python中SQLAlchemy排序的坑

前言

SQLAlchemy是Python程式語言下的一款ORM框架，該框架建立在資料庫API之上，使用關聯式物件映射進行資料庫操作，簡言之便是：將物件轉換成SQL，然後使用資料API執行SQL並取得執行結果。最近在使用SQLAlchemy排序時遇到了一個坑，所以想著總結下來，分享給更多的朋友，下面來一起看看吧。

坑的程式碼

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()

不坑的程式碼

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()

對，你沒看錯，就是那個橫槓，拉慢速度。改成desc() 函數速度能提高10倍

下面附上一個sqlalchemy 高效能隨機取出若干條資料

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()

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