MySQL gap锁

原创

mob64ca12f31496 2023-09-30 10:21:40 ©著作权

©著作权归作者所有：来自51CTO博客作者mob64ca12f31496的原创作品，请联系作者获取转载授权，否则将追究法律责任

MySQL Gap Lock

Introduction

In MySQL, a gap lock is a type of lock used to prevent phantom reads and ensure consistency in multi-version concurrency control (MVCC). It is a lock placed on a range of values between two existing rows in a database table. This article explains what a gap lock is, its purpose, and provides code examples to illustrate its use.

Purpose of Gap Locks

Gap locks are used to prevent phantom reads in concurrent transactions. A phantom read occurs when a transaction reads a set of rows based on a condition, but another transaction inserts or deletes a row that matches the condition before the first transaction completes. This can lead to inconsistent or incorrect results.

By using gap locks, MySQL ensures that no new rows are inserted or deleted in the range of values between two existing rows while a transaction is reading or modifying the data. This prevents phantom reads and maintains data consistency.

Gap Lock Example

Consider the following table named employees:

id	name	age
1	John	25
3	Alice	30
4	Bob	35

Suppose we want to select all employees between the ages of 25 and 35 (inclusive). We can use the following SQL query:

SELECT * FROM employees WHERE age BETWEEN 25 AND 35;

When executing this query, MySQL will use a gap lock to ensure that no new rows are inserted or deleted between the ages of 25 and 35 while the transaction is in progress. This prevents phantom reads.

Gap Locks and Indexes

Gap locks are closely related to index scans in MySQL. When performing an index scan, MySQL locks not only the rows that match the condition but also the gaps between the index entries. This ensures that no new rows are inserted or deleted in the range of values covered by the scan.

Let's consider the same employees table but with an index on the age column. Here's how we can create the index:

CREATE INDEX idx_age ON employees (age);

Now, when we execute the same query as before, MySQL will use the index and gap locks to prevent phantom reads:

SELECT * FROM employees WHERE age BETWEEN 25 AND 35;

Conclusion

Gap locks in MySQL are a powerful mechanism to prevent phantom reads and maintain data consistency in concurrent transactions. They ensure that no new rows are inserted or deleted within a specific range of values while a transaction is in progress. Gap locks work in conjunction with index scans to provide efficient and consistent query results.

By understanding gap locks and their purpose, developers can design their applications to handle concurrent transactions properly and avoid data inconsistencies.

![Pie Chart](