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id="go-tianli-blog">前往爱发电购买</div></div><script data-pjax src="/js/anzhiyu/ai_abstract.js"></script></div><article class="post-content" id="article-container" itemscope itemtype="https://ctext.top/4c2a.html"><header><a class="post-meta-categories" href="/categories/%E6%8A%80%E6%9C%AF%E6%95%99%E7%A8%8B/" itemprop="url">技术教程</a><a href="/tags/MYSQL/" tabindex="-1" itemprop="url">MYSQL</a><h1 id="CrawlerTitle" itemprop="name headline">索引下推</h1><span itemprop="author" itemscope itemtype="http://schema.org/Person">伴随</span><time itemprop="dateCreated datePublished" datetime="2025-01-02T08:21:35.000Z" title="发表于 2025-01-02 16:21:35">2025-01-02</time><time itemprop="dateCreated datePublished" datetime="2025-01-02T12:21:35.000Z" title="更新于 2025-01-02 20:21:35">2025-01-02</time></header><h1>什么是索引下推？(index condition pushdown)</h1>
<p>索引下推是MySQL中的一种查询优化技术，主要用于提高查询效率。它允许MySQL在存储引擎层使用索引来过滤数据，而不是将所有数据从存储引擎传输到服务器层 后再进行过滤。这样可以减少不必要的数据传输，从而提高查询性能。</p>
<h2 id="前期准备">前期准备</h2>
<ul>
<li>创建表</li>
</ul>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TABLE</span> orders (</span><br><span class="line"> id <span class="type">INT</span> <span class="keyword">PRIMARY</span> KEY, <span class="comment">-- 主键索引</span></span><br><span class="line"> user_id <span class="type">INT</span>, <span class="comment">-- 用户ID</span></span><br><span class="line"> status <span class="type">VARCHAR</span>(<span class="number">10</span>), <span class="comment">-- 订单状态</span></span><br><span class="line"> created_at DATETIME, <span class="comment">-- 创建时间</span></span><br><span class="line"> amount <span class="type">DECIMAL</span>(<span class="number">10</span>,<span class="number">2</span>), <span class="comment">-- 订单金额</span></span><br><span class="line"> INDEX idx_orders (user_id, status, created_at) <span class="comment">-- 联合索引</span></span><br><span class="line">);</span><br></pre></td></tr></table></figure>
<ul>
<li>查询语句</li>
</ul>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="operator">*</span> <span class="keyword">FROM</span> orders </span><br><span class="line"><span class="keyword">WHERE</span> user_id <span class="operator">=</span> <span class="number">100</span> <span class="keyword">AND</span> status <span class="operator">=</span> <span class="string">'shipped'</span> <span class="keyword">AND</span> created_at <span class="operator">></span> <span class="string">'2024-01-01'</span>;</span><br></pre></td></tr></table></figure>
<h2 id="SQL的执行逻辑">SQL的执行逻辑</h2>
<p>MySQL底层一共分为四层</p>
<ul>
<li>连接层（Client Connectors）：负责客户端与 MySQL 的通信（如 TCP/IP、Socket 等），处理连接、认证、权限校验等。</li>
<li>服务层（Server Layer）：解析 SQL、优化执行计划、调用存储引擎接口。</li>
<li>存储引擎层（Storage Engine Layer）：负责数据的存储和检索，支持插件式引擎（如 InnoDB、MyISAM）。</li>
<li>文件系统层（File System）：存储数据文件（.ibd、.frm）、日志文件（redo log、binlog）等物理文件。</li>
</ul>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre></td><td class="code"><pre><span class="line">客户端</span><br><span class="line"> │</span><br><span class="line"> ↓ 建立连接、发送 SQL</span><br><span class="line">连接层（认证、权限校验、连接池管理）</span><br><span class="line"> │</span><br><span class="line"> ↓ 传递 SQL</span><br><span class="line">服务层（解析器 → 优化器 → 执行器）</span><br><span class="line"> │</span><br><span class="line"> ↓ 调用存储引擎接口</span><br><span class="line">存储引擎层（InnoDB/MyISAM 读写数据）（回表）</span><br><span class="line"> │</span><br><span class="line"> ↓ 操作磁盘文件</span><br><span class="line">文件系统层（.ibd/.frm 文件、日志文件）</span><br><span class="line"> │</span><br><span class="line"> ↑ 返回数据</span><br><span class="line">存储引擎层 → 服务层 → 连接层 → 客户端</span><br></pre></td></tr></table></figure>
<p>MySQL 的查询执行过程可以分为 <mark class="hl-label green">解析、优化、执行</mark> 三个阶段。我们主要关注 执行阶段，并结合 索引下推 进行分析。</p>
<h3 id="1-判断索引的使用">1. 判断索引的使用</h3>
<p>MySQL 需要决定使用哪个索引，这里 WHERE 条件涉及 (user_id, status, created_at) 这三个列，而 idx_orders 这个索引正好覆盖了 user_id、status 和 created_at，所以 会使用该索引。</p>
<ul>
<li>索引匹配规则（最左前缀匹配）：</li>
</ul>
<blockquote>
<p>user_id = 100 可用索引 ✅<br>
status = ‘shipped’ 可用索引 ✅<br>
created_at > ‘2024-01-01’ 也可用索引（但范围查询会影响索引使用）</p>
</blockquote>
<p>因此，MySQL 选择 idx_orders 索引，并按以下步骤执行。</p>
<h3 id="2-具体执行步骤">2. 具体执行步骤</h3>
<ul>
<li>
<p>步骤 1：索引扫描</p>
<ul>
<li>MySQL 先利用 B+树索引，找到 user_id = 100 的数据范围。</li>
<li>然后在这个范围内，再筛选 status = ‘shipped’ 的记录。</li>
<li>如果<mark class="hl-label red">没有索引下推</mark> ，此时 MySQL 只会基于 user_id 和 status 进行索引扫描，找到符合条件的行，但 created_at > ‘2024-01-01’ 这个条件会在<mark class="hl-label red">回表后</mark> 再筛选。</li>
</ul>
</li>
<li>
<p>步骤 2：<mark class="hl-label orange">索引下推（如果启用）</mark> </p>
<ul>
<li>如果 MySQL 启用了索引下推（ICP），它会在索引扫描阶段就应用 created_at > ‘2024-01-01’ 的过滤条件</li>
<li>只有满足 user_id = 100、status = ‘shipped’ 且 created_at > ‘2024-01-01’ 的行会进入下一步，避免了不必要的回表操作。</li>
</ul>
</li>
<li>
<p>步骤 3：回表（如果需要）: <em>回表指的是如果索引列不完全包含所查询的字段，就需要回表，补全查询字段，然后返回</em></p>
<ul>
<li>由于 SELECT * 需要查询所有列（而索引 idx_orders 只包含 (user_id, status, created_at)），所以 MySQL 需要通过 主键回表 查询完整的行数据。</li>
<li>但 索引下推减少了不必要的回表，只对满足所有索引列筛选条件的行执行回表，从而提高了查询效率。</li>
</ul>
</li>
<li>
<p>步骤 4：返回数据</p>
<ul>
<li>读取回表数据后，MySQL 生成最终的结果集并返回给客户端。</li>
</ul>
</li>
</ul>
<h2 id="总结"><code>总结*</code></h2>
<h3 id="索引下推发生的条件">索引下推发生的条件</h3>
<ul>
<li>使用了<mark class="hl-label blue">二级索引</mark> （非主键索引）</li>
<li>查询条件涉及<mark class="hl-label blue">索引列</mark> </li>
<li>查询的字段<mark class="hl-label blue">不完全包含在索引中</mark> （可能回表）
<ul>
<li>如果查询字段包含在索引中，是否使用索引下推执行效率都是一样的，因为不需要回表</li>
</ul>
</li>
<li>索引列的筛选条件是<mark class="hl-label blue">范围查询</mark> (>, <, BETWEEN) 或 LIKE ‘xxx%’</li>
<li>数据库版本大于等于5.7</li>
</ul>
<h3 id="如何判断是否使用索引下推">如何判断是否使用索引下推</h3>
<p>使用EXPLAIN查询分析计划，查看Extra列，如果包含<code>Using index condition</code>说明使用了索引下推</p>
<h3 id="开关索引下推">开关索引下推</h3>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- 查询</span></span><br><span class="line"><span class="keyword">SHOW</span> VARIABLES <span class="keyword">LIKE</span> <span class="string">'optimizer_switch'</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">-- 开（全局）</span></span><br><span class="line"><span class="keyword">SET</span> <span class="keyword">GLOBAL</span> optimizer_switch <span class="operator">=</span> <span class="string">'index_condition_pushdown=on'</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">-- 关（全局）</span></span><br><span class="line"><span class="keyword">SET</span> <span class="keyword">GLOBAL</span> optimizer_switch <span class="operator">=</span> <span class="string">'index_condition_pushdown=off'</span>;</span><br></pre></td></tr></table></figure>
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class="toc-text">SQL的执行逻辑</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#1-%E5%88%A4%E6%96%AD%E7%B4%A2%E5%BC%95%E7%9A%84%E4%BD%BF%E7%94%A8"><span class="toc-number">1.2.1.</span> <span class="toc-text">1. 判断索引的使用</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#2-%E5%85%B7%E4%BD%93%E6%89%A7%E8%A1%8C%E6%AD%A5%E9%AA%A4"><span class="toc-number">1.2.2.</span> <span class="toc-text">2. 具体执行步骤</span></a></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#%E6%80%BB%E7%BB%93"><span class="toc-number">1.3.</span> <span class="toc-text">总结*</span></a><ol class="toc-child"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E7%B4%A2%E5%BC%95%E4%B8%8B%E6%8E%A8%E5%8F%91%E7%94%9F%E7%9A%84%E6%9D%A1%E4%BB%B6"><span class="toc-number">1.3.1.</span> <span class="toc-text">索引下推发生的条件</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%A6%82%E4%BD%95%E5%88%A4%E6%96%AD%E6%98%AF%E5%90%A6%E4%BD%BF%E7%94%A8%E7%B4%A2%E5%BC%95%E4%B8%8B%E6%8E%A8"><span class="toc-number">1.3.2.</span> <span class="toc-text">如何判断是否使用索引下推</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%BC%80%E5%85%B3%E7%B4%A2%E5%BC%95%E4%B8%8B%E6%8E%A8"><span class="toc-number">1.3.3.</span> <span class="toc-text">开关索引下推</span></a></li></ol></li></ol></li></ol></div></div><div class="card-widget card-recent-post"><div class="item-headline"><i class="anzhiyufont anzhiyu-icon-history"></i><span>最近发布</span></div><div class="aside-list"><div class="aside-list-item"><a class="thumbnail" href="/7ac.html" title="JAVA简易日志工具类"><img src= "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" onerror="this.onerror=null,this.src="https://up.ctext.top/default_bg.png"" data-lazy-src="https://up.ctext.top/article/2025/11/log.png" onerror="this.onerror=null;this.src='https://up.ctext.top/default_bg.png'" 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img.src = "https://up.ctext.top/happy.svg";
img.title = "下班了就该开开心心的玩耍，嘿嘿~";
img.alt = "下班了就该开开心心的玩耍，嘿嘿~";
}
currentTimeHtml = `本站居然运行了 ${dnum} 天<span id='runtime'> ${hnum} 小时 ${mnum} 分 ${snum} 秒 </span><i class='anzhiyufont anzhiyu-icon-heartbeat' style='color:red'></i>`;
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nick: '你的专属昵称',
nickError: '昵称太短啦！至少得有3个字符才能算数哦！',
mail: '你的电子小信箱',
mailError: '哎呀，这不是一个合格的邮箱地址哦，请再检查一遍！',
link: '你有个酷酷的网址吗？',
optional: '这个嘛，想填就填，不填也行！',
placeholder: '吐槽一下？',
sofa: '来，坐沙发上，发表你的评论吧！',
submit: '一键发送，让世界听到你的声音！',
like: '点个赞，让TA知道你的心意！',
cancelLike: '哎呀，不小心点错了，取消赞！',
reply: '回复TA，开启一场对话吧！',
cancelReply: '等等，我话还没说完，取消回复！',
comment: '来，说说你的看法，让评论区热闹起来！',
refresh: '刷新一下，看看有啥新动态！',
more: '哇塞，还有更多精彩内容等你来加载！',
preview: '先睹为快，预览一下你的大作吧！',
emoji: '来，加点表情，让你的评论更生动！',
uploadImage: '上传你的美照，让大家一起欣赏！',
seconds: '刚刚才发的，新鲜出炉！',
minutes: '几分钟前，还热乎着呢！',
hours: '几小时前，热度还在哦！',
days: '几天前，但依旧值得一看！',
now: '哇塞，刚刚发的，快抢沙发！',
uploading: '稍等片刻，你的图片正在飞奔而来！',
login: '登录一下，解锁更多功能！',
logout: '拜拜了您嘞，下次再见！',
admin: '博主大人在此，谁敢造次！',
sticky: '置顶一下，让更多人看到你的精彩内容！',
word: '字',
wordHint: '评论字数要在0到1字之间哦！\n现在你已经写了$2个字了，加油！',
anonymous: '神秘人出没，猜猜是谁？',
level0: '潜水员，深藏不露！',
level1: '冒泡小能手，偶尔露个头！',
level2: '吐槽大师，一针见血！',
level3: '活跃分子，无处不在！',
level4: '话痨达人，说个不停！',
level5: '传说级大神，膜拜一下！',
gif: '动态表情包，让你的评论更有趣！',
gifSearchPlaceholder: '快来搜索你喜欢的表情包，让评论更生动！',
profile: '看看我的个人资料，了解一下我吧！',
approved: '审核通过，可以展示啦！',
waiting: '正在等待审核，别急哦！',
spam: '垃圾内容，走开走开！',
unsticky: '换个位置看看！',
oldest: '看看最早的评论！',
latest: '看看最新的动态！',
hottest: '看看哪条评论最火！',
reactionTitle: '快来对这篇文章发表你的看法吧，让博主知道你的想法！😜'
};
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pageview: false,
dark: 'html[data-theme="dark"]',
path: window.location.pathname,
comment: false,
imageUploader: true,
comment: false,
copyright: false,
reaction: true,
locale
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waline.destroy()
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content = content.replace(/<pre><code>.*?<\/pre>/gi, '[代码]') // replace code
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generateHtml(walineArray)
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const $dom = document.querySelector('#card-newest-comments .aside-list')
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