浅谈MySQL对于SELECT语句是否会自动加锁

SELECT * FROM orders 
WHERE user_id = 5 
FOR UPDATE; -- 加 X 锁，阻塞其他写操作

SELECT * FROM products 
WHERE stock > 0 
LOCK IN SHARE MODE; -- 加 S 锁，允许并发读但阻塞写

DELETE FROM parent_table WHERE id = 10;
-- 子表的 SELECT 检查会加共享锁

INSERT INTO users (email) VALUES ('test@example.com');
-- 插入前的唯一性检查会加共享锁

INSERT INTO archive_users
SELECT * FROM users 
WHERE created_at < '2020-01-01'; 
-- 对源表 users 的扫描可能加锁

graph TD
    A[事务开始] --> B[创建Read View]
    B --> C{查询}
    C -->|读取| D[最新提交版本]
    C -->|修改| E[创建新版本]
    E --> F[写入Undo Log]

SELECT * FROM performance_schema.data_locks 
WHERE THREAD_ID = PS_CURRENT_THREAD_ID();

SELECT * FROM sys.innodb_lock_waits;

SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;

-- 仅当需要后续更新时加锁
START TRANSACTION;
SELECT balance FROM accounts WHERE id=1 FOR UPDATE;
UPDATE accounts SET balance = balance - 100 WHERE id=1;
COMMIT;

-- 设置锁等待超时
SET innodb_lock_wait_timeout = 30;

SELECT * FROM information_schema.INNODB_TRX 
ORDER BY TIME_MS DESC LIMIT 10;

-- 创建覆盖索引
ALTER TABLE orders ADD INDEX idx_user_status (user_id, status);
-- 使用覆盖索引查询
SELECT order_id FROM orders 
WHERE user_id = 5 AND status = 'pending';