简述SQLAlchemy中排序的容易犯的一个错误

这篇文章主要介绍了关于python中SQLAlchemy排序的一个坑，文中给出了详细的示例代码，需要的朋友可以参考借鉴，感兴趣的朋友们下面来一起学习学习吧。

前言

SQLAlchemy是Python编程语言下的一款ORM框架，该框架建立在数据库API之上，使用关系对象映射进行数据库操作，简言之便是：将对象转换成SQL，然后使用数据API执行SQL并获取执行结果。最近在使用SQLAlchemy排序遇到了一个坑，所以想着总结下来，分享给更多的朋友，下面来一起看看吧。

坑的代码

query = db_session.query(UserVideo.vid,         UserVideo.uid,         UserVideo.v_width,         UserVideo.v_height,         UserVideo.create_time,         UserVideo.cover,         UserVideo.source_url,         UserVideo.v_type,         UserVideo.category,         User.username,         User.sex,         UserExtraInfo.avatar,         UserExtraInfo.watermark)  query = query.filter(UserVideo.status == 1,        User.uid == UserVideo.uid,        UserExtraInfo.uid == UserVideo.uid)   query = query.filter(UserVideo.status == 1)  query = query.order_by(-UserVideo.vid)  query = query.limit(20).all()

不坑的代码

query = db_session.query(UserVideo.vid,         UserVideo.uid,         UserVideo.v_width,         UserVideo.v_height,         UserVideo.create_time,         UserVideo.cover,         UserVideo.source_url,         UserVideo.v_type,         UserVideo.category,         User.username,         User.sex,         UserExtraInfo.avatar,         UserExtraInfo.watermark)  query = query.filter(UserVideo.status == 1,        User.uid == UserVideo.uid,        UserExtraInfo.uid == UserVideo.uid) # .order_by(UserVideo.vid.desc()).limit(20).all()  query = query.filter(UserVideo.status == 1)  query = query.order_by(UserVideo.vid.desc())  query = query.limit(20).all()

对，你没看错，就是那个横杠，拉慢速度。改成 desc() 函数速度能提高10倍

下面附上一个 sqlalchemy 高性能随机取出若干条数据

query = db_session.query(UserVideo.vid,          UserVideo.uid,          UserVideo.v_width,          UserVideo.v_height,          UserVideo.create_time,          UserVideo.cover,          UserVideo.source_url,          UserVideo.v_type,          UserVideo.category,          User.username,          User.sex,          UserExtraInfo.avatar,          UserExtraInfo.watermark)   query = query.filter(UserVideo.status == 1, User.uid == UserVideo.uid, UserExtraInfo.uid == UserVideo.uid)rvid = db_session.query(func.round(random.random() * func.max(UserVideo.vid)).label('rvid')).subquery()query = query.filter(UserVideo.category == category)query_tail = queryquery_tail = query_tail.join(rvid, UserVideo.vid > rvid.c.rvid).limit(20).all()

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