blocks|key|3393898|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3393899|当您在LEFT+JOIN的“右”表上有约束时(参见WHERE)；优化器将把它转换成常规的JOIN。这允许它重新排列表。|unordered-list-item|offset|length|style|CODE|3393900|+PRIMARY+KEY通常是到表JOIN的最佳方式。|3393901|A“覆盖”INDEX是一个具有查询所需的所有列的方法。由于您指定了"covering".|3393902|The，所以请求从索引中缺少full_name，因此索引中列的顺序不是SELECT+*，这可能非常重要。(但在你的例子中并非如此。)这可能导致不使用复合索引，甚至可能是覆盖index.|3393903|Generally，--最好将=测试的列放在综合索引中。如果不是覆盖索引，优化器将停止在复合索引的第一个“范围”列上。(与example.)|3393904|IN(1,2,3,4)无关的优化可能像=或范围一样，这取决于组合索引组件在排列列时的moon.|3393905|Cardinality阶段。|3393906|如果优化器认为需要触及索引的20%25以上，它将避开索引并使用表扫描。|3393907|3393908|更多：http://mysql.rjweb.org/doc.php/index_cookbook_mysql和http://mysql.rjweb.org/doc.php/index1|3393909|entityMap|0|LINK|mutability|MUTABLE|url|http://mysql.rjweb.org/doc.php/index_cookbook_mysql|1|http://mysql.rjweb.org/doc.php/index1^0|0|3|9|P|5|18|4|0|1|B|H|4|0|5|5|0|E|9|Z|8|0|F|1|0|0|B|J|1|0|0|0|0|3|1F|0|1J|11|1|0^^$0|@$1|2|3|-4|4|5|6|18|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|19|7|@$D|1A|E|1B|F|G]|$D|1C|E|1D|F|G]|$D|1E|E|1F|F|G]]|8|@]|9|$]]|$1|H|3|I|4|C|6|1G|7|@$D|1H|E|1I|F|G]|$D|1J|E|1K|F|G]]|8|@]|9|$]]|$1|J|3|K|4|C|6|1L|7|@$D|1M|E|1N|F|G]]|8|@]|9|$]]|$1|L|3|M|4|C|6|1O|7|@$D|1P|E|1Q|F|G]|$D|1R|E|1S|F|G]]|8|@]|9|$]]|$1|N|3|O|4|C|6|1T|7|@$D|1U|E|1V|F|G]]|8|@]|9|$]]|$1|P|3|Q|4|C|6|1W|7|@$D|1X|E|1Y|F|G]|$D|1Z|E|20|F|G]]|8|@]|9|$]]|$1|R|3|S|4|C|6|21|7|@]|8|@]|9|$]]|$1|T|3|U|4|C|6|22|7|@]|8|@]|9|$]]|$1|V|3|-4|4|5|6|23|7|@]|8|@]|9|$]]|$1|W|3|X|4|5|6|24|7|@]|8|@$D|25|E|26|1|27]|$D|28|E|29|1|2A]]|9|$]]|$1|Y|3|-4|4|5|6|2B|7|@]|8|@]|9|$]]]|Z|$10|$4|11|12|13|9|$14|15]]|16|$4|11|12|13|9|$14|17]]]]

<ul>
<li>When you have constraints (see <code>WHERE</code>) on the 'right' table of a <code>LEFT JOIN</code>; the Optimizer will turn it into a regular <code>JOIN</code>. This allows it to rearrange the tables.</li>
<li>The <code>PRIMARY KEY</code> is usually the optimal way to <code>JOIN</code> to a table.</li>
<li>A "covering" <code>INDEX</code> is one that has all the columns needed for the query. Since you specified <code>SELECT *</code>, you are asking for <code>full_name</code>, which is missing from your index, therefore it is not "covering".</li>
<li>The order of the columns in an index may be very important. (But not in your example.) This could lead to not using a composite index, and perhaps even a covering index.</li>
<li>Generally, it is best to put columns tested by <code>=</code> first in a composite index.</li>
<li>If it is not a covering index, the Optimizer will stop at the first "range" column in a composite index. (Not relevant in your example.)</li>
<li><code>IN(1,2,3,4)</code> might be optimized like <code>=</code> or like a range, depending on the phase of the moon.</li>
<li>Cardinality of the components of a composite index does not matter when arranging the columns.</li>
<li>If the Optimizer thinks that it needs to touch more than something like 20% of an index, it will shun the index and use a table scan.</li>
</ul>

More: <a href="http://mysql.rjweb.org/doc.php/index_cookbook_mysql" rel="nofollow noreferrer">http://mysql.rjweb.org/doc.php/index_cookbook_mysql</a> and <a href="http://mysql.rjweb.org/doc.php/index1" rel="nofollow noreferrer">http://mysql.rjweb.org/doc.php/index1</a>

I have two tables as per below

<a href="https://i.stack.imgur.com/WMHUi.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/WMHUi.png" alt="enter image description here"></a>

I have composite index on both tables as per below:

<ul>
<li>users table has a composite index: (login_type_id, language_id, enabled, level) </li>
<li>address table has a composite index: (user_id, city_id, enabled)</li>
</ul>

I would like to execute the following query

<pre><code>select * from users u
left join address adr
on u.id = adr.user_id
where u.login_type_id = 1
and u.langauge_id = 1
and u.enabled = true
and adr.city_id in (1,2,3,4,5)
and adr.enabled = true
order by u.level
</code></pre>

when I execute the query, I am expecting MySQL to use the composite index on both tables since all the fields in where clauses are indexed. But MySql uses the address table as base and only used the primary key of users to join. 

Does MySql ignore a composite index when joining two tables?

Can you guys give me an idea of why MySql does not use composite index on both tables?

To make the query fastest, which fields should I create indexes on, please?

Thank you.

Explain SQL
<a href="https://i.stack.imgur.com/il0zb.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/il0zb.png" alt="enter image description here"></a>

Please note that actual database is different from the database in the question but the relationship and index are the same.

How composite index and left join work in MySql?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

文章

问答

视频

教程

学习中心

腾讯云实验室

直播

竞赛

腾讯云代码分析专区

腾讯iOA零信任安全管理系统专区

腾讯云架构师技术同盟交流圈

腾讯云数据库专区

腾讯云智能顾问专区

腾讯云原生专区

腾讯混元专区

腾讯云TCE专区

腾讯云Lighthouse专区

腾讯云HAI专区

腾讯云Edgeone专区

腾讯云存储专区

腾讯云智能专区

腾讯轻联专区 

腾讯云开发专区

TAPD专区

腾讯轻量云游戏服专区

EdgeOne AI 安全实战专区

腾讯云最具价值专家

腾讯云架构师技术同盟

腾讯云创作之星

腾讯云开发者先锋

腾讯云AI代码助手

云原生构建

TAPD 敏捷项目管理

Cloud Studio

SDK中心

API中心

命令行工具

功能1上新10个字符

功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符功能2描述100个字符。

功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符功能2上新100个字符。

功能5描述100个字符功能5描述100个字符功能5描述100个字符功能5描述100个字符功能5描述100个字符功能5描述100个字符

功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符功能5上新100个字符

功能4上新

文章&问答评论现已支持表情

全新交互，全新视觉，新增快捷键、悬浮工具栏、高亮块等功能并同时优化现有功能，全面提升创作效率和体验

社区富文本编辑器全新改版！诚邀体验～ 

精选全网热门MCP server，让你的AI更好用 🚀

💥开发者 MCP广场重磅上线！

涵盖代码开发、场景应用、自动测试全流程，助你从零构建专属AI助手

一站式MCP教程库，解锁AI应用新玩法

聚焦“写作效率、视觉美观与运行性能”三方面进行全面升级，为您提供更高效、稳定的创作环境

社区富文本&Markdown编辑器全新改版上线，欢迎大家体验!

诚挚邀请您参与本次调研，分享您的真实使用感受与建议。您的反馈至关重要，感谢您的支持与参与！

社区新版编辑器体验调研

我有两张桌子如下所示​​我在两个表上都有如下所示的综合索引：users表有一个复合索引：(login_type_id，language_id，enabled，level) 地址表有一个复合索引：(user_id，city_id，enabled)我想执行以下查询select * from users uleft join address adron u.id = adr.user_idwhere u

问复合索引和左联接如何在MySql中工作？
EN

回答 1

Stack Overflow用户

社区

活动

圈层

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问复合索引和左联接如何在MySql中工作？EN