SQL 性能优化：索引、执行计划与常见反模式

-- 组合索引顺序很重要：等值优先，范围在后
CREATE INDEX idx_user_status_created ON users(status, created_at);

-- 覆盖索引：减少回表
SELECT id, status FROM users WHERE status = 'active';

-- 避免在列上做函数运算导致索引失效
WHERE DATE(created_at) = '2025-09-01' -- ❌
WHERE created_at >= '2025-09-01' AND created_at < '2025-09-02' -- ✅