python sqlalchemy原理_浅析python中SQLAlchemy排序的一个坑

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 = query

query_tail = query_tail.join(rvid, UserVideo.vid > rvid.c.rvid).limit(20).all()

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作能带来一定的帮助，如果有疑问大家可以留言交流，谢谢大家对脚本之家的支持。总结