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HomeDatabaseMysql Tutorial关于MYSQL索引优化和in or替换为union all的问题_MySQL

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一个文章库,里面有两个表:category和article。category里面有10条分类数据。article里面有 20万条。

article里面有一个"article_category"字段是与category里的"category_id"字段相对应的。 article表里面已经把 article_category字义为了索引。数据库大小为1.3G。
 问题描述:
 执行一个很普通的查询: Select * FROM `article` Where article_category=11 orDER BY article_id DESC LIMIT 5 。执行时间大约要5秒左右 解决方案:
 建一个索引:create index idx_u on article (article_category,article_id);Select * FROM `article` Where article_category=11 orDER BY article_id DESC LIMIT 5 减少到0.0027秒     继续问题:Select * FROM `article` Where article_category IN (2,3) orDER BY article_id DESC LIMIT 5 执行时间要11.2850秒。
 使用OR:select * from articlewhere article_category=2or article_category=3order by article_id desclimit 5执行时间:11.0777   解决方案:避免使用in 或者 or (or会导致扫表),使用union all 使用UNION ALL:(select * from article where article_category=2 order by article_id desc limit 5)UNION ALL (select * from article where article_category=3 order by article_id desc limit 5)orDER BY article_id desclimit 5执行时间:0.0261
  作者 adparking bitsCN.com

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