浅析python中SQLAlchemy排序的一个坑
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()
总结
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