MySQL Table Locking

Last Updated: 02 July, 2023

In MySQL, table locking is a mechanism used to control concurrent access to database tables. When multiple transactions or queries are executed simultaneously, table locking ensures that only one transaction can modify a specific table at a time to maintain data integrity.

MySQL supports various types of table locking, which can be broadly categorized into two main types:

1. Table-level locking
2. Row-level locking

Table-level locking

In this type of locking, the entire table is locked for exclusive access, meaning only one user or process can modify the table at a time while others have to wait. Table-level locking can be useful when certain operations require consistency or when we want to prevent potential race conditions.

• READ Lock: A READ lock allows multiple sessions to read from the table simultaneously but prevents any session from acquiring a WRITE lock (for modifications) until all READ locks are released.
• WRITE Lock: A WRITE lock gives exclusive access to a single session, allowing it to modify the table. No other session can acquire any type of lock (read or write) until the WRITE lock is released.

Row-level locking:

This type of locking is more granular, where individual rows in a table are locked rather than the entire table. Row-level locking allows multiple sessions to access different rows simultaneously, improving concurrency and reducing contention. MySQL's default storage engine, InnoDB, supports row-level locking. Other storage engines, like MyISAM, primarily support table-level locking.

Locking can be managed explicitly by the application or can be automatic based on the transaction isolation level. The transaction isolation level determines the visibility of changes made by one transaction to other transactions. MySQL provides four standard isolation levels:

1. READ UNCOMMITTED: Lowest level, allows dirty reads, non-repeatable reads, and phantom reads.
2. READ COMMITTED: Prevents dirty reads but allows non-repeatable reads and phantom reads.
3. REPEATABLE READ: Prevents dirty reads and non-repeatable reads but allows phantom reads.
4. SERIALIZABLE: Highest level, prevents dirty reads, non-repeatable reads, and phantom reads by applying shared and exclusive locks.

By default, MySQL uses the REPEATABLE READ isolation level.

It's essential to use locking judiciously, as overusing locks can lead to performance issues and contention among users. Careful design of the application and proper understanding of the concurrency requirements are vital to strike a balance between data consistency and system performance.