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<h3 id="题目介绍"><a href="#题目介绍" class="headerlink" title="题目介绍"></a>题目介绍</h3><p>A <strong>path</strong> in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence <strong>at most once</strong>. Note that the path does not need to pass through the root.</p>
<p>The <strong>path sum</strong> of a path is the sum of the node’s values in the path.</p>
<p>Given the <code>root</code> of a binary tree, return the maximum <strong>path sum</strong> of any path.</p>
<p><strong>路径</strong> 被定义为一条从树中任意节点出发，沿父节点-子节点连接，达到任意节点的序列。该路径 <strong>至少包含一个</strong> 节点，且不一定经过根节点。</p>
<p><strong>路径和</strong> 是路径中各节点值的总和。</p>
<p>给你一个二叉树的根节点 <code>root</code> ，返回其 <strong>最大路径和</strong></p>
<h3 id="简要分析"><a href="#简要分析" class="headerlink" title="简要分析"></a>简要分析</h3><p>其实这个题目会被误解成比较简单，左子树最大的，或者右子树最大的，或者两边加一下，仔细想想都不对，其实有可能是产生于左子树中，或者右子树中，这两个都是指跟左子树根还有右子树根没关系的，这么说感觉不太容易理解，画个图<br><img data-src="https://gitee.com/nicksxs/images/raw/master/uPic/SfxIEy.png" alt=""><br>可以看到图里，其实最长路径和是左边这个子树组成的，跟根节点还有右子树完全没关系，然后再想一种情况，如果是整棵树就是图中的左子树，那么这个最长路径和就是左子树加右子树加根节点了，所以不是我一开始想得那么简单，在代码实现中也需要一些技巧</p>
<h3 id="代码"><a href="#代码" class="headerlink" title="代码"></a>代码</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">int</span> ansNew = Integer.MIN_VALUE;</span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">maxPathSum</span><span class="params">(TreeNode root)</span> </span>&#123;</span><br><span class="line"> maxSumNew(root);</span><br><span class="line"> <span class="keyword">return</span> ansNew;</span><br><span class="line"> &#125;</span><br><span class="line"> </span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">maxSumNew</span><span class="params">(TreeNode root)</span> </span>&#123;</span><br><span class="line"> <span class="keyword">if</span> (root == <span class="keyword">null</span>) &#123;</span><br><span class="line"> <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="comment">// 这里是个简单的递归，就是去递归左右子树，但是这里其实有个概念，当这样处理时，其实相当于把子树的内部的最大路径和已经算出来了</span></span><br><span class="line"> <span class="keyword">int</span> left = maxSumNew(root.left);</span><br><span class="line"> <span class="keyword">int</span> right = maxSumNew(root.right);</span><br><span class="line"> <span class="comment">// 这里前面我有点没想明白，但是看到 ansNew 的比较，其实相当于，返回的是三种情况里的最大值，一个是左子树+根，一个是右子树+根，一个是单独根节点，</span></span><br><span class="line"> <span class="comment">// 这样这个递归的返回才会有意义，不然像原来的方法，它可能是跳着的，但是这种情况其实是借助于 ansNew 这个全局的最大值，因为原来我觉得要比较的是</span></span><br><span class="line"> <span class="comment">// left, right, left + root , right + root, root, left + right + root 这些的最大值，这里是分成了两个阶段，left 跟 right 的最大值已经在上面的</span></span><br><span class="line"> <span class="comment">// 调用过程中赋值给 ansNew 了 </span></span><br><span class="line"> <span class="keyword">int</span> currentSum = Math.max(Math.max(root.val + left , root.val + right), root.val);</span><br><span class="line"> <span class="comment">// 这边返回的是 currentSum，然后再用它跟 left + right + root 进行对比，然后再去更新 ans</span></span><br><span class="line"> <span class="comment">// PS: 有个小点也是这边的破局点，就是这个 ansNew</span></span><br><span class="line"> <span class="keyword">int</span> res = Math.max(left + right + root.val, currentSum);</span><br><span class="line"> ans = Math.max(res, ans);</span><br><span class="line"> <span class="keyword">return</span> currentSum;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这里非常重要的就是 ansNew 是最后的一个结果，而对于 maxSumNew 这个函数的返回值其实是需要包含了一个连续结果，因为要返回继续去算路径和，所以返回的是 currentSum，最终结果是 ansNew</p>
<h3 id="结果图"><a href="#结果图" class="headerlink" title="结果图"></a>结果图</h3><p>难得有个 100%，贴个图哈哈<br><img data-src="https://gitee.com/nicksxs/images/raw/master/uPic/iipgv0.png" alt=""></p>
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<p>今天真的是被气得不轻，情况是碰到一个有 70 多秒的直行红灯，然后直行就排了很长的队，但是左转车道没车，就有好几辆车占着左转车道，准备往直行车道插队加塞，一般这种加塞的，会挑个不太计较的，如果前面一辆不让的话就再等等，我因为赶着回家，就不想让，结果那辆车几次车头直接往里冲，当时怒气值基本已经蓄满了，我真的是分毫都不想让，如果路上都是让着这种人的，那么这种情况只会越来越严重，我理解的这种心态，就赌你怕麻烦，多一事不如少一事，结果就是每次都能顺利插队加塞，其实延伸到我们社会中的种种实质性的排队或者等同于排队的情况，都已经有这种惯有思维，一方面这种不符合规则，可能在严重程度上容易被很多人所忽视，基本上已经被很多人当成是“合理”行为，另一方面，对于这些“微小”的违规行为，本身管理层面也基本没有想要管的意思，就更多的成为了纵容这些行为的导火索，并且大多数人都是想着如果不让，发生点小剐小蹭的要浪费很多时间精力来处理，甚至会觉得会被别人觉得自己太小气等等，诸多内外成本结合起来，会真的去硬刚的可能少之又少了，这样也就让更多的人觉得这种行为是被默许的，再举个非常小的例子，以我们公司疫情期间的盒饭发放为例，有两个比较“有意思”的事情，第一个就是因为疫情，本来是让排队要间隔一米，但是可能除了我比较怕死会跟前面的人保持点距离基本没别人会不挨着前面的人，甚至我跟我前面的人保持点距离，后面的同学会推着我让我上去；第二个是关于拿饭，这么多人排着队拿饭，然后有部分同学，一个人拿好几份，帮组里其他人的都拿了，有些甚至一个人拿十份，假如这个盒饭发放是说明了可以按部门直接全领了那就没啥问题，但是当时的状况是个人排队领自己的那一份，如果一个同学直接帮着组里十几个人都拿了，后面排队的人是什么感受呢，甚至有些是看到队伍排长了，就找队伍里自己认识的比较靠前的人说你帮我也拿一份，其实作为我这个比较按规矩办事的“愣头青”来说，我是比较不能接受这两件小事里的行为的，再往下说可能就有点偏激了，先说到这~</p>
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<h2 id="题目介绍"><a href="#题目介绍" class="headerlink" title="题目介绍"></a>题目介绍</h2><p>写一个程序找出两个单向链表的交叉起始点，可能是我英语不好，图里画的其实还有一点是交叉以后所有节点都是相同的<br>Write a program to find the node at which the intersection of two singly linked lists begins.</p>
<p>For example, the following two linked lists:<br><img data-src="https://gitee.com/nicksxs/images/raw/master/uPic/nkha2z.png" alt=""><br>begin to intersect at node c1.</p>
<h3 id="Example-1"><a href="#Example-1" class="headerlink" title="Example 1:"></a>Example 1:</h3><p><img data-src="https://gitee.com/nicksxs/images/raw/master/uPic/1TrhYe.png" alt=""></p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">Input: intersectVal &#x3D; 8, listA &#x3D; [4,1,8,4,5], listB &#x3D; [5,6,1,8,4,5], skipA &#x3D; 2, skipB &#x3D; 3</span><br><span class="line">Output: Reference of the node with value &#x3D; 8</span><br><span class="line">Input Explanation: The intersected node&#39;s value is 8 (note that this must not be 0 if the two lists intersect). From the head of A, it reads as [4,1,8,4,5]. From the head of B, it reads as [5,6,1,8,4,5]. There are 2 nodes before the intersected node in A; There are 3 nodes before the intersected node in B.</span><br></pre></td></tr></table></figure>
<h2 id="分析题解"><a href="#分析题解" class="headerlink" title="分析题解"></a>分析题解</h2><p>一开始没什么头绪，感觉只能最原始的遍历，后来看了一些文章，发现比较简单的方式就是先找两个链表的长度差，因为从相交点开始肯定是长度一致的，这是个很好的解题突破口，找到长度差以后就是先跳过长链表的较长部分，然后开始同步遍历比较 A，B 链表；</p>
<h3 id="代码"><a href="#代码" class="headerlink" title="代码"></a>代码</h3><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> ListNode <span class="title">getIntersectionNode</span><span class="params">(ListNode headA, ListNode headB)</span> </span>&#123;</span><br><span class="line"> <span class="keyword">if</span> (headA == <span class="keyword">null</span> || headB == <span class="keyword">null</span>) &#123;</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">null</span>;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="comment">// 算 A 的长度</span></span><br><span class="line"> <span class="keyword">int</span> countA = <span class="number">0</span>;</span><br><span class="line"> ListNode tailA = headA;</span><br><span class="line"> <span class="keyword">while</span> (tailA != <span class="keyword">null</span>) &#123;</span><br><span class="line"> tailA = tailA.next;</span><br><span class="line"> countA++;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="comment">// 算 B 的长度</span></span><br><span class="line"> <span class="keyword">int</span> countB = <span class="number">0</span>;</span><br><span class="line"> ListNode tailB = headB;</span><br><span class="line"> <span class="keyword">while</span> (tailB != <span class="keyword">null</span>) &#123;</span><br><span class="line"> tailB = tailB.next;</span><br><span class="line"> countB++;</span><br><span class="line"> &#125;</span><br><span class="line"> tailA = headA;</span><br><span class="line"> tailB = headB;</span><br><span class="line"> <span class="comment">// 依据长度差，先让长的链表 tail 指针往后移</span></span><br><span class="line"> <span class="keyword">if</span> (countA &gt; countB) &#123;</span><br><span class="line"> <span class="keyword">while</span> (countA &gt; countB) &#123;</span><br><span class="line"> tailA = tailA.next;</span><br><span class="line"> countA--;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125; <span class="keyword">else</span> <span class="keyword">if</span> (countA &lt; countB) &#123;</span><br><span class="line"> <span class="keyword">while</span> (countA &lt; countB) &#123;</span><br><span class="line"> tailB = tailB.next;</span><br><span class="line"> countB--;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="comment">// 然后以相同速度遍历两个链表比较</span></span><br><span class="line"> <span class="keyword">while</span> (tailA != <span class="keyword">null</span>) &#123;</span><br><span class="line"> <span class="keyword">if</span> (tailA == tailB) &#123;</span><br><span class="line"> <span class="keyword">return</span> tailA;</span><br><span class="line"> &#125; <span class="keyword">else</span> &#123;</span><br><span class="line"> tailA = tailA.next;</span><br><span class="line"> tailB = tailB.next;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">null</span>;</span><br><span class="line"> &#125;</span><br></pre></td></tr></table></figure>
<h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><p>可能缺少这种思维，做的还是比较少，所以没法一下子反应过来，需要锻炼，我的第一反应是两重遍历，不过那样复杂度就高了，这里应该是只有 O(N) 的复杂度。</p>
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<p>Java 中的这个话题也是比较常遇到的，关于这块原先也是比较忽略的，但是仔细想想又有点遗忘了就在这里记一下<br>简单看下代码<br><code>java.lang.Object#equals</code></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">equals</span><span class="params">(Object obj)</span> </span>&#123;</span><br><span class="line"> <span class="keyword">return</span> (<span class="keyword">this</span> == obj);</span><br><span class="line"> &#125;</span><br></pre></td></tr></table></figure>
<p>对于所有对象的父类，<code>equals</code> 方法其实对比的就是对象的地址，也就是是否是同一个对象，试想如果像 Integer 或者 String 这种，我们没有重写 <code>equals</code>，那其实就等于是在用<code>==</code>，可能就没法达到我们的目的，所以像 String 这种常用的 jdk 类都是默认重写了<br><code>java.lang.String#equals</code></p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">boolean</span> <span class="title">equals</span><span class="params">(Object anObject)</span> </span>&#123;</span><br><span class="line"> <span class="keyword">if</span> (<span class="keyword">this</span> == anObject) &#123;</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="keyword">if</span> (anObject <span class="keyword">instanceof</span> String) &#123;</span><br><span class="line"> String anotherString = (String)anObject;</span><br><span class="line"> <span class="keyword">int</span> n = value.length;</span><br><span class="line"> <span class="keyword">if</span> (n == anotherString.value.length) &#123;</span><br><span class="line"> <span class="keyword">char</span> v1[] = value;</span><br><span class="line"> <span class="keyword">char</span> v2[] = anotherString.value;</span><br><span class="line"> <span class="keyword">int</span> i = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">while</span> (n-- != <span class="number">0</span>) &#123;</span><br><span class="line"> <span class="keyword">if</span> (v1[i] != v2[i])</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line"> i++;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">true</span>;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">false</span>;</span><br><span class="line"> &#125;</span><br></pre></td></tr></table></figure>
<p>然后呢就是为啥一些书或者 <code>effective java</code> 中写了 <code>equals</code> 跟 <code>hashCode</code> 要一起重写，这里涉及到当对象作为 <code>HashMap</code> 的 <code>key</code> 的时候<br>首先 <code>HashMap</code> 会使用 <code>hashCode</code> 去判断是否在同一个槽里，然后在通过 <code>equals</code> 去判断是否是同一个 <code>key</code>，是的话就替换，不是的话就链表接下去，如果不重写 <code>hashCode</code> 的话，默认的 <code>object</code> 的<code>hashCode</code> 是 <code>native</code> 方法，根据对象的地址生成的，这样其实对象的值相同的话，因为地址不同，<code>HashMap</code> 也会出现异常，所以需要重写，同时也需要重写 <code>equals</code> 方法，才能确认是同一个 <code>key</code>，而不是落在同一个槽的不同 <code>key</code>.</p>
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<p>前几天同事问了我个 mysql 索引的问题，虽然大概知道，但是还是想来实践下，就是 is null，is not null 这类查询是否能用索引，可能之前有些网上的文章说都是不能用索引，但是其实不是，我们来看个小试验</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TABLE</span> <span class="string">`null_index_t`</span> (</span><br><span class="line"> <span class="string">`id`</span> <span class="built_in">int</span>(<span class="number">10</span>) <span class="keyword">unsigned</span> <span class="keyword">NOT</span> <span class="literal">NULL</span> AUTO_INCREMENT,</span><br><span class="line"> <span class="string">`null_key`</span> <span class="built_in">varchar</span>(<span class="number">255</span>) <span class="keyword">DEFAULT</span> <span class="literal">NULL</span>,</span><br><span class="line"> <span class="string">`null_key1`</span> <span class="built_in">varchar</span>(<span class="number">255</span>) <span class="keyword">DEFAULT</span> <span class="literal">NULL</span>,</span><br><span class="line"> <span class="string">`null_key2`</span> <span class="built_in">varchar</span>(<span class="number">255</span>) <span class="keyword">DEFAULT</span> <span class="literal">NULL</span>,</span><br><span class="line"> PRIMARY <span class="keyword">KEY</span> (<span class="string">`id`</span>),</span><br><span class="line"> <span class="keyword">KEY</span> <span class="string">`idx_1`</span> (<span class="string">`null_key`</span>) <span class="keyword">USING</span> BTREE,</span><br><span class="line"> <span class="keyword">KEY</span> <span class="string">`idx_2`</span> (<span class="string">`null_key1`</span>) <span class="keyword">USING</span> BTREE,</span><br><span class="line"> <span class="keyword">KEY</span> <span class="string">`idx_3`</span> (<span class="string">`null_key2`</span>) <span class="keyword">USING</span> BTREE</span><br><span class="line">) <span class="keyword">ENGINE</span>=<span class="keyword">InnoDB</span> <span class="keyword">DEFAULT</span> <span class="keyword">CHARSET</span>=utf8mb4;</span><br></pre></td></tr></table></figure>
<p>用个存储过程来插入数据</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line"></span><br><span class="line">delimiter $ <span class="comment">#以delimiter来标记用$表示存储过程结束</span></span><br><span class="line"><span class="keyword">create</span> <span class="keyword">procedure</span> nullIndex1()</span><br><span class="line"><span class="keyword">begin</span></span><br><span class="line"><span class="keyword">declare</span> i <span class="built_in">int</span>; </span><br><span class="line"><span class="keyword">declare</span> j <span class="built_in">int</span>; </span><br><span class="line"><span class="keyword">set</span> i=<span class="number">1</span>;</span><br><span class="line"><span class="keyword">set</span> j=<span class="number">1</span>;</span><br><span class="line">while(i&lt;=100) do </span><br><span class="line"> while(j&lt;=100) do </span><br><span class="line"> IF (i % 3 = 0) THEN</span><br><span class="line"> <span class="keyword">INSERT</span> <span class="keyword">INTO</span> null_index_t ( <span class="string">`null_key`</span>, <span class="string">`null_key1`</span>, <span class="string">`null_key2`</span> ) <span class="keyword">VALUES</span> (<span class="literal">null</span> , <span class="keyword">LEFT</span>(<span class="keyword">MD5</span>(<span class="keyword">RAND</span>()), <span class="number">8</span>), <span class="keyword">LEFT</span>(<span class="keyword">MD5</span>(<span class="keyword">RAND</span>()), <span class="number">8</span>));</span><br><span class="line"> ELSEIF (i % 3 = 1) THEN</span><br><span class="line"> <span class="keyword">INSERT</span> <span class="keyword">INTO</span> null_index_t ( <span class="string">`null_key`</span>, <span class="string">`null_key1`</span>, <span class="string">`null_key2`</span> ) <span class="keyword">VALUES</span> (<span class="keyword">LEFT</span>(<span class="keyword">MD5</span>(<span class="keyword">RAND</span>()), <span class="number">8</span>), <span class="literal">NULL</span>, <span class="keyword">LEFT</span>(<span class="keyword">MD5</span>(<span class="keyword">RAND</span>()), <span class="number">8</span>));</span><br><span class="line"> ELSE</span><br><span class="line"> <span class="keyword">INSERT</span> <span class="keyword">INTO</span> null_index_t ( <span class="string">`null_key`</span>, <span class="string">`null_key1`</span>, <span class="string">`null_key2`</span> ) <span class="keyword">VALUES</span> (<span class="keyword">LEFT</span>(<span class="keyword">MD5</span>(<span class="keyword">RAND</span>()), <span class="number">8</span>), <span class="keyword">LEFT</span>(<span class="keyword">MD5</span>(<span class="keyword">RAND</span>()), <span class="number">8</span>), <span class="literal">NULL</span>);</span><br><span class="line"> <span class="keyword">END</span> <span class="keyword">IF</span>;</span><br><span class="line"> <span class="keyword">set</span> j=j+<span class="number">1</span>;</span><br><span class="line"> <span class="keyword">end</span> <span class="keyword">while</span>;</span><br><span class="line"> <span class="keyword">set</span> i=i+<span class="number">1</span>;</span><br><span class="line"> <span class="keyword">set</span> j=<span class="number">1</span>; </span><br><span class="line"><span class="keyword">end</span> <span class="keyword">while</span>;</span><br><span class="line"><span class="keyword">end</span> </span><br><span class="line">$</span><br><span class="line"><span class="keyword">call</span> nullIndex1();</span><br></pre></td></tr></table></figure>
<p>然后看下我们的 is null 查询</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">EXPLAIN</span> <span class="keyword">select</span> * <span class="keyword">from</span> null_index_t <span class="keyword">WHERE</span> null_key <span class="keyword">is</span> <span class="literal">null</span>;</span><br></pre></td></tr></table></figure>
<p><img data-src="https://gitee.com/nicksxs/images/raw/master/uPic/IejArR.png" alt=""><br>再来看看另一个</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">EXPLAIN</span> <span class="keyword">select</span> * <span class="keyword">from</span> null_index_t <span class="keyword">WHERE</span> null_key <span class="keyword">is</span> <span class="keyword">not</span> <span class="literal">null</span>;</span><br></pre></td></tr></table></figure>
<p><img data-src="https://gitee.com/nicksxs/images/raw/master/uPic/vwttcE.png" alt=""><br>从这里能看出来啥呢，可以思考下</p>
<p>从上面可以发现，<code>is null</code>应该是用上了索引了，所以至少不是一刀切不能用，但是看着<code>is not null</code>好像不太行额<br>我们在做一点小改动，把这个表里的数据改成 9100 条是 null，剩下 900 条是有值的，然后再执行下<br><img data-src="https://gitee.com/nicksxs/images/raw/master/uPic/McIoej.png" alt=""><br>然后再来看看执行结果</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">EXPLAIN</span> <span class="keyword">select</span> * <span class="keyword">from</span> null_index_t <span class="keyword">WHERE</span> null_key <span class="keyword">is</span> <span class="literal">null</span>;</span><br></pre></td></tr></table></figure>
<p><img data-src="https://gitee.com/nicksxs/images/raw/master/uPic/i4ki84.png" alt=""></p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">EXPLAIN</span> <span class="keyword">select</span> * <span class="keyword">from</span> null_index_t <span class="keyword">WHERE</span> null_key <span class="keyword">is</span> <span class="keyword">not</span> <span class="literal">null</span>;</span><br></pre></td></tr></table></figure>
<p><img data-src="https://gitee.com/nicksxs/images/raw/master/uPic/1HKVQH.png" alt=""><br>是不是不一样了，这里再补充下我试验使用的 mysql 是 5.7 的，不保证在其他版本的一致性，<br>其实可以看出随着数据量的变化，mysql 会不会使用索引是会变化的，不是说 is not null 一定会使用，也不是一定不会使用，而是优化器会根据查询成本做个预判，这个预判尽可能会减小查询成本，主要包括回表啥的，但是也不一定完全准确。</p>
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class="tag-list-item"><a class="tag-list-link" href="/tags/Mybatis/" rel="tag">Mybatis</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Mysql/" rel="tag">Mysql</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/NameServer/" rel="tag">NameServer</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/PHP/" rel="tag">PHP</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Preorder-Traversal/" rel="tag">Preorder Traversal</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/RPC/" rel="tag">RPC</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Redis/" rel="tag">Redis</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/RocketMQ/" rel="tag">RocketMQ</a><span class="tag-list-count">4</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/SPI/" rel="tag">SPI</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Servlet/" rel="tag">Servlet</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Singleton/" rel="tag">Singleton</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Spring/" rel="tag">Spring</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Sql%E6%B3%A8%E5%85%A5/" rel="tag">Sql注入</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Stream/" rel="tag">Stream</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Thread-dump/" rel="tag">Thread dump</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Tomcat/" rel="tag">Tomcat</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Web/" rel="tag">Web</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/Webhook/" rel="tag">Webhook</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/aqs/" rel="tag">aqs</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/await/" rel="tag">await</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/bloom-filter/" rel="tag">bloom filter</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/c/" rel="tag">c++</a><span class="tag-list-count">14</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/cgroup/" rel="tag">cgroup</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/cluster/" rel="tag">cluster</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/condition/" rel="tag">condition</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/docker/" rel="tag">docker</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/echo/" rel="tag">echo</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/environment/" rel="tag">environment</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/gap-lock/" rel="tag">gap lock</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/grep/" rel="tag">grep</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/hadoop/" rel="tag">hadoop</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/icu4c/" rel="tag">icu4c</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/im/" rel="tag">im</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/is-not-null/" rel="tag">is not null</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/is-null/" rel="tag">is null</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/j-u-c/" rel="tag">j.u.c</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/java/" rel="tag">java</a><span class="tag-list-count">11</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/leetcode/" rel="tag">leetcode</a><span class="tag-list-count">21</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/leetcode-155/" rel="tag">leetcode 155</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/linked-list/" rel="tag">linked list</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/linux/" rel="tag">linux</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/lock/" rel="tag">lock</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/mfc/" rel="tag">mfc</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/min-stack/" rel="tag">min stack</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/mq/" rel="tag">mq</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/mvcc/" rel="tag">mvcc</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/mysql/" rel="tag">mysql</a><span class="tag-list-count">5</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/namespace/" rel="tag">namespace</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/next-key-lock/" rel="tag">next-key lock</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/nginx/" rel="tag">nginx</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/nullsfirst/" rel="tag">nullsfirst</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/openresty/" rel="tag">openresty</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/php/" rel="tag">php</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/procedure/" rel="tag">procedure</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/python/" rel="tag">python</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/read-view/" rel="tag">read view</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/redis/" rel="tag">redis</a><span class="tag-list-count">8</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/signal/" rel="tag">signal</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/sort/" rel="tag">sort</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/spark/" rel="tag">spark</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/stack/" rel="tag">stack</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/string/" rel="tag">string</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/swoole/" rel="tag">swoole</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/uname/" rel="tag">uname</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/unlock/" rel="tag">unlock</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/value/" rel="tag">value</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/websocket/" rel="tag">websocket</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/zsh/" rel="tag">zsh</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%89%E9%98%B6%E6%AE%B5%E6%8F%90%E4%BA%A4/" rel="tag">三阶段提交</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%A4%E9%98%B6%E6%AE%B5%E6%8F%90%E4%BA%A4/" rel="tag">两阶段提交</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%AD%E5%BA%8F/" rel="tag">中序</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%B8%AD%E9%97%B4%E4%BB%B6/" rel="tag">中间件</a><span class="tag-list-count">4</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%BA%8C%E5%8F%89%E6%A0%91/" rel="tag">二叉树</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E4%BA%92%E6%96%A5%E9%94%81/" rel="tag">互斥锁</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%85%AC%E4%BA%A4/" rel="tag">公交</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%85%AC%E4%BA%A4%E8%BD%A6/" rel="tag">公交车</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%87%8F%E8%82%A5/" rel="tag">减肥</a><span class="tag-list-count">6</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%88%86%E5%B8%83%E5%BC%8F%E4%BA%8B%E5%8A%A1/" rel="tag">分布式事务</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%88%86%E5%B8%83%E5%BC%8F%E9%94%81/" rel="tag">分布式锁</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%89%8A%E5%B3%B0%E5%A1%AB%E8%B0%B7/" rel="tag">削峰填谷</a><span class="tag-list-count">4</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%89%8D%E5%BA%8F/" rel="tag">前序</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8A%A0%E5%A1%9E/" rel="tag">加塞</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8D%95%E4%BE%8B/" rel="tag">单例</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8D%9A%E5%AE%A2%EF%BC%8C%E6%96%87%E7%AB%A0/" rel="tag">博客，文章</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8F%91%E8%A1%8C%E7%89%88/" rel="tag">发行版</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%8F%A3%E7%BD%A9/" rel="tag">口罩</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%90%90%E6%A7%BD/" rel="tag">吐槽</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%9F%BA%E7%A1%80%E8%AE%BE%E6%96%BD/" rel="tag">基础设施</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%AE%B9%E9%94%99%E6%9C%BA%E5%88%B6/" rel="tag">容错机制</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%AF%84%E7%94%9F%E8%99%AB/" rel="tag">寄生虫</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B0%8F%E6%8A%80%E5%B7%A7/" rel="tag">小技巧</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B8%83%E9%9A%86%E8%BF%87%E6%BB%A4%E5%99%A8/" rel="tag">布隆过滤器</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B9%B2%E6%B4%BB/" rel="tag">干活</a><span class="tag-list-count">5</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B9%B4%E4%B8%AD%E6%80%BB%E7%BB%93/" rel="tag">年中总结</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B9%B4%E7%BB%88%E6%80%BB%E7%BB%93/" rel="tag">年终总结</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B9%B6%E5%8F%91/" rel="tag">并发</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B9%B8%E7%A6%8F%E4%BA%86%E5%90%97/" rel="tag">幸福了吗</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%B9%BB%E8%AF%BB/" rel="tag">幻读</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%BC%80%E8%BD%A6/" rel="tag">开车</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E5%BD%B1%E8%AF%84/" rel="tag">影评</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%89%93%E5%8D%A1/" rel="tag">打卡</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%8E%92%E5%BA%8F/" rel="tag">排序</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84/" rel="tag">数据结构</a><span class="tag-list-count">11</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%9C%80%E5%B0%8F%E6%A0%88/" rel="tag">最小栈</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%9D%80%E4%BA%BA%E8%AF%9B%E5%BF%83/" rel="tag">杀人诛心</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%9D%AD%E5%B7%9E/" rel="tag">杭州</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%B3%A8%E8%A7%A3/" rel="tag">注解</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%B6%88%E6%81%AF%E9%98%9F%E5%88%97/" rel="tag">消息队列</a><span class="tag-list-count">4</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%BA%90%E7%A0%81/" rel="tag">源码</a><span class="tag-list-count">11</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E6%BA%90%E7%A0%81%E8%A7%A3%E6%9E%90/" rel="tag">源码解析</a><span class="tag-list-count">3</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E7%94%9F%E6%B4%BB/" rel="tag">生活</a><span class="tag-list-count">13</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E7%96%AB%E6%83%85/" rel="tag">疫情</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E7%B3%9F%E5%BF%83%E4%BA%8B/" rel="tag">糟心事</a><span class="tag-list-count">2</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E7%B4%A2%E5%BC%95/" rel="tag">索引</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E7%BC%93%E5%AD%98/" rel="tag">缓存</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E7%BC%93%E5%AD%98%E5%87%BB%E7%A9%BF/" rel="tag">缓存击穿</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E7%BC%93%E5%AD%98%E7%A9%BF%E9%80%8F/" rel="tag">缓存穿透</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a class="tag-list-link" href="/tags/%E7%BC%93%E5%AD%98%E9%9B%AA%E5%B4%A9/" rel="tag">缓存雪崩</a><span class="tag-list-count">1</span></li><li class="tag-list-item"><a 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