【MySQL】索引相关

"<div class="htmledit_views" id="content_views"> <p id="main-toc"><strong>目录</strong></p>html

<p id="%E6%99%AE%E9%80%9A%E7%B4%A2%E5%BC%95-toc" style="margin-left:80px;"><a href="#%E6%99%AE%E9%80%9A%E7%B4%A2%E5%BC%95" rel="nofollow" target="_self">普通索引</a></p>mysql

<p id="%E5%94%AF%E4%B8%80%E7%B4%A2%E5%BC%95-toc" style="margin-left:80px;"><a href="#%E5%94%AF%E4%B8%80%E7%B4%A2%E5%BC%95" rel="nofollow" target="_self">惟一索引</a></p>算法

<p id="%E4%B8%BB%E9%94%AE%E7%B4%A2%E5%BC%95-toc" style="margin-left:80px;"><a href="#%E4%B8%BB%E9%94%AE%E7%B4%A2%E5%BC%95" rel="nofollow" target="_self">主键索引</a></p>sql

<p id="%E7%BB%84%E5%90%88%E7%B4%A2%E5%BC%95-toc" style="margin-left:80px;"><a href="#%E7%BB%84%E5%90%88%E7%B4%A2%E5%BC%95" rel="nofollow" target="_self">组合索引</a></p>数据库

<p id="%E6%AD%A3%E7%A1%AE%E4%BD%BF%E7%94%A8%E7%B4%A2%E5%BC%95%E7%9A%84%E6%83%85%E5%86%B5-toc" style="margin-left:40px;"><a href="#%E6%AD%A3%E7%A1%AE%E4%BD%BF%E7%94%A8%E7%B4%A2%E5%BC%95%E7%9A%84%E6%83%85%E5%86%B5" rel="nofollow" target="_self">正确使用索引的状况</a></p>服务器

<p id="%E7%B4%A2%E5%BC%95%E7%9A%84%E6%B3%A8%E6%84%8F%E4%BA%8B%E9%A1%B9-toc" style="margin-left:40px;"><a href="#%E7%B4%A2%E5%BC%95%E7%9A%84%E6%B3%A8%E6%84%8F%E4%BA%8B%E9%A1%B9" rel="nofollow" target="_self">索引的注意事项</a></p>数据结构

<p id="%E6%89%A7%E8%A1%8C%E8%AE%A1%E5%88%92%20axplain-toc" style="margin-left:40px;"><a href="#%E6%89%A7%E8%A1%8C%E8%AE%A1%E5%88%92%20axplain" rel="nofollow" target="_self">执行计划 axplain</a></p>函数

<p id="%E6%85%A2%E6%97%A5%E5%BF%97%E8%AE%B0%E5%BD%95-toc" style="margin-left:40px;"><a href="#%E6%85%A2%E6%97%A5%E5%BF%97%E8%AE%B0%E5%BD%95" rel="nofollow" target="_self">慢日志记录</a></p>性能

<p id="%E5%88%86%E9%A1%B5%E6%80%A7%E8%83%BD%E7%9B%B8%E5%85%B3%E6%96%B9%E6%A1%88-toc" style="margin-left:40px;"><a href="#%E5%88%86%E9%A1%B5%E6%80%A7%E8%83%BD%E7%9B%B8%E5%85%B3%E6%96%B9%E6%A1%88" rel="nofollow" target="_self">分页性能相关方案</a></p>测试

<hr id="hr-toc"><p><strong>索引是数据库中专门用于帮助用户快速查找数据的一种数据结构.</strong><br> 相似于字典中的目录，查找字典内容能够根据目录查找到数据的存放位置，而后直接获取.</p>

<p><span style="color:#f33b45;"><strong>做用：约束和加速查找</strong></span></p>

<blockquote> <p><strong>常见的几种索引：</strong></p>

<p><strong><span style="color:#7c79e5;">- 普通索引<br> - 惟一索引<br> - 主键索引<br> - 联合索引（多列）<br>         -- 联合主键索引<br>        -- 联合惟一索引<br>        -- 联合普通索引</span></strong></p> </blockquote>

<blockquote> <p><strong>无索引和有索引的区别：</strong></p>

<p><span style="color:#f33b45;"><strong>无索引：</strong></span><strong>从前日后一条一条查询.</strong><br><strong><span style="color:#f33b45;">有索引：</span>建立索引的本质，就是建立额外的文件，以某种格式存储，查询的时候，先去额外的文件找，肯定了位置，而后再去原始表中直接查询，可是建立的索引越多，越会对硬盘有损耗.</strong></p>

<p><strong>创建索引的目的：</strong></p>

<ol><li><span style="color:#7c79e5;">额外的文件保存特殊的数据结构</span></li> <li><span style="color:#7c79e5;">查询快，可是插入更新删除依旧慢</span></li> <li><span style="color:#7c79e5;">建立索引以后，必须命中索引才能有效</span></li> </ol></blockquote>

<blockquote> <p><strong>索引的种类：</strong></p>

<p><span style="color:#f33b45;"><strong>hash索引：</strong></span><span style="color:#7c79e5;"><strong>查询单条快，范围查询慢</strong></span></p>

<p><span style="color:#f33b45;"><strong>btre类索引：</strong></span><span style="color:#7c79e5;"><strong>b+树，层数增多，数据量指数级增加</strong></span>（InnoDB默认支持btree索引，这里就使用它）</p> </blockquote>

<p><strong><span style="color:#f33b45;">覆盖索引：</span></strong><span style="color:#7c79e5;">在索引文件中直接获取数据</span><br><span style="color:#3399ea;">（例如：select name from userinfo where name = 'zyk';)</span></p>

<p><span style="color:#f33b45;"><strong>索引合并：</strong></span><span style="color:#7c79e5;"><strong>把多个单列索引合并使用</strong></span><br><span style="color:#3399ea;">（例如：select name from userinfo where name = 'zyk' and sex = 'boy';）</span></p> </blockquote>

<hr><pre class="has" name="code" onclick="hljs.copyCode(event)"><code class="language-sql hljs"><ol class="hljs-ln"><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="1"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># 查看索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="2"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"><span class="hljs-keyword">show</span> <span class="hljs-keyword">index</span> <span class="hljs-keyword">from</span> 表名;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="3"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="4"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># Key_name字段为定义的索引名</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="5"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># Column_name字段显示的是建立了索引的字段</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="6"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># 注意：一个表中默认会有一个主键，主键默认为主键索引.</div></div></li></ol></code><div class="hljs-button" data-title="复制"></div></pre>

<p><span style="color:#f33b45;"><strong>做用：加速查找和惟一约束（包含null）</strong></span></p>

<p><span style="color:#f33b45;"><strong>做用：加速查找和惟一约束（不包含null）</strong></span></p>

<p><span style="color:#f33b45;"><strong>组合索引是将n个列组合成一个索引</strong></span><br> 应用场景：频繁的同时使用n列来进行查询（如：where name = 'zyk' and sex = 'boy'）</p>

<hr><h2 id="%E6%AD%A3%E7%A1%AE%E4%BD%BF%E7%94%A8%E7%B4%A2%E5%BC%95%E7%9A%84%E6%83%85%E5%86%B5"><a name="t4"></a><span style="color:#ffbb66;">正确使用索引的状况</span></h2>

<p><strong>数据库中添加索引后确实会让查询速度起飞，但前提必须是正确的使用索引来查询，若是以错误的方式使用，则即便创建索引也不会奏效.</strong></p>

<p><strong>使用索引，咱们必须知道：</strong><br><span style="color:#f33b45;">(1) 建立索引<br> (2) 命中索引<br> (3) 正确使用索引</span></p>

<pre class="has" name="code" onclick="hljs.copyCode(event)"><code class="language-sql hljs"><ol class="hljs-ln"><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="1"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">#1. 准备表</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="2"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"><span class="hljs-keyword">create</span> <span class="hljs-keyword">table</span> userinfo(</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="3"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">id</span> <span class="hljs-built_in">int</span>,</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="4"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">name</span> <span class="hljs-built_in">varchar</span>(<span class="hljs-number">20</span>),</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="5"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> gender <span class="hljs-built_in">char</span>(<span class="hljs-number">6</span>),</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="6"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> email <span class="hljs-built_in">varchar</span>(<span class="hljs-number">50</span>)</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="7"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> );</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="8"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="9"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="10"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># 建立存储过程，实现批量插入记录</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="11"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">delimiter $$ # 声明存储过程的结束符号为$$</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="12"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"><span class="hljs-keyword">create</span> <span class="hljs-keyword">procedure</span> auto_insert1() # 定义存储过程名为：auto_insert1()</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="13"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"><span class="hljs-keyword">begin</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="14"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">declare</span> i <span class="hljs-built_in">int</span> <span class="hljs-keyword">default</span> <span class="hljs-number">1</span>; # 定义i为1</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="15"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> while (i<1000000) <span class="hljs-keyword">do</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="16"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">insert</span> <span class="hljs-keyword">into</span> userinfo</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="17"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"><span class="hljs-keyword">values</span>(i, <span class="hljs-keyword">concat</span>(<span class="hljs-string">'zyk'</span>, i), <span class="hljs-string">'male'</span>, <span class="hljs-keyword">concat</span>(<span class="hljs-string">'zyk'</span>, i, <span class="hljs-string">'@oldboy'</span>));</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="18"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">set</span> i = i + <span class="hljs-number">1</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="19"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">end</span> <span class="hljs-keyword">while</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="20"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"><span class="hljs-keyword">end</span> $$</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="21"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">delimiter ; 从新声明分号为结束符号</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="22"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="23"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="24"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># 查看存储过程</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="25"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"><span class="hljs-keyword">show</span> <span class="hljs-keyword">create</span> <span class="hljs-keyword">procedure</span> auto_insert\G</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="26"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="27"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># 调用存储过程</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="28"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"><span class="hljs-keyword">call</span> auto_insert1();</div></div></li></ol></code><div class="hljs-button" data-title="复制"></div></pre>

<pre class="has" name="code" onclick="hljs.copyCode(event)"><code class="language-sql hljs"><ol class="hljs-ln" style="width:892px"><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="1"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">- like '%xx'</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="2"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> <span class="hljs-keyword">like</span> <span class="hljs-string">'%al'</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="3"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">- 使用函数</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="4"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">reverse</span>(<span class="hljs-keyword">name</span>) = <span class="hljs-string">'alex333'</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="5"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">- or</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="6"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">id</span> = <span class="hljs-number">1</span> <span class="hljs-keyword">or</span> email = <span class="hljs-string">'alex122@oldbody'</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="7"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 特别的：当or条件中有未创建索引的列才失效，如下会走索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="8"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">id</span> = <span class="hljs-number">1</span> <span class="hljs-keyword">or</span> <span class="hljs-keyword">name</span> = <span class="hljs-string">'alex1222'</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="9"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">id</span> = <span class="hljs-number">1</span> <span class="hljs-keyword">or</span> email = <span class="hljs-string">'alex122@oldbody'</span> <span class="hljs-keyword">and</span> <span class="hljs-keyword">name</span> = <span class="hljs-string">'alex112'</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="10"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">- 类型不一致</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="11"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 若是列是字符串类型，传入条件是必须用引号引发来，否则...</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="12"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> = <span class="hljs-number">999</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="13"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">- !=</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="14"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> <span class="hljs-keyword">count</span>(*) <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> != <span class="hljs-string">'alex'</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="15"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 特别的：若是是主键，则仍是会走索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="16"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> <span class="hljs-keyword">count</span>(*) <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">id</span> != <span class="hljs-number">123</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="17"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">- ></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="18"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> > <span class="hljs-string">'alex'</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="19"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 特别的：若是是主键或索引是整数类型，则仍是会走索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="20"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">id</span> > <span class="hljs-number">123</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="21"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">num</span> > <span class="hljs-number">123</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="22"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">- <span class="hljs-keyword">order</span> <span class="hljs-keyword">by</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="23"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> email <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">order</span> <span class="hljs-keyword">by</span> <span class="hljs-keyword">name</span> <span class="hljs-keyword">desc</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="24"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 当根据索引排序时候，选择的映射若是不是索引，则不走索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="25"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 特别的：若是对主键排序，则仍是走索引：</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="26"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">order</span> <span class="hljs-keyword">by</span> nid <span class="hljs-keyword">desc</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="27"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="28"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">- 组合索引最左前缀</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="29"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 若是组合索引为：(name,email)</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="30"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> name and email <span class="hljs-comment">-- 使用索引</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="31"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> name <span class="hljs-comment">-- 使用索引</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="32"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> email <span class="hljs-comment">-- 不使用索引</span></div></div></li></ol></code><div class="hljs-button" data-title="复制"></div></pre>

<blockquote> <p><strong>什么是最左前缀：</strong></p>

<pre class="has" name="code" onclick="hljs.copyCode(event)"><code class="language-sql hljs"><ol class="hljs-ln" style="width:1170px"><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="1"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># 创建组合索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="2"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"><span class="hljs-keyword">create</span> <span class="hljs-keyword">index</span> ix_name_email <span class="hljs-keyword">on</span> <span class="hljs-keyword">user</span> info(<span class="hljs-keyword">name</span>, email);</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="3"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="4"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># where name：使用索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="5"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">mysql> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> <span class="hljs-keyword">like</span> <span class="hljs-string">'zyk123456%'</span>; +<span class="hljs-comment">--------+-----------+--------+------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="6"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| id | name | gender | email |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="7"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">--------+-----------+--------+------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="8"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| 123456 | zyk123456 | male | zyk123456@oldboy |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="9"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">--------+-----------+--------+------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="10"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">1 row in <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) # <span class="hljs-number">0.00</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="11"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="12"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> <span class="hljs-keyword">and</span> email：使用索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="13"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">mysql> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> <span class="hljs-keyword">like</span> <span class="hljs-string">'zyk123456%'</span> <span class="hljs-keyword">and</span> email <span class="hljs-keyword">like</span> <span class="hljs-string">'zyk123456%'</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="14"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">--------+-----------+--------+------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="15"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| id | name | gender | email |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="16"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">--------+-----------+--------+------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="17"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| 123456 | zyk123456 | male | zyk123456@oldboy |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="18"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">--------+-----------+--------+------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="19"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">1 row in <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec) # <span class="hljs-number">0.00</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="20"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="21"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># <span class="hljs-keyword">where</span> email：不使用索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="22"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">mysql> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> email <span class="hljs-keyword">like</span> <span class="hljs-string">'zyk123456%'</span>; +<span class="hljs-comment">--------+-----------+--------+------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="23"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| id | name | gender | email |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="24"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">--------+-----------+--------+------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="25"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| 123456 | zyk123456 | male | zyk123456@oldboy |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="26"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">--------+-----------+--------+------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="27"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">1 row in <span class="hljs-keyword">set</span> (<span class="hljs-number">0.60</span> sec) # 注意：<span class="hljs-number">0.60</span></div></div></li></ol></code><div class="hljs-button" data-title="复制"></div></pre>

<p><span style="color:#f33b45;"><strong>若是使用组合索引如上，name和email组合索引以后，查询：</strong></span><br><strong>（1）name and email </strong>       <span style="color:#3399ea;"># 使用索引</span><br><strong>（2）name  </strong>      <span style="color:#3399ea;"># 使用索引</span><br><strong>（3）email </strong>       <span style="color:#3399ea;"># 不使用索引</span></p>

<p><strong>对于同时搜索n个条件，组合索引的性能好于多个单列索引.</strong></p> </blockquote>

<hr><h2 id="%E7%B4%A2%E5%BC%95%E7%9A%84%E6%B3%A8%E6%84%8F%E4%BA%8B%E9%A1%B9"><a name="t5"></a><span style="color:#ffbb66;">索引的注意事项</span></h2>

<ol><li><span style="color:#f33b45;">不要使用select *        # 对性能的损耗太大</span></li> <li><s>应使用 count(1) 或 count(列名) 代替 count(*)</s>  后在查资料发现这些如今都没有区别了</li> <li>建立表时尽可能使用char代替varchar        # 查询速度</li> <li>表的字段顺序：固定长度的字段优先</li> <li>常用多个条件查询时，应创建组合索引代替多个单列索引</li> <li>尽可能使用短索引（create index ix_title on tb(title(16)); 特殊的数据类型，如text类型）</li> <li>使用连接 join 来代替子查询</li> <li>连表时注意条件类型需一致</li> <li>索引散列（重复少）不适用于建索引，例如：性别不适合建索引</li> </ol><hr><h2 id="%E6%89%A7%E8%A1%8C%E8%AE%A1%E5%88%92%20axplain"><a name="t6"></a><span style="color:#ffbb66;">执行计划 axplain</span></h2>

<p><strong>explain + 查询SQL：</strong>用于显示SQL执行信息参数，根据参考信息能够进行SQL优化.</p>

<pre class="has" name="code" onclick="hljs.copyCode(event)"><code class="language-sql hljs"><ol class="hljs-ln" style="width:1128px"><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="1"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">mysql> <span class="hljs-keyword">explain</span> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="2"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="3"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="4"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="5"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| 1 | SIMPLE | userinfo | NULL | ALL | NULL | NULL | NULL | NULL | 997307 | 100.00 | NULL |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="6"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="7"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">1 row in <span class="hljs-keyword">set</span>, <span class="hljs-number">1</span> <span class="hljs-keyword">warning</span> (<span class="hljs-number">0.00</span> sec)</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="8"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="9"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">mysql> <span class="hljs-keyword">explain</span> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> (<span class="hljs-keyword">select</span> <span class="hljs-keyword">id</span>,<span class="hljs-keyword">name</span> <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">id</span> < <span class="hljs-number">20</span>) <span class="hljs-keyword">as</span> A;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="10"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="11"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="12"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="13"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| 1 | SIMPLE | userinfo | NULL | ALL | NULL | NULL | NULL | NULL | 997307 | 33.33 | Using where |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="14"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">----+-------------+----------+------------+------+---------------+------+---------+------+--------+----------+-------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="15"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">1 row in <span class="hljs-keyword">set</span>, <span class="hljs-number">1</span> <span class="hljs-keyword">warning</span> (<span class="hljs-number">0.00</span> sec)</div></div></li></ol></code><div class="hljs-button" data-title="复制"></div></pre>

<p><strong>各字段参数说明：</strong></p>

<pre class="has" name="code" onclick="hljs.copyCode(event)"><code class="language-sql hljs"><ol class="hljs-ln" style="width:2002px"><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="1"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">select_type：</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="2"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 查询类型</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="3"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> SIMPLE 简单查询</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="4"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> PRIMARY 最外层查询</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="5"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> SUBQUERY 映射为子查询</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="6"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> DERIVED 子查询</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="7"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> UNION 联合</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="8"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> UNION RESULT 使用联合的结果</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="9"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">table：</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="10"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 正在访问的表名</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="11"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">type：</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="12"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 查询时的访问方式，性能：all < index < range < index_merge < ref_or_null < ref < eq_ref < system/const</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="13"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> ALL 全表扫描，对于数据表从头至尾找一遍</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="14"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="15"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 特别的：若是有limit限制，则找到以后就不在继续向下扫描</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="16"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> email = <span class="hljs-string">'alex112@oldboy'</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="17"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> email = <span class="hljs-string">'alex112@oldboy'</span> <span class="hljs-keyword">limit</span> <span class="hljs-number">1</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="18"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 虽然上述两个语句都会进行全表扫描，第二句使用了limit，则找到一个后就再也不继续扫描。</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="19"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="20"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">INDEX ：全索引扫描，对索引从头至尾找一遍</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="21"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> nid <span class="hljs-keyword">from</span> userinfo;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="22"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="23"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">RANGE：对索引列进行范围查找</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="24"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> < <span class="hljs-string">'alex'</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="25"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> PS:</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="26"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> between and</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="27"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> in</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="28"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> > >= < <= 操做</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="29"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 注意：!= 和 > 符号</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="30"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="31"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="32"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">INDEX_MERGE：合并索引，使用多个单列索引搜索</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="33"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> = <span class="hljs-string">'alex'</span> <span class="hljs-keyword">or</span> nid <span class="hljs-keyword">in</span> (<span class="hljs-number">11</span>,<span class="hljs-number">22</span>,<span class="hljs-number">33</span>);</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="34"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="35"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">REF：根据索引查找一个或多个值</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="36"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">name</span> = <span class="hljs-string">'alex112'</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="37"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="38"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">EQ_REF：链接时使用primary key 或 unique类型</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="39"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> userinfo2.id,userinfo.name <span class="hljs-keyword">from</span> userinfo2 <span class="hljs-keyword">left</span> <span class="hljs-keyword">join</span> tuserinfo <span class="hljs-keyword">on</span> userinfo2.id = userinfo.id;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="40"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="41"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="42"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="43"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">CONST：常量</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="44"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 表最多有一个匹配行,由于仅有一行,在这行的列值可被优化器剩余部分认为是常数,const表很快,由于它们只读取一次。</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="45"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> <span class="hljs-keyword">id</span> <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">id</span> = <span class="hljs-number">2</span> ;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="46"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="47"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">SYSTEM：系统</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="48"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 表仅有一行(=系统表)。这是const联接类型的一个特例。</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="49"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> (<span class="hljs-keyword">select</span> <span class="hljs-keyword">id</span> <span class="hljs-keyword">from</span> userinfo <span class="hljs-keyword">where</span> <span class="hljs-keyword">id</span> = <span class="hljs-number">1</span>) <span class="hljs-keyword">as</span> A;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="50"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="51"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="52"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">possible_keys：可能使用的索引</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="53"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="54"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">key：真实使用的</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="55"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="56"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">key_len：　　MySQL中使用索引字节长度</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="57"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="58"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">rows： mysql估计为了找到所需的行而要读取的行数 <span class="hljs-comment">------ 只是预估值</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="59"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="60"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">extra：</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="61"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 该列包含MySQL解决查询的详细信息</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="62"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> “Using index”</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="63"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 此值表示mysql将使用覆盖索引，以免访问表。不要把覆盖索引和index访问类型弄混了。</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="64"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> “Using where”</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="65"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 这意味着mysql服务器将在存储引擎检索行后再进行过滤，许多where条件里涉及索引中的列，当（而且若是）它读取索引时，就能被存储引擎检验，所以不是全部带where子句的查询都会显示“Using where”。有时“Using where”的出现就是一个暗示：查询可受益于不一样的索引。</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="66"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> “Using temporary”</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="67"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 这意味着mysql在对查询结果排序时会使用一个临时表。</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="68"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> “Using filesort”</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="69"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 这意味着mysql会对结果使用一个外部索引排序，而不是按索引次序从表里读取行。mysql有两种文件排序算法，这两种排序方式均可以在内存或者磁盘上完成，<span class="hljs-keyword">explain</span>不会告诉你mysql将使用哪种文件排序，也不会告诉你排序会在内存里仍是磁盘上完成。</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="70"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> “<span class="hljs-keyword">Range</span> checked <span class="hljs-keyword">for</span> <span class="hljs-keyword">each</span> <span class="hljs-built_in">record</span>(<span class="hljs-keyword">index</span> <span class="hljs-keyword">map</span>: N)”</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="71"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> 这个意味着没有好用的索引，新的索引将在联接的每一行上从新估算，N是显示在possible_keys列中索引的位图，而且是冗余的</div></div></li></ol></code><div class="hljs-button" data-title="复制"></div></pre>

<hr><h2 id="%E6%85%A2%E6%97%A5%E5%BF%97%E8%AE%B0%E5%BD%95"><a name="t7"></a><span style="color:#ffbb66;">慢日志记录</span></h2>

<p><strong>开启慢查询日志，可让MySQL记录查询时长超过指定时间的语句，经过定位分析性能的瓶颈，能更好的优化数据库系统的性能.</strong></p>

<pre class="has" name="code" onclick="hljs.copyCode(event)"><code class="language-sql hljs"><ol class="hljs-ln"><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="1"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">(1) 查询是否开启了慢查询日志</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="2"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">mysql> <span class="hljs-keyword">show</span> <span class="hljs-keyword">variables</span> <span class="hljs-keyword">like</span> <span class="hljs-string">'slow_query%'</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="3"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">---------------------+----------------------------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="4"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| Variable_name | Value |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="5"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">---------------------+----------------------------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="6"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| slow_query_log | OFF |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="7"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| slow_query_log_file | /usr/local/mysql/data/MacBook-slow.log |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="8"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">---------------------+----------------------------------------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="9"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">2 rows in <span class="hljs-keyword">set</span> (<span class="hljs-number">0.01</span> sec)</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="10"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"># 参数解释：</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="11"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> slow_query_log：慢查询开启状态：<span class="hljs-keyword">OFF</span>为关闭 <span class="hljs-keyword">ON</span>为开启</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="12"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> slow_query_log_file：慢查询日志存放的位置（注意用户权限）</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="13"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="14"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">(<span class="hljs-number">2</span>) 查看慢查询超时时间</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="15"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">mysql> <span class="hljs-keyword">show</span> <span class="hljs-keyword">variables</span> <span class="hljs-keyword">like</span> <span class="hljs-string">'long%'</span>;</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="16"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">-----------------+-----------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="17"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| Variable_name | Value |</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="18"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">-----------------+-----------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="19"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">| long_query_time | 10.000000 | # 默认为10妙</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="20"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">+<span class="hljs-comment">-----------------+-----------+</span></div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="21"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">1 row in <span class="hljs-keyword">set</span> (<span class="hljs-number">0.00</span> sec)</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="22"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="23"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">(<span class="hljs-number">3</span>) 开启慢日志</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="24"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">mysql> <span class="hljs-keyword">set</span> <span class="hljs-keyword">global</span> slow_query_log = <span class="hljs-number">1</span>; # 1表示开启，0表示关闭</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="25"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">Query OK, 0 rows affected (0.01 sec)</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="26"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line"> </div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="27"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">(4) 修改查询超时时间</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="28"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">mysql> <span class="hljs-keyword">set</span> long_query_time = <span class="hljs-number">1</span>; # 直接指定秒</div></div></li><li><div class="hljs-ln-numbers"><div class="hljs-ln-line hljs-ln-n" data-line-number="29"></div></div><div class="hljs-ln-code"><div class="hljs-ln-line">Query OK, 0 rows affected (0.00 sec)</div></div></li></ol></code><div class="hljs-button" data-title="复制"></div></pre>

<p><strong>经过配置文件开启慢日志（推荐）：</strong></p>

<hr><h2 id="%E5%88%86%E9%A1%B5%E6%80%A7%E8%83%BD%E7%9B%B8%E5%85%B3%E6%96%B9%E6%A1%88"><a name="t8"></a><span style="color:#ffbb66;">分页性能相关方案</span></h2>

<blockquote> <p><strong>(1) 只有上一页和下一页：</strong><br> 作一个记录：<span style="color:#7c79e5;">记录当前页的最大id或最小id</span></p>

<ul><li><strong>下一页：</strong><span style="color:#3399ea;">select * from userinfo where id>max_id limit 10;</span></li> <li><strong>上一页：</strong><span style="color:#3399ea;">select * from userinfo where id<min_id order by id desc limit 10;</span></li> </ul><p><strong>(2) 中间有页码的状况</strong></p>

</blockquote>

<hr><hr><p> </p> </div> "