In mysql, deadlock is a concurrency control problem. It usually occurs when two or more transactions are waiting for their own resources. Deadlock will cause transactions to fail to continue, and system resources will be blocked. To understand and solve the mysql database deadlock problem, you need to ensure the performance and reliability of the database.

First understand the specific concept of deadlock, deadlock is a special concurrency problem, generally occurs when two or more transactions hold the lock required by each other, resulting in the relationship can not continue to execute. In plain English, a deadlock is a recurring wait between transactions. For example, in A transaction A and B, transaction A locks resource R1 and transaction B locks resource R2. When transaction A tries to lock resource R1 and transaction B tries to lock resource R1, both are waiting for the other to release both resources, resulting in a deadlock.

In this case, neither transaction can continue, and the database management system needs to check and resolve the deadlock.

MySQL uses a locking mechanism to manage multiple types of transactions. MySQL supports different types of locks, such as row locks, table locks, and gap locks. A shared lock allows a transaction to read a record, but prevents other transactions from modifying the record, and a separate lock allows a transaction to read and modify the row of records, preventing other transactions from accessing the record.

MySQL resolves deadlocks through an internal deadlock checking mechanism, which automatically rolls back one of the transactions and releases the lock held, allowing the other transactions to continue executing.

In order to prevent and handle deadlocks more effectively, the following strategies can be adopted, such as selecting the appropriate isolation level for application requirements for balance and data consistency, Read Uncomied (Read Uncomied) : The lowest isolation level, the transaction allows reading of uncommitted data, prone to read, fantasy read and other problems. Read Commied: Only data that has been committed can be read, preventing it from being read, but repeated unreadable may occur. Repeatable Read: prevents and cannot be read repeatedly, but phantom reads may occur. InnoDB's default isolation level. Serializable: Highest level of isolation, enforces serial execution of transactions, avoids all concurrency problems, but has lower performance.

-- Set the global transaction isolation level to repeatable read

SET GLOBAL TRANSACTION ISOLATION LEVEL REPEATABLE READ;

Set the transaction isolation level for the current session to serializable

SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Transactions occupy resources for a long time and are prone to deadlock triggering locks. Minimize transaction execution time and lock holding time.

START TRANSACTION;

-- Perform the necessary operations

COMMIT;   Commit the transaction as soon as possible

Reasonably design index, speed up query, reduce lock contention, and use index field in query statement.

-- Create an index

CREATE INDEX idx_name ON table_name(column_name);

-- Queries that use indexes

SELECT * FROM table_name WHERE column_name = 'value';

Ensure that all transactions access resources in the same order, avoiding circular waits. For example, first lock table A and then lock table B.

Use the EXPLAIN command to analyze the execution plan of the SQL statement and search for possible lock contention.

EXPLAIN SELECT * FROM table_name WHERE column_name = 'value';

Use SHOW ENGINE INNODB STATUS

This command can view the current state of the InnoDB engine, including deadlock information.

SHOW ENGINE INNODB STATUS\G;

In the LATEST DETECTED DEADLOCK section of the output, you can see information about the most recent deadlock that occurred, including the transactions and locks involved.

MySQL's performance_schema library provides some views for monitoring lock and transaction activity

-- Queries the transaction that currently holds the lock

SELECT * FROM performance_schema.data_locks;

-- Queries transactions waiting for a lock

SELECT * FROM performance_schema.data_lock_waits;

The concurrency control of MySQL is an important part of database management. Deadlock problem can be effectively prevented and solved by designing database structure reasonably, optimizing SQL statement, using appropriate isolation level and locking strategy.