【MySQL】索引相關

"<div class="htmledit_views" id="content_views"> <p id="main-toc"><strong>目錄</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">主鍵索引</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">組合索引</a></p>數據庫

<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">正確使用索引的狀況</a></p>服務器

<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">索引的注意事項</a></p>數據結構

<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">執行計劃 axplain</a></p>函數

<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">慢日誌記錄</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">分頁性能相關方案</a></p>測試

<hr id="hr-toc"><p><strong>索引是數據庫中專門用於幫助用戶快速查找數據的一種數據結構.</strong><br> 相似於字典中的目錄，查找字典內容能夠根據目錄查找到數據的存放位置，而後直接獲取.</p>

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

<blockquote> <p><strong>常見的幾種索引：</strong></p>

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

<blockquote> <p><strong>無索引和有索引的區別：</strong></p>

<p><span style="color:#f33b45;"><strong>無索引：</strong></span><strong>從前日後一條一條查詢.</strong><br><strong><span style="color:#f33b45;">有索引：</span>建立索引的本質，就是建立額外的文件，以某種格式存儲，查詢的時候，先去額外的文件找，肯定了位置，而後再去原始表中直接查詢，可是建立的索引越多，越會對硬盤有損耗.</strong></p>

<p><strong>創建索引的目的：</strong></p>

<ol><li><span style="color:#7c79e5;">額外的文件保存特殊的數據結構</span></li> <li><span style="color:#7c79e5;">查詢快，可是插入更新刪除依舊慢</span></li> <li><span style="color:#7c79e5;">建立索引以後，必須命中索引纔能有效</span></li> </ol></blockquote>

<blockquote> <p><strong>索引的種類：</strong></p>

<p><span style="color:#f33b45;"><strong>hash索引：</strong></span><span style="color:#7c79e5;"><strong>查詢單條快，範圍查詢慢</strong></span></p>

<p><span style="color:#f33b45;"><strong>btre類索引：</strong></span><span style="color:#7c79e5;"><strong>b+樹，層數增多，數據量指數級增加</strong></span>（InnoDB默認支持btree索引，這裏就使用它）</p> </blockquote>

<p><strong><span style="color:#f33b45;">覆蓋索引：</span></strong><span style="color:#7c79e5;">在索引文件中直接獲取數據</span><br><span style="color:#3399ea;">（例如：select name from userinfo where name = 'zyk';)</span></p>

<p><span style="color:#f33b45;"><strong>索引合併：</strong></span><span style="color:#7c79e5;"><strong>把多個單列索引合併使用</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字段爲定義的索引名</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字段顯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：一個表中默認會有一個主鍵，主鍵默認爲主鍵索引.</div></div></li></ol></code><div class="hljs-button" data-title="複製"></div></pre>

<p><span style="color:#f33b45;"><strong>做用：加速查找和惟一約束（包含null）</strong></span></p>

<p><span style="color:#f33b45;"><strong>做用：加速查找和惟一約束（不包含null）</strong></span></p>

<p><span style="color:#f33b45;"><strong>組合索引是將n個列組合成一個索引</strong></span><br> 應用場景：頻繁的同時使用n列來進行查詢（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;">正確使用索引的狀況</span></h2>

<p><strong>數據庫中添加索引後確實會讓查詢速度起飛，但前提必須是正確的使用索引來查詢，若是以錯誤的方式使用，則即便創建索引也不會奏效.</strong></p>

<p><strong>使用索引，咱們必須知道：</strong><br><span style="color:#f33b45;">(1) 建立索引<br> (2) 命中索引<br> (3) 正確使用索引</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. 準備表</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"># 建立存儲過程，實現批量插入記錄</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 $$ # 聲明存儲過程的結束符號爲$$</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() # 定義存儲過程名爲：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>; # 定義i爲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 ; 從新聲明分號爲結束符號</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"># 查看存儲過程</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"># 調用存儲過程</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="複製"></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">- 使用函數</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"> 特別的：當or條件中有未創建索引的列才失效，如下會走索引</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">- 類型不一致</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"> 若是列是字符串類型，傳入條件是必須用引號引發來，否則...</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"> 特別的：若是是主鍵，則仍是會走索引</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"> 特別的：若是是主鍵或索引是整數類型，則仍是會走索引</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"> 當根據索引排序時候，選擇的映射若是不是索引，則不走索引</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"> 特別的：若是對主鍵排序，則仍是走索引：</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">- 組合索引最左前綴</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"> 若是組合索引爲：(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="複製"></div></pre>

<blockquote> <p><strong>什麼是最左前綴：</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"># 創建組合索引</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="複製"></div></pre>

<p><span style="color:#f33b45;"><strong>若是使用組合索引如上，name和email組合索引以後，查詢：</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>對於同時搜索n個條件，組合索引的性能好於多個單列索引.</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;">索引的注意事項</span></h2>

<ol><li><span style="color:#f33b45;">不要使用select *        # 對性能的損耗太大</span></li> <li><s>應使用 count(1) 或 count(列名) 代替 count(*)</s>  後在查資料發現這些如今都沒有區別了</li> <li>建立表時儘可能使用char代替varchar        # 查詢速度</li> <li>表的字段順序：固定長度的字段優先</li> <li>常用多個條件查詢時，應創建組合索引代替多個單列索引</li> <li>儘可能使用短索引（create index ix_title on tb(title(16)); 特殊的數據類型，如text類型）</li> <li>使用連接 join 來代替子查詢</li> <li>連表時注意條件類型需一致</li> <li>索引散列（重複少）不適用於建索引，例如：性別不適合建索引</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;">執行計劃 axplain</span></h2>

<p><strong>explain + 查詢SQL：</strong>用於顯示SQL執行信息參數，根據參考信息能夠進行SQL優化.</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="複製"></div></pre>

<p><strong>各字段參數說明：</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"> 查詢類型</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 簡單查詢</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 最外層查詢</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 映射爲子查詢</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 子查詢</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 聯合</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 使用聯合的結果</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"> 正在訪問的表名</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"> 查詢時的訪問方式，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 全表掃描，對於數據表從頭至尾找一遍</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"> 特別的：若是有limit限制，則找到以後就不在繼續向下掃描</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"> 雖然上述兩個語句都會進行全表掃描，第二句使用了limit，則找到一個後就再也不繼續掃描。</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 ：全索引掃描，對索引從頭至尾找一遍</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：對索引列進行範圍查找</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 > 符號</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：合併索引，使用多個單列索引搜索</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：根據索引查找一個或多個值</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：鏈接時使用primary key 或 unique類型</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"> 表最多有一個匹配行,由於僅有一行,在這行的列值可被優化器剩餘部分認爲是常數,const表很快,由於它們只讀取一次。</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：系統</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"> 表僅有一行(=系統表)。這是const聯接類型的一個特例。</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：真實使用的</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中使用索引字節長度</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估計爲了找到所需的行而要讀取的行數 <span class="hljs-comment">------ 只是預估值</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"> 該列包含MySQL解決查詢的詳細信息</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將使用覆蓋索引，以免訪問表。不要把覆蓋索引和index訪問類型弄混了。</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"> 這意味着mysql服務器將在存儲引擎檢索行後再進行過濾，許多where條件裏涉及索引中的列，當（而且若是）它讀取索引時，就能被存儲引擎檢驗，所以不是全部帶where子句的查詢都會顯示「Using where」。有時「Using where」的出現就是一個暗示：查詢可受益於不一樣的索引。</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"> 這意味着mysql在對查詢結果排序時會使用一個臨時表。</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"> 這意味着mysql會對結果使用一個外部索引排序，而不是按索引次序從表裏讀取行。mysql有兩種文件排序算法，這兩種排序方式均可以在內存或者磁盤上完成，<span class="hljs-keyword">explain</span>不會告訴你mysql將使用哪種文件排序，也不會告訴你排序會在內存裏仍是磁盤上完成。</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"> 這個意味着沒有好用的索引，新的索引將在聯接的每一行上從新估算，N是顯示在possible_keys列中索引的位圖，而且是冗餘的</div></div></li></ol></code><div class="hljs-button" data-title="複製"></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;">慢日誌記錄</span></h2>

<p><strong>開啓慢查詢日誌，可讓MySQL記錄查詢時長超過指定時間的語句，經過定位分析性能的瓶頸，能更好的優化數據庫系統的性能.</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) 查詢是否開啓了慢查詢日誌</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"># 參數解釋：</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：慢查詢開啓狀態：<span class="hljs-keyword">OFF</span>爲關閉 <span class="hljs-keyword">ON</span>爲開啓</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：慢查詢日誌存放的位置（注意用戶權限）</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>) 查看慢查詢超時時間</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 | # 默認爲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>) 開啓慢日誌</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表示開啓，0表示關閉</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) 修改查詢超時時間</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="複製"></div></pre>

<p><strong>經過配置文件開啓慢日誌（推薦）：</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;">分頁性能相關方案</span></h2>

<blockquote> <p><strong>(1) 只有上一頁和下一頁：</strong><br> 作一個記錄：<span style="color:#7c79e5;">記錄當前頁的最大id或最小id</span></p>

<ul><li><strong>下一頁：</strong><span style="color:#3399ea;">select * from userinfo where id>max_id limit 10;</span></li> <li><strong>上一頁：</strong><span style="color:#3399ea;">select * from userinfo where id<min_id order by id desc limit 10;</span></li> </ul><p><strong>(2) 中間有頁碼的狀況</strong></p>

</blockquote>

<hr><hr><p> </p> </div> "