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<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div></div> <!----></nav> <!----> </aside> <div class="password-shadow password-wrapper-in" style="display:none;" data-v-59e6cb88 data-v-7dd95ae2><h3 class="title" data-v-59e6cb88>SQL优化</h3> <!----> <label id="box" class="inputBox" data-v-59e6cb88><input type="password" value="" data-v-59e6cb88> <span data-v-59e6cb88>Konck! Knock!</span> <button data-v-59e6cb88>OK</button></label> <div class="footer" data-v-59e6cb88><span data-v-59e6cb88><i class="iconfont reco-theme" data-v-59e6cb88></i> <a target="blank" href="https://vuepress-theme-reco.recoluan.com" data-v-59e6cb88>vuePress-theme-reco</a></span> <span data-v-59e6cb88><i class="iconfont reco-copyright" data-v-59e6cb88></i> <a data-v-59e6cb88><span data-v-59e6cb88>apan</span>
<span data-v-59e6cb88>2015 - </span>
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</a></span></div></div> <div data-v-7dd95ae2><div data-v-7dd95ae2><main class="page"><section style="display:;"><div class="page-title"><h1 class="title">SQL优化</h1> <div data-v-8a445198><i class="iconfont reco-account" data-v-8a445198><span data-v-8a445198>apan</span></i> <i class="iconfont reco-date" data-v-8a445198><span data-v-8a445198>11/22/2022</span></i> <i class="iconfont reco-eye" data-v-8a445198><span id="/8.html" data-flag-title="Your Article Title" class="leancloud-visitors" data-v-8a445198><a class="leancloud-visitors-count" style="font-size:.9rem;font-weight:normal;color:#999;"></a></span></i> <i class="tags iconfont reco-tag" data-v-8a445198><span class="tag-item" data-v-8a445198>sql</span><span class="tag-item" data-v-8a445198>后端</span></i></div></div> <div class="theme-reco-content content__default"><div class="adswrapper"><!----> <div style="display:none;"></div> <ins data-ad-layout="in-article" data-ad-format="fluid" data-ad-client="ca-pub-3196441204212737" data-ad-slot="7162284157" data-ad-test="" data-ad-region="" class="adsbygoogle" style="display:block;"></ins> <!----> <div style="display:none;"> (adsbygoogle = window.adsbygoogle || []).push({}); </div></div> <h2 id="索引"><a href="#索引" class="header-anchor">#</a> 索引</h2> <p>空间换时间，索引的体积大约是表的1.5倍</p> <p>一般设置在表的某些列上</p> <p>两个列合起来索引是复合索引</p> <p>索引只能放在硬盘中，所以磁盘的IO次数决定了查询性能</p> <h4 id="二叉树"><a href="#二叉树" class="header-anchor">#</a> 二叉树</h4> <p>B树(Balance Tree)</p> <p>高度小于二叉树</p> <p>P表示指针</p> <p>B+树</p> <h4 id="应该加索引"><a href="#应该加索引" class="header-anchor">#</a> 应该加索引</h4> <ol><li><p>经常被查询的列并且数据量比较大</p></li> <li><p>自增主键默认索引</p></li></ol> <h4 id="不需要加索引的列"><a href="#不需要加索引的列" class="header-anchor">#</a> 不需要加索引的列</h4> <p>数据离散值低(数据重复率高，例如性别)</p> <h3 id="测试"><a href="#测试" class="header-anchor">#</a> 测试</h3> <p>添加索引前，大约0.5s</p> <p>添加索引后，大约25ms，差了20倍</p> <p>默认添加的索引是BTREE</p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">from</span> product <span class="token keyword">WHERE</span> title <span class="token operator">=</span> <span class="token string">"998083趾饭"</span>
<span class="token keyword">CREATE</span> <span class="token keyword">INDEX</span> index_title <span class="token keyword">on</span> product<span class="token punctuation">(</span>title<span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h3 id="hash索引"><a href="#hash索引" class="header-anchor">#</a> HASH索引</h3> <p>Hash函数： 值 ==> 数</p> <p>Hash索引： 值 ==> 桶 ==> 数据行</p> <p>应用条件更为苛刻</p> <p>最常用的索引也就是B-tree索引和Hash索引，且只有<code>Memory</code>，<code>NDB</code>两种引擎支持Hash索引。</p> <p><img src="C:%5CUsers%5CPanXin%5CAppData%5CRoaming%5Cmarktext%5Cimages%5C2022-11-21-10-29-16-image.png" alt=""></p> <h3 id="联合索引"><a href="#联合索引" class="header-anchor">#</a> 联合索引</h3> <p>两个列合起来是一个索引</p> <h4 id="联合索引的最左原则"><a href="#联合索引的最左原则" class="header-anchor">#</a> 联合索引的最左原则：</h4> <p>一定要按照顺序查询，最左侧列一定要出现</p> <h3 id="索引失效"><a href="#索引失效" class="header-anchor">#</a> 索引失效</h3> <ol><li><p>%%,双%like会使索引失效</p></li> <li><p>右侧单%，索引正常</p></li> <li><p>or的左右两侧有一个没有索引会失效</p></li> <li><p>!= > <</p></li> <li><p>索引提高了查询的效率，但也降低了增删改的效率</p></li> <li><p>尽量使用数字类型字段</p></li></ol> <h3 id="执行计划"><a href="#执行计划" class="header-anchor">#</a> 执行计划</h3> <p>语句前加<code>explain</code>可以查看执行时是否有索引等信息</p> <ol><li><p>rows 扫描行数，不准，一般值越小越好</p></li> <li></li></ol> <p>EXPLAIN SELECT * from product WHERE title = "998105驼跺"</p> <p>id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE product ref index_title index_title 767 const 1 Using where</p> <h3 id="_5-性能分析"><a href="#_5-性能分析" class="header-anchor">#</a> 5. 性能分析</h3> <p>show global status like "com_______"</p> <h4 id="慢查询日志"><a href="#慢查询日志" class="header-anchor">#</a> 慢查询日志</h4> <p>SHOW GLOBAL VARIABLES LIKE '%log%';</p> <p>开启方法</p> <h3 id="_6-mysql查询优化方法-重点"><a href="#_6-mysql查询优化方法-重点" class="header-anchor">#</a> 6. Mysql查询优化方法<mark> 重点</mark></h3> <p>思路：便面全表扫描</p> <ol><li><p>禁止用* 来查询，需要指定字段</p></li> <li><p><mark>in的个数在1000个以内</mark></p></li> <li><p>查询一条数据使用limit 1</p></li> <li><p>尽量使用inner join 避免使用left join</p></li> <li><p>!= 用 > or < 替换</p></li> <li><p>between代替in</p></li> <li><p>exist 代替in</p></li></ol> <p>例如：优化前10s，优化后5s</p> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">from</span> product <span class="token keyword">WHERE</span> title <span class="token operator">IN</span> <span class="token punctuation">(</span><span class="token keyword">SELECT</span> title <span class="token keyword">FROM</span> product <span class="token keyword">WHERE</span> title <span class="token operator">=</span> <span class="token string">'24吻'</span> <span class="token operator">or</span> title <span class="token operator">=</span> <span class="token string">'25折'</span><span class="token punctuation">)</span>
<span class="token keyword">SELECT</span> <span class="token operator">*</span> <span class="token keyword">FROM</span> product p <span class="token keyword">WHERE</span> <span class="token keyword">EXISTS</span> <span class="token punctuation">(</span><span class="token keyword">SELECT</span> <span class="token number">0</span> <span class="token keyword">FROM</span> product t <span class="token keyword">WHERE</span> p<span class="token punctuation">.</span>id <span class="token operator">=</span> t<span class="token punctuation">.</span>id <span class="token operator">and</span> <span class="token punctuation">(</span>p<span class="token punctuation">.</span>title <span class="token operator">=</span> <span class="token string">'24吻'</span> <span class="token operator">or</span> p<span class="token punctuation">.</span>title <span class="token operator">=</span><span class="token string">'25折'</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><ol start="8"><li><p>left josin 代替in</p></li> <li><p>尽量不适用mysql自己的函数</p></li> <li><p>使用union all 代替 union</p></li></ol> <h2 id="主从备份"><a href="#主从备份" class="header-anchor">#</a> 主从备份</h2> <p>双机热备就是使用MySQL提供的一种主从备份机制实现。所谓双机热备其实是一个复制的过程，复制过程中一个服务器充当主服务器，一个或多个服务器充当从服务。这个复制的过程实质上是从服务器复制主服务器上MySQL的二进制日志（bin-log），并在从服务器上还原主服务器上的操作。</p> <h3 id="要求"><a href="#要求" class="header-anchor">#</a> 要求</h3> <ol><li><p>MySQL的版本高于3.2</p></li> <li><p>从服务器的MySQL版本不可以低于主服务器的数据库版本</p></li></ol> <h3 id="操作"><a href="#操作" class="header-anchor">#</a> 操作</h3> <h4 id="_1-主服务器授权"><a href="#_1-主服务器授权" class="header-anchor">#</a> 1. 主服务器授权</h4> <div class="language-sql line-numbers-mode"><pre class="language-sql"><code><span class="token keyword">GRANT</span> <span class="token keyword">FILE</span><span class="token punctuation">,</span><span class="token keyword">SELECT</span><span class="token punctuation">,</span><span class="token keyword">REPLICATION</span> SLAVE <span class="token keyword">ON</span> <span class="token operator">*</span><span class="token punctuation">.</span><span class="token operator">*</span> <span class="token keyword">TO</span> <span class="token keyword">backup</span><span class="token variable">@192.168.11.40</span> IDENTIFIED <span class="token keyword">BY</span> <span class="token string">'123456'</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div><h4 id="_2-同步数据"><a href="#_2-同步数据" class="header-anchor">#</a> 2. 同步数据</h4> <h4 id="_3-配置主服务器"><a href="#_3-配置主服务器" class="header-anchor">#</a> 3. 配置主服务器</h4> <p>停止MySQL服务，修改主的my.ini，找到mysqId</p> <div class="language-ini line-numbers-mode"><pre class="language-ini"><code><span class="token section"><span class="token punctuation">[</span><span class="token section-name selector">mysqld</span><span class="token punctuation">]</span></span>
<span class="token key attr-name">server-id</span><span class="token punctuation">=</span><span class="token value attr-value">1</span>
<span class="token comment">#是1~2^23-1内的唯一值且不能与B或其它slave服务中的配置相同</span>
<span class="token key attr-name">log-bin</span> <span class="token punctuation">=</span> <span class="token value attr-value">bin_log</span>
<span class="token comment">#日志文件以bin_log为前缀,如果不给log-bin赋值,日志文件将以#master-server-hostname为前缀</span>
<span class="token key attr-name">binlog-ignore-db</span><span class="token punctuation">=</span> <span class="token value attr-value">test</span>
<span class="token comment">#日志文件跳过的数据库(可选属性 ),如果有多个数据库，可以重复配置这个属性</span>
<span class="token key attr-name">binlog-do-db</span><span class="token punctuation">=</span> <span class="token value attr-value">db_test</span>
<span class="token comment">#日志文件操作的数据库(可选属性 ，默认所有数据库的相关操作都写入二进制日志文件) ,如果有多个数据库，可以重复配置这个属性</span>
<span class="token key attr-name">expire_logs_days</span><span class="token punctuation">=</span><span class="token value attr-value">30</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div><p>启动MySQL</p> <h4 id="_4-修改从服务器"><a href="#_4-修改从服务器" class="header-anchor">#</a> 4. 修改从服务器</h4> <p>修改/etc/my.cnf</p> <div class="language-ini line-numbers-mode"><pre class="language-ini"><code><span class="token key attr-name">server-id</span><span class="token punctuation">=</span><span class="token value attr-value">2</span>
<span class="token comment">#唯一并与主服务器上的server-id不同。</span>
<span class="token key attr-name">replicate-do-db</span><span class="token punctuation">=</span><span class="token value attr-value">db_test</span>
<span class="token comment">#复制操作要针对的数据库(可选，默认为全部)，同样如果有多个数据库则可配置多次本属性。</span>
<span class="token key attr-name">expire_logs_days</span><span class="token punctuation">=</span><span class="token value attr-value">30</span>
<span class="token comment">#设置bin-log日志保存的天数</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><div class="language-bash line-numbers-mode"><pre class="language-bash"><code>systemctl restart mysqld.service <span class="token comment">#重启 mysql</span>
用mysql <span class="token parameter variable">-u</span> root <span class="token parameter variable">-p</span> 登录MySQL，并执行如下命令：
mysql<span class="token operator">></span>change master to <span class="token punctuation">\</span>
-<span class="token operator">></span>master_host<span class="token operator">=</span><span class="token string">'192.168.11.184'</span>,
-<span class="token operator">></span>master_user<span class="token operator">=</span><span class="token string">'backup'</span>,
-<span class="token operator">></span>master_password<span class="token operator">=</span><span class="token string">'123456'</span><span class="token punctuation">;</span>
systemctl restart mysqld.service <span class="token comment">#重启 mysql</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><ol><li><p>start slave;</p></li> <li><p>检查状态show slave status \G；</p></li></ol> <p>必须两个都是yes，如果是connecting，关闭下防火墙试一下，进一步在主数据库插入一条数据后在从数据库检查下有没有数据</p> <p>Slave_IO_Running: Yes
Slave_SQL_Running: Yes</p> <div class="language-pure line-numbers-mode"><pre class="language-pure"><code><span class="token operator">***************************</span> <span class="token number">1</span><span class="token operator">.</span> <span class="token function">row</span> <span class="token operator">***************************</span>
Slave_IO_State<span class="token operator">:</span> Waiting for master to send event
Master_Host<span class="token operator">:</span> <span class="token number">192.168</span><span class="token number">.11</span><span class="token number">.184</span>
Master_User<span class="token operator">:</span> backup
Master_Port<span class="token operator">:</span> <span class="token number">3306</span>
Connect_Retry<span class="token operator">:</span> <span class="token number">60</span>
Master_Log_File<span class="token operator">:</span> bin_log<span class="token operator">.</span><span class="token number">000001</span>
Read_Master_Log_Pos<span class="token operator">:</span> <span class="token number">107</span>
Relay_Log_File<span class="token operator">:</span> localhost<span class="token operator">-</span>relay<span class="token operator">-</span>bin<span class="token operator">.</span><span class="token number">000004</span>
Relay_Log_Pos<span class="token operator">:</span> <span class="token number">302</span>
Relay_Master_Log_File<span class="token operator">:</span> bin_log<span class="token operator">.</span><span class="token number">000001</span>
Slave_IO_Running<span class="token operator">:</span> Yes
Slave_SQL_Running<span class="token operator">:</span> Yes
Replicate_Do_DB<span class="token operator">:</span> security
Replicate_Ignore_DB<span class="token operator">:</span>
Replicate_Do_Table<span class="token operator">:</span>
Replicate_Ignore_Table<span class="token operator">:</span>
Replicate_Wild_Do_Table<span class="token operator">:</span>
Replicate_Wild_Ignore_Table<span class="token operator">:</span>
Last_Errno<span class="token operator">:</span> <span class="token number">0</span>
Last_Error<span class="token operator">:</span>
Skip_Counter<span class="token operator">:</span> <span class="token number">0</span>
Exec_Master_Log_Pos<span class="token operator">:</span> <span class="token number">107</span>
Relay_Log_Space<span class="token operator">:</span> <span class="token number">513</span>
Until_Condition<span class="token operator">:</span> None
Until_Log_File<span class="token operator">:</span>
Until_Log_Pos<span class="token operator">:</span> <span class="token number">0</span>
Master_SSL_Allowed<span class="token operator">:</span> No
Master_SSL_CA_File<span class="token operator">:</span>
Master_SSL_CA_Path<span class="token operator">:</span>
Master_SSL_Cert<span class="token operator">:</span>
Master_SSL_Cipher<span class="token operator">:</span>
Master_SSL_Key<span class="token operator">:</span>
Seconds_Behind_Master<span class="token operator">:</span> <span class="token number">0</span>
Master_SSL_Verify_Server_Cert<span class="token operator">:</span> No
Last_IO_Errno<span class="token operator">:</span> <span class="token number">0</span>
Last_IO_Error<span class="token operator">:</span>
Last_SQL_Errno<span class="token operator">:</span> <span class="token number">0</span>
Last_SQL_Error<span class="token operator">:</span>
Replicate_Ignore_Server_Ids<span class="token operator">:</span>
Master_Server_Id<span class="token operator">:</span> <span class="token number">1</span>
Master_UUID<span class="token operator">:</span>
Master_Info_File<span class="token operator">:</span> <span class="token operator">/</span><span class="token function">var</span><span class="token operator">/</span>lib<span class="token operator">/</span>mysql<span class="token operator">/</span>master<span class="token operator">.</span>info
SQL_Delay<span class="token operator">:</span> <span class="token number">0</span>
SQL_Remaining_Delay<span class="token operator">:</span> <span class="token keyword">NULL</span>
Slave_SQL_Running_State<span class="token operator">:</span> Slave has read <span class="token function">all</span> relay log<span class="token punctuation">;</span> waiting for more updates
Master_Retry_Count<span class="token operator">:</span> <span class="token number">86400</span>
Master_Bind<span class="token operator">:</span>
Last_IO_Error_Timestamp<span class="token operator">:</span>
Last_SQL_Error_Timestamp<span class="token operator">:</span>
Master_SSL_Crl<span class="token operator">:</span>
Master_SSL_Crlpath<span class="token operator">:</span>
Retrieved_Gtid_Set<span class="token operator">:</span>
Executed_Gtid_Set<span class="token operator">:</span>
Auto_Position<span class="token operator">:</span> <span class="token number">0</span>
Replicate_Rewrite_DB<span class="token operator">:</span>
Channel_Name<span class="token operator">:</span>
Master_TLS_Version<span class="token operator">:</span>
<span class="token number">1</span> <span class="token function">row</span> in set <span class="token punctuation">(</span><span class="token number">0.00</span> sec<span class="token punctuation">)</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span 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class="line-number">50</span><br><span class="line-number">51</span><br><span class="line-number">52</span><br><span class="line-number">53</span><br><span class="line-number">54</span><br><span class="line-number">55</span><br><span class="line-number">56</span><br><span class="line-number">57</span><br><span class="line-number">58</span><br><span class="line-number">59</span><br></div></div><h3 id="失败的解决办法"><a href="#失败的解决办法" class="header-anchor">#</a> 失败的解决办法</h3> <p><img src="https://jsd.cdn.zzko.cn/gh/pxpy/img@master/lQLPJxbk9Cy0gznNAw7NA0iwDiEo4xB9fiMDeC8mxoCbAA_840_782.6gxpbjllu2g0.webp" alt="lQLPJxbk9Cy0gznNAw7NA0iwDiEo4xB9fiMDeC8mxoCbAA_840_782"></p> <p><a href="https://blog.csdn.net/zzddada/article/details/113352717" target="_blank" rel="nofollow noopener noreferrer">MySQL主从同步故障：Slave_SQL_Running:No 两种解决办法_Lucky@Dong的博客-CSDN博客<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a><a href="https://blog.csdn.net/zzddada/article/details/113352717" target="_blank" rel="nofollow noopener noreferrer">MySQL主从同步故障：Slave_SQL_Running:No 两种解决办法_Lucky@Dong的博客-CSDN博客<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <h3 id="spring-boot-读写分离"><a href="#spring-boot-读写分离" class="header-anchor">#</a> spring boot 读写分离</h3> <p><a href="https://gitee.com/baomidou/dynamic-datasource-spring-boot-starter" target="_blank" rel="nofollow noopener noreferrer">dynamic-datasource-spring-boot-starter: 基于 SpringBoot 多数据源 动态数据源 主从分离 快速启动器 支持分布式事务 (gitee.com)<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></p> <p>如果数据库挂了一个，例如从服务器，会一直报错，项目也不能启动，需要注释不能使用的数据源</p> 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