MySQL回表机制的原理及优化实战

CREATE TABLE `user` (
  `id` int PRIMARY KEY,
  `name` varchar(20),
  `age` int,
  `city` varchar(20),
  KEY `idx_city_age` (`city`, `age`)
) ENGINE=InnoDB;

-- 只需city和age字段，都在二级索引中
EXPLAIN SELECT city, age FROM user WHERE city = '北京';

-- 需要name字段，不在二级索引中
EXPLAIN SELECT name, city FROM user WHERE city = '北京';

-- 原始索引
ALTER TABLE user ADD INDEX idx_city (city);

-- 优化为覆盖索引
ALTER TABLE user ADD INDEX idx_city_name_age (city, name, age);

-- 好的联合索引示例
ALTER TABLE orders ADD INDEX idx_user_status_ctime (user_id, status, create_time);

-- 优于 WHERE name='张三'（如果name是二级索引）
SELECT * FROM user WHERE id = 123;

-- 不推荐
SELECT * FROM user WHERE city = '上海';

-- 推荐
SELECT id, name, city FROM user WHERE city = '上海';

-- 启用ICP（默认开启）
SET optimizer_switch = 'index_condition_pushdown=on';

-- 启用MRR
SET optimizer_switch = 'mrr=on';
SET optimizer_switch = 'mrr_cost_based=off';

SELECT product_name, price, detail 
FROM products 
WHERE category = '电子产品' 
AND price BETWEEN 1000 AND 5000;

-- 低效写法（深度分页回表）
SELECT * FROM user_friends 
WHERE user_id = 123 
ORDER BY create_time DESC 
LIMIT 10000, 20;

-- 优化写法（先查主键再join）
SELECT a.* FROM user_friends a
JOIN (
    SELECT id FROM user_friends
    WHERE user_id = 123
    ORDER BY create_time DESC
    LIMIT 10000, 20
) b ON a.id = b.id;

-- 查看索引使用情况
SHOW INDEX FROM user;

-- 分析查询开销
EXPLAIN ANALYZE SELECT * FROM user WHERE city = '北京';

-- 查找可能需要的覆盖索引
SELECT TABLE_NAME, COLUMN_NAME
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = 'your_db'
AND COLUMN_NAME IN ('city', 'age', 'name');

-- 设置监控
UPDATE performance_schema.setup_instruments
SET ENABLED = 'YES'
WHERE NAME LIKE '%handler%';

-- 查看统计
SELECT * FROM performance_schema.events_statements_summary_by_digest
WHERE DIGEST_TEXT LIKE '%SELECT%user%';

-- 函数索引示例(MySQL 8.0+)
ALTER TABLE user ADD INDEX idx_name_upper ((UPPER(name)));