MySQL间隙锁与排他锁的区别全解析

-- 示例：对uid=8的行加排他锁
SELECT * FROM student WHERE uid = 8 FOR UPDATE;
-- 或
UPDATE student SET age = 60 WHERE uid = 8;

-- 示例：锁定uid在(5, 10)之间的所有间隙
SELECT * FROM student WHERE uid BETWEEN 6 AND 9 FOR UPDATE;
-- 或
SELECT * FROM student WHERE uid > 5 AND uid < 10 FOR UPDATE;

uid: 1, 2, 3, 4, 5, 7, 8, 10, 11

-- 事务一
BEGIN;
SELECT * FROM student WHERE age BETWEEN 20 AND 30 FOR UPDATE;
-- 锁定所有age在20-30之间的间隙
-- 事务二尝试插入age=25的新记录（会被阻塞）
INSERT INTO student (name, age, sex) VALUES ('new_student', 25, 'M');

MySQL 实际上经常使用 **临键锁 = 记录锁 + 间隙锁**
-- 锁定uid=8的行及其前后的间隙
SELECT * FROM student WHERE uid = 8 FOR UPDATE;
-- 实际锁定：(7, 8] 和 (8, 10)

-- 如果name有唯一索引，不会加间隙锁
SELECT * FROM student WHERE name = 'bbbb' FOR UPDATE;
-- 只对'bbbb'这一行加排他锁
-- 如果name没有索引或不是唯一索引，可能会锁全表间隙
SELECT * FROM student WHERE name LIKE 'b%' FOR UPDATE;

-- 查看当前的锁（MySQL 8.0+）
SELECT 
    engine_transaction_id as trx_id,
    lock_type,
    lock_mode,
    lock_data,
    index_name
FROM performance_schema.data_locks 
WHERE object_name = 'student';

-- 事务A
BEGIN;
SELECT * FROM student WHERE uid = 15 FOR UPDATE; -- 间隙锁(10, +∞)
-- 事务B  
BEGIN;
SELECT * FROM student WHERE uid = 16 FOR UPDATE; -- 间隙锁(10, +∞)
-- 事务A
INSERT INTO student (uid, name) VALUES (15, 'test'); -- 等待B
-- 事务B
INSERT INTO student (uid, name) VALUES (16, 'test'); -- 等待A → 死锁！

-- 好的设计：使用唯一索引避免不必要的间隙锁
CREATE UNIQUE INDEX idx_uid ON student(uid);
-- 查询时使用唯一索引
SELECT * FROM student WHERE uid = 8 FOR UPDATE; -- 只锁一行

-- 如果不需要防止幻读，使用读已提交
SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
-- 此时不会加间隙锁

-- 尽快提交事务，减少锁持有时间
BEGIN;
-- 立即执行需要加锁的操作
SELECT ... FOR UPDATE;
UPDATE ...;
-- 尽快提交
COMMIT;