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What are the transaction isolation levels in MySQL, and which is the default?
What are the transaction isolation levels in MySQL, and which is the default?
Jun 23, 2025 pm 03:05 PM
MySQL's default transaction isolation level is Repeatable Read, which prevents dirty reads and non-repeatable reads through MVCC and gap locks, and avoids phantom reading in most cases; other major levels include Read Uncommitted, allowing dirty reads but the fastest performance, 1. Read Committed ensures that the submitted data is read but may encounter non-repeatable reads and phantom readings, 2. Repeatable Read default level, ensuring the consistency of multiple reads within the transaction, 3. Serialization is the highest level, preventing other transactions from modifying data through locks, ensuring data integrity but sacrificing performance; SELECT @@tx_isolation; and SET SESSION TRANSACTION ISOLATION LEVEL to view or set isolation levels.
MySQL supports several transaction isolation levels, which determine how transactions interact with each other and how data consistency is maintained. The default isolation level in MySQL is REPEATABLE READ .
Here's a breakdown of the main isolation levels you might encounter:
Read Uncommitted – Lowest Isolation
This level allows one transaction to read uncommitted changes made by another transaction. It can lead to "dirty reads," where data that hasn't been finalized (and might be rolled back) is read.
- Not commonly used in practice due to risk of inconsistency
- Fastest performance-wise, but least safe
Example:
If Transaction A updates a row but hasn't committed yet, Transaction B can see that update immediately—even if it gets rolled back later.
Read Committed – Avoids Dirty Reads
This level ensures that any data read during a transaction is committed at the moment it is read.
- Prevents dirty reads
- May still allow non-repeatable reads and phantom reads
In some databases like Oracle, this is the default, but not in MySQL.
Use case:
When you want to make sure you're only reading committed data, but don't need strict consistency across the entire transaction.
Repeatable Read – Default in MySQL
At this level, MySQL ensures that if a transaction reads the same data multiple times, it will see the same values ??throughout the transaction.
- Prevents dirty reads and non-repeatable reads
- Also prevents most phantom reads (though technically possible in some edge cases)
- Uses multi-version concurrency control (MVCC) and next-key locks to achieve this
Because it's the default, you'll often find it being used without explicit configuration.
Common behavior:
- If two transactions try to modify the same rows, one will wait or fail depending on locking
- Consistent reads within a transaction use a snapshot of the database
Serializable – Highest Isolation
This is the strictest isolation level. It places a lock on the rows being read, preventing other transactions from modifying them until the current transaction completes.
- Prevents dirty reads, non-repeatable reads, and phantom reads
- Can significantly reduce performance due to increased locking
It's typically used when absolute data integrity is required and performance isn't a top priority.
Summary of Isolation Levels and Behavior
| Isolation Level | Dirty Reads | Non-Repeatable Reads | Phantom Reads |
|---|---|---|---|
| Read Uncommitted | Possible | Possible | Possible |
| Read Committed | No | Possible | Possible |
| Repeatable Read | No | No | Rare* |
| Serializable | No | No | No |
*MySQL uses next-key locking in Repeatable Read to prevent most phantom reads.
You can check or change the isolation level using SQL commands like:
SELECT @@tx_isolation; SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
Just keep in mind that changing the isolation level affects how your transactions behave under load and concurrency.
Basically that's it.
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