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<a class="post-title-link" href="/2018/04/25/sql查询优化---order by /" itemprop="url">sql查询优化-- 索引命中</a></h1>

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<p>　　最近遇到一个sql优化的场景，记录分享一下。</p>

<p> <strong>下图是我们服务的详情页，出现的问题是什么呢？</strong>在用户查询发送短信详情的情况下，用户的页面请求迟迟不反回，然后报错。</p>

<p>　<strong>问题的原因是什么呢</strong>，保障的客户是我们服务的大客户，消息量非常大，　目前为止已经这张详情表已经有５千万左右的数据量。这个页面我们的处理有两个，一个是查数据，另一个就是count一个总条数，在现网分别分析了下两条sql都在几百秒。因此问题很明显在sql这两条语句上面, 那么一个一个分析。<br>　 <strong>第一条慢sql如下</strong><br> <figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">SELECT ENDPOINT, NATION_CODE, MESSAGE_ID, SIGN_ID, CREATE_TIME, DELIVER_TIME, STATUS, CODE WHERE OWNER_ID = ? AND CREATE_TIME > ? AND CREAT_TIME < ? ORDER BY ID DESC LIMIT ? OFFSET ?;</span><br></pre></td></tr></table></figure></p>

<pre><code>　　这张表当前索引情况：、、、、

　　explan 发现 seq scan，没命中索引，在这种数据量下全表扫肯定不行。另一个问题是这个表的排序用的是quicksort, 理论上b+tree是递增排序的，因此可以省去排序的时间。

</code></pre><p>　<strong>解决办法</strong><br>　　1. 添加一条（CREAT_TIME, OWNER_ID）的联合索引<br>　　２. Order by ID DESC 改为 Order by CREAT_TIME DESC<br>　　改第一条的原因是因为主要产生筛选作用的是CREATE_TIME, 为了命中索引因此做这个改进。<br>　　改第二条的原因是当Order by 的关键字在Where中有，才不会filesort.<br>　<strong>第二条慢sql</strong><br>　　产生第二条慢sql的原因就是因为使用count，对于带where的count的sql语句的优化空间非常小，因为是实时的计数，扫描数据量根据查询范围可能非常大。<br>　<strong>解决办法</strong><br>　这块的解决办法有很多，首先使用缓存count是不行的，因为查询中带实时性。那么剩下的解决办法就是去掉这个count，不给用户反馈了。 </p>

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