淺析python中SQLAlchemy排序的一個坑，pythonsqlalchemy

淺析python中SQLAlchemy排序的一個坑，pythonsqlalchemy

前言

SQLAlchemy是Python程式設計語言下的一款ORM架構，該架構建立在資料庫API之上，使用關聯性物件映射進行資料庫操作，簡言之便是：將對象轉換成SQL，然後使用資料API執行SQL並擷取執行結果。最近在使用SQLAlchemy排序遇到了一個坑，所以想著總結下來，分享給更多的朋友，下面來一起看看吧。

坑的代碼

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()

不坑的代碼

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，就是那個橫杠，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 = queryquery_tail = query_tail.join(rvid, UserVideo.vid > rvid.c.rvid).limit(20).all()

總結

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