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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