Mysql联表查询索引失效的几种问题解决

SELECT o.*, u.name 
FROM orders o 
JOIN users u ON o.user_id = u.id  -- user_id或u.id缺少索引
WHERE o.create_time > '2023-01-01'

-- 单列索引示例
CREATE INDEX idx_orders_user_id ON orders(user_id);

-- 复合索引示例（多列连接条件）
CREATE INDEX idx_order_composite ON orders(user_id, product_id);

-- orders.user_id是VARCHAR，而users.id是INT
SELECT * FROM orders o JOIN users u ON o.user_id = u.id

-- 修改表结构统一类型
ALTER TABLE orders MODIFY user_id INT;

-- 或者使用显式转换（不推荐，影响性能）
SELECT * FROM orders o JOIN users u ON CAST(o.user_id AS SIGNED) = u.id

-- 存在索引idx_status_create_time(status, create_time)
SELECT * FROM orders WHERE create_time > '2023-01-01'  -- 无法使用索引

-- 调整查询顺序
SELECT * FROM orders WHERE status = 1 AND create_time > '2023-01-01'

-- 或创建新的复合索引
CREATE INDEX idx_create_time_status ON orders(create_time, status);

-- 原始查询
SELECT o.id, o.order_no, u.name, p.product_name
FROM orders o
JOIN users u ON o.user_id = u.id
JOIN products p ON o.product_id = p.id
WHERE o.status = 1
ORDER BY o.create_time DESC;

-- 创建覆盖索引
CREATE INDEX idx_order_covering ON orders(
    status, 
    create_time DESC, 
    user_id, 
    product_id
) INCLUDE (id, order_no);

SELECT o.*, u.name, p.product_name
FROM (
    SELECT * FROM orders 
    WHERE status = 1
    ORDER BY create_time DESC
    LIMIT 1000
) o
JOIN users u ON o.user_id = u.id
JOIN products p ON o.product_id = p.id;

-- 不推荐
SELECT * FROM orders 
WHERE user_id IN (SELECT id FROM users WHERE vip = 1);

-- 推荐
SELECT o.* FROM orders o
JOIN users u ON o.user_id = u.id AND u.vip = 1;

# MySQL配置示例
join_buffer_size = 8M  # 增大连接缓冲区
sort_buffer_size = 4M  # 排序缓冲区
read_rnd_buffer_size = 4M  # 随机读缓冲区
optimizer_switch = 'index_merge=on'  # 启用索引合并优化

SELECT o.*, u.*, p.*
FROM orders o
LEFT JOIN users u ON o.user_id = u.id
LEFT JOIN products p ON o.product_id = p.id
WHERE o.status IN (2,3,5)
AND u.vip_level > 3
AND p.category_id = 10
ORDER BY o.create_time DESC
LIMIT 50;

SELECT o.*, u.*, p.*
FROM (
    SELECT * FROM orders 
    WHERE status IN (2,3,5)
    ORDER BY create_time DESC
    LIMIT 50
) o
JOIN users u ON o.user_id = u.id AND u.vip_level > 3
JOIN products p ON o.product_id = p.id AND p.category_id = 10;

CREATE INDEX idx_orders_status_time ON orders(status, create_time DESC);
CREATE INDEX idx_users_vip ON users(vip_level, id);
CREATE INDEX idx_products_category ON products(category_id, id);

ANALYZE TABLE orders;
ANALYZE TABLE users;
ANALYZE TABLE products;

ALTER TABLE orders ENGINE=InnoDB;  -- 重建表整理碎片

-- 启用慢查询日志
SET GLOBAL slow_query_log = 'ON';
SET GLOBAL long_query_time = 1;