MySQL SET TRANSACTION Statements

The `SET TRANSACTION` statement in MySQL is used to specify the transaction characteristics for the current transaction. It allows you to define the isolation level and access mode for transactions to ensure data integrity and consistency.

Usage

The `SET TRANSACTION` statement is used before starting a transaction to control its behavior, particularly its isolation level and read/write permissions. This is crucial when handling concurrent transactions to prevent anomalies.

sql
SET TRANSACTION [READ WRITE | READ ONLY]
[ISOLATION LEVEL {READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE}];

In this syntax, you specify the transaction mode (`READ WRITE` or `READ ONLY`) and the isolation level to control how changes made by concurrent transactions are visible. Note that `SET TRANSACTION` affects only the next transaction started within the session and does not alter the session or global defaults unless `SET SESSION` or `SET GLOBAL` is used.

Examples

1. Setting Read-Only Transaction

sql
SET TRANSACTION READ ONLY;

This sets the current transaction to be read-only, ensuring that no data modifications can occur until the transaction is committed or rolled back.

2. Setting Isolation Level to Read Committed

sql
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;

This sets the isolation level to `READ COMMITTED`, allowing each transaction to see only the data committed before the query's start, thus preventing dirty reads. It provides a balance between consistency and performance.

3. Setting Read-Write with Serializable Isolation Level

sql
SET TRANSACTION READ WRITE
ISOLATION LEVEL SERIALIZABLE;

This example sets the transaction to `READ WRITE` mode with a `SERIALIZABLE` isolation level, ensuring full transaction isolation by preventing phantoms, dirty reads, and non-repeatable reads. This is the strictest isolation level, providing maximum data integrity at the cost of performance.

Tips and Best Practices

• Set characteristics early. Always set the transaction characteristics before starting the transaction to avoid unintended behavior.
• Understand isolation levels. Choose the correct isolation level based on your application's concurrency requirements and tolerance for phenomena like dirty reads (uncommitted data), non-repeatable reads (data inconsistency between reads), or phantom reads (new rows appearing).
• Limit read-write transactions. Use `READ WRITE` mode only when necessary to minimize the potential for conflicts and increase performance.
• Consider default settings. Be aware of MySQL's default transaction isolation level (`REPEATABLE READ`) and adjust only when necessary for specific use cases.
• Scope Awareness. Remember that `SET TRANSACTION` only applies to the next transaction and doesn't change session or global settings unless explicitly specified.
• Error Handling. Ensure compatibility of characteristics, as setting conflicting options may result in errors. Handle such errors by reviewing transaction requirements or consulting MySQL logs for troubleshooting.