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<p> 今天查看数据库 CSVLOG， 有个慢查询语句, 这个优化有点意思，记录下来，下面是详细的步骤。</p>
<p>1 数据库CSVLOG<br><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></pre></td><td class="code"><pre><span class="line">2011-03-01 17:24:49.442 CST,"skytf","skytf",21500,"192.168.164.37:39154",4d1d3830.53fc,7950,"<span class="keyword">SELECT</span><span class="string">",2010-12-31 09:56:00 CST,16/3143301,0,LOG,00000,"</span><span class="keyword">duration</span>: <span class="number">1701.007</span> ms <span class="keyword">execute</span> <unnamed>: </span><br><span class="line"><span class="keyword">SELECT</span> <span class="keyword">id</span>, modifier, gmt_create <span class="keyword">as</span> gmtCreate, gmt_modified <span class="keyword">as</span> gmtModified, creator, </span><br><span class="line">is_deleted <span class="keyword">as</span> isDeleted, mpxyzdatas, skyid, wap_token <span class="keyword">as</span> wapToken, app_token <span class="keyword">as</span> appToken, <span class="keyword">source</span>, </span><br><span class="line">username, passwd, nickname, mpxyzdataf, mpxyzdatag, phone, phone2, mpxyzdatae, f <span class="keyword">FROM</span> tmp_user </span><br><span class="line"><span class="keyword">WHERE</span> is_deleted=<span class="string">'n'</span> <span class="keyword">AND</span> mpxyzdatas=$<span class="number">1</span> </span><br><span class="line"><span class="keyword">order</span> <span class="keyword">by</span> <span class="keyword">id</span> <span class="keyword">desc</span> <span class="string">","</span><span class="keyword">parameters</span>: $<span class="number">1</span> = <span class="string">')dzyDlyyLdyyfjzy)vzy'</span><span class="string">",,,,,,,,""</span></span><br></pre></td></tr></table></figure></p>
<p>根据 duration: 1701.007 ，这个语句执行时间在1.7 s左右，而且数据库日志抛出大量这个慢SQL语句。</p>
<p>2 慢查询 SQL<br><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">SELECT</span> <span class="keyword">id</span>, modifier, gmt_create <span class="keyword">as</span> gmtCreate, gmt_modified <span class="keyword">as</span> gmtModified, creator, is_deleted <span class="keyword">as</span> isDeleted, </span><br><span class="line">mpxyzdatas, skyid, wap_token <span class="keyword">as</span> wapToken, app_token <span class="keyword">as</span> appToken, <span class="keyword">source</span>, username, passwd, nickname, </span><br><span class="line">mpxyzdataf, mpxyzdatag, phone, phone2, mpxyzdatae, f <span class="keyword">FROM</span> tmp_user </span><br><span class="line"><span class="keyword">WHERE</span> is_deleted=<span class="string">'n'</span> <span class="keyword">AND</span> mpxyzdatas=$<span class="number">1</span> <span class="keyword">order</span> <span class="keyword">by</span> <span class="keyword">id</span> <span class="keyword">desc</span> ; </span><br><span class="line">parameters: $1 = ')dzyL5zyzpzy)rzy!Pzy'",,,,,,,,"</span><br></pre></td></tr></table></figure></p>
<p>3 老的执行计划<br><figure class="highlight clean"><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></pre></td><td class="code"><pre><span class="line">skytf=> explain analyze SELECT id, modifier, gmt_create <span class="keyword">as</span> gmtCreate, gmt_modified <span class="keyword">as</span> gmtModified, creator, is_deleted <span class="keyword">as</span> isDeleted, </span><br><span class="line">skytf-> mpxyzdatas, skyid, wap_token <span class="keyword">as</span> wapToken, app_token <span class="keyword">as</span> appToken, source, username, passwd, nickname, </span><br><span class="line">skytf-> mpxyzdataf, mpxyzdatag, phone, phone2, mpxyzdatae, f FROM tmp_user WHERE is_deleted=<span class="string">'n'</span> AND </span><br><span class="line">skytf-> mpxyzdatas=<span class="string">')dzyL5zyzpzy)rzy!Pzy'</span> order by id desc ; </span><br><span class="line"> QUERY PLAN </span><br><span class="line">--------------------------------------------------------------------------------------------------------------------- </span><br><span class="line">Sort (cost=<span class="number">139298.73</span>..<span class="number">139298.74</span> rows=<span class="number">1</span> width=<span class="number">226</span>) (actual time=<span class="number">1552.541</span>..<span class="number">1552.542</span> rows=<span class="number">1</span> loops=<span class="number">1</span>) </span><br><span class="line"> Sort Key: id </span><br><span class="line"> Sort Method: quicksort Memory: <span class="number">25</span>kB </span><br><span class="line"> -> Seq Scan on tmp_user (cost=<span class="number">0.00</span>..<span class="number">139298.72</span> rows=<span class="number">1</span> width=<span class="number">226</span>) (actual time=<span class="number">1352.316</span>..<span class="number">1552.524</span> rows=<span class="number">1</span> loops=<span class="number">1</span>) </span><br><span class="line"> Filter: ((is_deleted = <span class="string">'n'</span>::bpchar) AND ((mpxyzdatas)::text = <span class="string">')dzyL5zyzpzy)rzy!Pzy'</span>::text))Total runtime: <span class="number">1552.583</span> ms </span><br><span class="line">(<span class="number">6</span> rows)</span><br><span class="line">Time: <span class="number">1566.089</span> ms</span><br></pre></td></tr></table></figure></p>
<p>这个PLAN比较简单，很明了，”Seq Scan”，整个消耗 1.5s</p>
<p>4 表的相关信息<br><figure class="highlight coq"><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></pre></td><td class="code"><pre><span class="line">skytf=> select pg_size_pretty(pg_relation_size('tmp_user')); </span><br><span class="line">pg_size_pretty </span><br><span class="line">---------------- </span><br><span class="line"><span class="number">573</span> MB </span><br><span class="line">(<span class="number">1</span> row)</span><br><span class="line"></span><br><span class="line">skytf=> \d tmp_user </span><br><span class="line"> <span class="keyword">Table</span> <span class="string">"skytf.tmp_user"</span> </span><br><span class="line"> Column | <span class="type">Type</span> | <span class="type">Modifiers</span> </span><br><span class="line">--------------+-----------------------------+------------------------------ </span><br><span class="line">id | <span class="type">integer</span> | <span class="type">not</span> null </span><br><span class="line">modifier | <span class="type">integer</span> | <span class="type"></span></span><br><span class="line"><span class="type">gmt_create</span> | <span class="type">timestamp</span> without <span class="built_in">time</span> zone | <span class="type"></span></span><br><span class="line"><span class="type">gmt_modified</span> | <span class="type">timestamp</span> without <span class="built_in">time</span> zone | <span class="type"></span></span><br><span class="line"><span class="type">creator</span> | <span class="type">integer</span> | <span class="type"></span></span><br><span class="line"><span class="type">is_deleted</span> | <span class="type">character</span>(<span class="number">1</span>) | <span class="type">not</span> null default 'n'::bpchar </span><br><span class="line">mpxyzdatas | <span class="type">character</span> varying | <span class="type"></span></span><br><span class="line"><span class="type">skyid</span> | <span class="type">bigint</span> | <span class="type">not</span> null </span><br><span class="line">wap_token | <span class="type">character</span> varying(<span class="number">100</span>) | <span class="type"></span></span><br><span class="line"><span class="type">app_token</span> | <span class="type">character</span> varying(<span class="number">100</span>) | <span class="type"></span></span><br><span class="line"><span class="type">source</span> | <span class="type">character</span>(<span class="number">10</span>) | <span class="type">default</span> 'applist'::bpchar </span><br><span class="line">username | <span class="type">character</span> varying | <span class="type"></span></span><br><span class="line"><span class="type">passwd</span> | <span class="type">character</span> varying | <span class="type"></span></span><br><span class="line"><span class="type">nickname</span> | <span class="type">character</span> varying | <span class="type"></span></span><br><span class="line"><span class="type">mpxyzdataf</span> | <span class="type">character</span> varying | <span class="type"></span></span><br><span class="line"><span class="type">mpxyzdatag</span> | <span class="type">character</span> varying | <span class="type"></span></span><br><span class="line"><span class="type">phone</span> | <span class="type">character</span> varying(<span class="number">15</span>) | <span class="type"></span></span><br><span class="line"><span class="type">mpxyzdatae</span> | <span class="type">character</span> varying | <span class="type"></span></span><br><span class="line"><span class="type">f</span> | <span class="type">character</span> varying | <span class="type"></span></span><br><span class="line"><span class="type">phone2</span> | <span class="type">character</span> varying(<span class="number">15</span>) | <span class="type"></span></span><br><span class="line"><span class="type">Indexes</span>: </span><br><span class="line"> <span class="string">"tmp_user_new_pkey"</span> PRIMARY KEY, btree (id) </span><br><span class="line"> <span class="string">"unique_skyid1"</span> UNIQUE, btree (skyid) </span><br><span class="line"> <span class="string">"idx_mpxyzdatas"</span> btree (mpxyzdatas) </span><br><span class="line"> <span class="string">"index_gmt_create1"</span> btree (date(gmt_create))</span><br></pre></td></tr></table></figure></p>
<p>从上面信息可以看出，表tmp_user不太大，才 573 MB , 但查询条件 is_deleted 和 mpxyzdatas 都没有建索引, 根据直觉，像这种查询，建个联合索引 (is_deleted,mpxyzdatas ) 最合适不过了，先别着急，下面深入分析下。</p>
<p>5 查询列的 distinct 情况<br><figure class="highlight coq"><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></pre></td><td class="code"><pre><span class="line">skytf=> select tablename,attname,n_distinct from pg_stats <span class="keyword">where</span> tablename='tmp_user' and attname <span class="built_in">in</span> ('is_deleted','mpxyzdatas'); </span><br><span class="line">tablename | <span class="type">attname</span> | <span class="type">n_distinct</span> </span><br><span class="line">-----------+------------+------------ </span><br><span class="line">tmp_user | <span class="type">is_deleted</span> | <span class="type">1</span> </span><br><span class="line">tmp_user | <span class="type">mpxyzdatas</span> | <span class="type">-1</span></span><br></pre></td></tr></table></figure></p>
<p>pg_stats.n_distinct值为正数时，表时distinct实际值，当 n_distinct为 “ -1 “ 时，表示此列具有非常好的 distinct属性，即为 unique. 也就是说 表 tmp_user的列 is_deleted 只有一个 distinct 值, 而 mpxyzdatas 字段却为唯一。</p>
<p>6 设想几种创建索引的方法 </p>
<ol>
<li><code>create index idx_is_deleted_mpxyzdatas on skytf.tmp_user using btree (is_deleted,mpxyzdatas );</code> </li>
<li><code>create index idx_is_mpxyzdatas_id on skytf.tmp_user using btree (mpxyzdatas, id desc );</code> </li>
<li><code>create index idx_mpxyzdatas on skytf.tmp_user using btree (mpxyzdatas );</code></li>
</ol>
<p>现在权衡一下上面三种方法，方法一创建联合索引通常情况下适合这种QUERY 场合，但由于列 is_deleted 只有一个 distinct 值，所以这个字段上不适合建索引。于是排除方法一。 而方法二建了个(mpxyzdatas, id desc )索引，第二个方法主要是考虑到查询条件中按ID 进行降序排序，但由于 id 已经是 pk,并且排序代价不大，这可以从步骤3的 plan来查看，Sort (cost=139298.73..139298.74 rows=1 width=226) (actual time=1552.541..1552.542 rows=1 loops=1) 根据 cost=139298.73..139298或者 actual time=1552.541..1552.542 这个结点的起动代价，完成代价，起动时间完成时间可以非常明显地看出这个排序步骤代价非常小。而方法三，仅在列 mpxyzdatas 上创建索引，看上去不可行， </p>
<p>下面准备在测试库上测试下，接着把生产的这张表倒到测试环境下去验证一下，我的测试库主机，配置和生产的几乎相同,导表的步骤这里就省略了，通过 pg_dump备份表，pg_restore 还原表就行。</p>
<p>在测试环境上测试</p>
<p>7 先取部分数据，做测试用。<br><figure class="highlight shell"><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></pre></td><td class="code"><pre><span class="line">skytf=> select mpxyzdatas from tmp_user limit 10; </span><br><span class="line"> mpxyzdatas </span><br><span class="line">---------------------- </span><br><span class="line"><span class="meta">)dzyK5zy$</span><span class="bash">vzyGdzyLNzy </span></span><br><span class="line">)dzyKBzyL1zy(~zy)dzy </span><br><span class="line"><span class="meta">)dzyD5zyP1yy%</span><span class="bash">xzyL9zy </span></span><br><span class="line"><span class="meta">)dzyLlyy)5zz$</span><span class="bash">NzyfFzy </span></span><br><span class="line"><span class="meta">)dzyDlzyb#</span><span class="bash">zybvzy)^zy </span></span><br><span class="line">)dzyD5yyD1zyC3zyP@zy </span><br><span class="line">)dzyH5yyK9zyy)zzz1yy </span><br><span class="line">)dzyL5yy~7zz(fzyL5yz </span><br><span class="line">)dzyD5yyKJzye3zyG7zy </span><br><span class="line"><span class="meta">)dzyKlzy$</span><span class="bash">jzyDtyy)Dzy </span></span><br><span class="line">(10 rows)</span><br></pre></td></tr></table></figure></p>
<p>8 创建单列索引<br><figure class="highlight subunit"><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">skytf=> create index idx_mpxyzdatas on skytf.tmp_user using btree (mpxyzdatas ); </span><br><span class="line">CREATE INDEX </span><br><span class="line"><span class="keyword">Time:</span> 18191.498 ms</span><br></pre></td></tr></table></figure></p>
<p>9 创建索引后的 plan<br><figure class="highlight clean"><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></pre></td><td class="code"><pre><span class="line">skytf=> explain analyze SELECT id, modifier, gmt_create <span class="keyword">as</span> gmtCreate, gmt_modified <span class="keyword">as</span> gmtModified, creator, is_deleted <span class="keyword">as</span> isDeleted, </span><br><span class="line">skytf-> mpxyzdatas, skyid, wap_token <span class="keyword">as</span> wapToken, app_token <span class="keyword">as</span> appToken, source, username, passwd, nickname, </span><br><span class="line">skytf-> mpxyzdataf, mpxyzdatag, phone, phone2, mpxyzdatae, f FROM tmp_user WHERE is_deleted=<span class="string">'n'</span> AND </span><br><span class="line">skytf-> mpxyzdatas=<span class="string">')dzyDlzyb#zybvzy)^zy'</span> order by id desc ; </span><br><span class="line"> QUERY PLAN </span><br><span class="line">--------------------------------------------------------------------------------------------------------------------------------- </span><br><span class="line">Sort (cost=<span class="number">4.71</span>..<span class="number">4.72</span> rows=<span class="number">1</span> width=<span class="number">226</span>) (actual time=<span class="number">0.100</span>..<span class="number">0.101</span> rows=<span class="number">1</span> loops=<span class="number">1</span>) </span><br><span class="line"> Sort Key: id </span><br><span class="line"> Sort Method: quicksort Memory: <span class="number">25</span>kB </span><br><span class="line"> -> Index Scan using idx_mpxyzdatas on tmp_user (cost=<span class="number">0.00</span>..<span class="number">4.70</span> rows=<span class="number">1</span> width=<span class="number">226</span>) (actual time=<span class="number">0.088</span>..<span class="number">0.090</span> rows=<span class="number">1</span> loops=<span class="number">1</span>) </span><br><span class="line"> Index Cond: ((mpxyzdatas)::text = <span class="string">')dzyDlzyb#zybvzy)^zy'</span>::text) </span><br><span class="line"> Filter: (is_deleted = <span class="string">'n'</span>::bpchar) </span><br><span class="line">Total runtime: <span class="number">0.131</span> ms </span><br><span class="line">(<span class="number">7</span> rows)</span><br><span class="line">Time: <span class="number">0.459</span> ms</span><br></pre></td></tr></table></figure></p>
<p>可以看出,plan 走了索引 idx_mpxyzdatas, 花费的 cost 大大降低，执行时间也仅为 0.459 ms。在测试环境上通过后，于是将方法三的索引加到生产环境下，慢查询消失。 </p>
<p>总结</p>
<ol>
<li><p>创建精简的索引对性能影响是很大的，一方面保证了查询的速度，另一方面降低了索引的维护成本，当表上有insert,update操作时，相比联合索引，维护代价更低。 </p>
</li>
<li><p>引用一句德哥的话，在这节能俭排的年代，能够利用最小的代价实现最大的价值，何乐而不为呢。</p>
</li>
</ol>
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<a href="/20190117151500.html" title="EnterpriseDB: 使用 edbstart and edbstop 启停数据库" target="_blank">EnterpriseDB: 使用 edbstart and edbstop 启停数据库</a>
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<a href="/20190116103400.html" title="EnterpriseDB: 无人值守安装" target="_blank">EnterpriseDB: 无人值守安装</a>
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<a href="/20190115143600.html" title="EnterpriseDB: 交互式命令行安装" target="_blank">EnterpriseDB: 交互式命令行安装</a>
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<a href="/20190107095300.html" title="Hexo: 添加Valine评论(邮件通知、评论列表头像)" target="_blank">Hexo: 添加Valine评论(邮件通知、评论列表头像)</a>
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