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索引的思考.html
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<h3 id="试着用字典来讲索引"><a href="#试着用字典来讲索引" class="headerlink" title="试着用字典来讲索引"></a>试着用字典来讲索引</h3><blockquote>
<p>关键字： 缩小范围，变随机为顺序，磁盘IO，<a href="https://zhuanlan.zhihu.com/p/30127242" target="_blank" rel="noopener">局部预读性原理</a>，</p>
<p>$h = log_aN$ ($h$ 代表b+树的高度，$a$ 代表每个磁盘块可以放的数据的大小，$N$ 代表总的数据量，磁盘块的大小 = $N/a$ )，范围查询时索引到该字段处失效，区分度高，保持列干净，通用公式$count(distinct col)/count(*)$）</p>
</blockquote>
<hr>
<h5 id="为什么把真实数据都放在叶子节点上？"><a href="#为什么把真实数据都放在叶子节点上？" class="headerlink" title="为什么把真实数据都放在叶子节点上？"></a>为什么把真实数据都放在叶子节点上？</h5><h5 id="为什么索引的字段要尽量的小"><a href="#为什么索引的字段要尽量的小" class="headerlink" title="为什么索引的字段要尽量的小"></a>为什么索引的字段要尽量的小</h5>
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