITTED: Common in high-throughput OLTP web applications (e.g., social feeds or analytics trackers) where non-repeatable reads are harmless and reducing lock contention is paramount. Many frameworks (like Ruby on Rails or Django with PostgreSQL) default to Read Committed.
Use REPEATABLE READ: Best for financial ledgers, billing calculations, and e-commerce inventory management where snapshot consistency across multiple queries is essential.
Use SERIALIZABLE: Used in ultra-strict compliance operations. It forces all plain SELECT queries to execute as SELECT ... FOR SHARE, heavily locking rows and serializing transactions sequentially at the cost of high deadlock probability.

Interactive Quiz• Knowledge Check

Multiple Choice Questions

Question 1

What is the default transaction isolation level in MySQL InnoDB?

Question 2

What is a "Dirty Read"?

Question 3

Which isolation level completely eliminates all read anomalies by converting standard SELECTs into locking reads?

Question 4

Which anomaly occurs when re-executing a range query returns newly inserted rows committed by another transaction?

Question 5

Which SQL command configures the isolation level for the current session?

Next Lesson

Creating & Managing MySQL Users

Continue learning with hands-on practice, examples, and exercises in the upcoming topic.

Related Lessons

Previous Lesson	Next Lesson
Using SAVEPOINT & ROLLBACK TO SAVEPOINT	Creating & Managing MySQL Users

Practice Quiz

Test your understanding of this lesson with 5 questions. Each question has one correct answer.

Previous Next