What are the transaction isolation levels in MariaDB and how to choose the appropriate one?

Transaction isolation levels in MariaDB determine visibility between transactions and concurrency control behavior. There are four isolation levels:

1. READ UNCOMMITTED

Features:

• Can read uncommitted data from other transactions
• May experience dirty reads, non-repeatable reads, phantom reads
• Best performance but worst data consistency

Use cases: Almost never used, only for special performance requirements

2. READ COMMITTED

Features:

• Can only read committed data from other transactions
• Avoids dirty reads but may experience non-repeatable reads, phantom reads
• Default isolation level for Oracle, PostgreSQL

Use cases: Most business scenarios, balances performance and consistency

3. REPEATABLE READ

Features:

• Ensures consistent results when reading the same data multiple times in the same transaction
• Avoids dirty reads, non-repeatable reads but may experience phantom reads
• MariaDB's default isolation level
• Implemented through MVCC (Multi-Version Concurrency Control)

Use cases: Applications requiring higher data consistency

4. SERIALIZABLE

Features:

• Highest isolation level, completely isolates transactions
• Avoids all concurrency issues (dirty reads, non-repeatable reads, phantom reads)
• Worst performance, may cause lock contention

Use cases: Financial transactions, inventory management, scenarios requiring extremely high consistency

Setting Isolation Levels

sql
-- View current isolation level
SELECT @@tx_isolation;

-- Set global isolation level
SET GLOBAL tx_isolation = 'READ-COMMITTED';

-- Set session isolation level
SET SESSION tx_isolation = 'REPEATABLE-READ';

-- Set within a transaction
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
START TRANSACTION;

Concurrency Issues Explained

• Dirty Read: Reading uncommitted data from other transactions
• Non-Repeatable Read: Getting different results when reading the same data multiple times in the same transaction
• Phantom Read: Getting different numbers of records when querying multiple times in the same transaction

Selection Recommendations

1. Default to REPEATABLE READ: MariaDB's default choice, suitable for most scenarios
2. READ COMMITTED: Suitable for read-heavy, low consistency requirement scenarios
3. SERIALIZABLE: Only for critical business requiring extremely high consistency
4. Avoid READ UNCOMMITTED: Unless there are special performance requirements

Choosing the appropriate isolation level helps find the best balance between performance and data consistency.