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<h1 class="post-title" itemprop="name headline">Hash 索引创建奇慢</h1>
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<p>今天学习了下 Hash 索引，在了解HASH索引的应用场合后，打算实践一下，但在创建过程中奇慢，有个测试表才 1.5G 大，在创建了两小时候还没结束，而且系统上也没有锁。于是猜测 HASH 索引创建应该比 btree 索引创建慢得多，下面是做的一些测试，用来比较不同数据量环境下创建 btree 索引和 HASH 索引的速度。</p>
<h1 id="机器配置"><a href="#机器配置" class="headerlink" title="机器配置"></a>机器配置</h1><p>8 CPU 8G 内存<br>OS Red Hat Enterprise Linux Server release 5.5</p>
<h1 id="场景一：创建表test-a-插入1000条数据"><a href="#场景一：创建表test-a-插入1000条数据" class="headerlink" title="场景一：创建表test_a, 插入1000条数据"></a>场景一：创建表test_a, 插入1000条数据</h1><figure class="highlight n1ql"><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">skytf=> <span class="keyword">create</span> table test_a (id integer,name varchar(<span class="number">32</span>)); </span><br><span class="line"><span class="keyword">CREATE</span> TABLE</span><br><span class="line">skytf=> <span class="keyword">insert</span> <span class="keyword">into</span> test_a <span class="keyword">select</span> generate_series(<span class="number">1</span>,<span class="number">1000</span>),<span class="string">'francs'</span>; </span><br><span class="line"><span class="keyword">INSERT</span> <span class="number">0</span> <span class="number">1000</span></span><br><span class="line">skytf=> <span class="keyword">create</span> <span class="keyword">index</span> idx_btree_test_a_name <span class="keyword">on</span> test_a <span class="keyword">using</span> btree (name); </span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">INDEX</span> </span><br><span class="line">Time: <span class="number">35.931</span> ms</span><br><span class="line">skytf=> <span class="keyword">create</span> <span class="keyword">index</span> idx_hash_test_a_name <span class="keyword">on</span> test_a <span class="keyword">using</span> hash (name); </span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">INDEX</span> </span><br><span class="line">Time: <span class="number">2.578</span> ms</span><br></pre></td></tr></table></figure>
<p>在 1000条数据下，创建 btree索引花了 35.9 ms , 创建 hash索引花了2.6ms </p>
<h1 id="场景二：创建表-test-b-插入1万条数据"><a href="#场景二：创建表-test-b-插入1万条数据" class="headerlink" title="场景二：创建表 test_b, 插入1万条数据"></a>场景二：创建表 test_b, 插入1万条数据</h1><figure class="highlight n1ql"><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">skytf=> <span class="keyword">create</span> table test_b (id integer,name varchar(<span class="number">32</span>)); </span><br><span class="line"><span class="keyword">CREATE</span> TABLE</span><br><span class="line">skytf=> <span class="keyword">insert</span> <span class="keyword">into</span> test_a <span class="keyword">select</span> generate_series(<span class="number">1</span>,<span class="number">10000</span>),<span class="string">'francs'</span>; </span><br><span class="line"><span class="keyword">INSERT</span> <span class="number">0</span> <span class="number">10000</span></span><br><span class="line">skytf=> <span class="keyword">create</span> <span class="keyword">index</span> idx_btree_test_b_name <span class="keyword">on</span> test_b <span class="keyword">using</span> btree (name); </span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">INDEX</span> </span><br><span class="line">Time: <span class="number">56.990</span> ms</span><br><span class="line">skytf=> <span class="keyword">create</span> <span class="keyword">index</span> idx_hash_test_b_name <span class="keyword">on</span> test_b <span class="keyword">using</span> hash (name); </span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">INDEX</span> </span><br><span class="line">Time: <span class="number">56.261</span> ms</span><br></pre></td></tr></table></figure>
<p>在 1万 条数据下，创建 btree索引花了 56.9ms , 创建 hash索引花了 56.2 ms </p>
<h1 id="场景三：创建表-test-c-插入10万条数据"><a href="#场景三：创建表-test-c-插入10万条数据" class="headerlink" title="场景三：创建表 test_c, 插入10万条数据"></a>场景三：创建表 test_c, 插入10万条数据</h1><figure class="highlight n1ql"><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">skytf=> <span class="keyword">create</span> table test_c (id integer,name varchar(<span class="number">32</span>)); </span><br><span class="line"><span class="keyword">CREATE</span> TABLE</span><br><span class="line">skytf=> <span class="keyword">insert</span> <span class="keyword">into</span> test_a <span class="keyword">select</span> generate_series(<span class="number">1</span>,<span class="number">100000</span>),<span class="string">'francs'</span>; </span><br><span class="line"><span class="keyword">INSERT</span> <span class="number">0</span> <span class="number">100000</span></span><br><span class="line">skytf=> <span class="keyword">create</span> <span class="keyword">index</span> idx_btree_test_c_name <span class="keyword">on</span> test_c <span class="keyword">using</span> btree (name); </span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">INDEX</span> </span><br><span class="line">Time: <span class="number">181.033</span> ms</span><br><span class="line">skytf=> <span class="keyword">create</span> <span class="keyword">index</span> idx_hash_test_c_name <span class="keyword">on</span> test_c <span class="keyword">using</span> hash (name); </span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">INDEX</span> </span><br><span class="line">Time: <span class="number">4643.801</span> ms</span><br></pre></td></tr></table></figure>
<p>在 10万 条数据下，创建 btree索引花了 181.0 ms , 创建 hash索引花了 4643 ms </p>
<h1 id="场景四：创建表-test-d-插入100万条数据"><a href="#场景四：创建表-test-d-插入100万条数据" class="headerlink" title="场景四：创建表 test_d, 插入100万条数据"></a>场景四：创建表 test_d, 插入100万条数据</h1><figure class="highlight n1ql"><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">skytf=> <span class="keyword">create</span> table test_d (id integer,name varchar(<span class="number">32</span>)); </span><br><span class="line"><span class="keyword">CREATE</span> TABLE</span><br><span class="line">skytf=> <span class="keyword">insert</span> <span class="keyword">into</span> test_d <span class="keyword">select</span> generate_series(<span class="number">1</span>,<span class="number">1000000</span>),<span class="string">'francs'</span>; </span><br><span class="line"><span class="keyword">INSERT</span> <span class="number">0</span> <span class="number">1000000</span></span><br><span class="line">skytf=> <span class="keyword">create</span> <span class="keyword">index</span> idx_btree_test_d_name <span class="keyword">on</span> test_d <span class="keyword">using</span> btree (name); </span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">INDEX</span> </span><br><span class="line">Time: <span class="number">1909.001</span> ms</span><br><span class="line">skytf=> <span class="keyword">create</span> <span class="keyword">index</span> idx_hash_test_d_name <span class="keyword">on</span> test_d <span class="keyword">using</span> hash (name); </span><br><span class="line"><span class="keyword">CREATE</span> <span class="keyword">INDEX</span> </span><br><span class="line">Time: <span class="number">591817.814</span> ms</span><br></pre></td></tr></table></figure>
<p>在 100万 条数据下，创建 btree索引花了 1909.0 ms , 创建 hash索引花了 591817 ms 在这种情况下，创建HASH索引明显很慢，创建HASH索引的时间是创建btree 索引的近 31倍。</p>
<h1 id="四种场景时间统计"><a href="#四种场景时间统计" class="headerlink" title="四种场景时间统计"></a>四种场景时间统计</h1><table>
<thead>
<tr>
<th style="text-align:center">表名</th>
<th style="text-align:center">记录数</th>
<th style="text-align:center">创建 Btree 索引耗时(ms)</th>
<th style="text-align:center">创建 Hash 索引耗时 (ms)</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align:center">table_a</td>
<td style="text-align:center">1000</td>
<td style="text-align:center">35.9</td>
<td style="text-align:center">2.6</td>
</tr>
<tr>
<td style="text-align:center">table_b</td>
<td style="text-align:center">1万</td>
<td style="text-align:center">56.9</td>
<td style="text-align:center">56.2</td>
</tr>
<tr>
<td style="text-align:center">table_c</td>
<td style="text-align:center">10万</td>
<td style="text-align:center">181</td>
<td style="text-align:center">4643</td>
</tr>
<tr>
<td style="text-align:center">table_d</td>
<td style="text-align:center">100万</td>
<td style="text-align:center">1909</td>
<td style="text-align:center">591817</td>
</tr>
</tbody>
</table>
<h1 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h1><ol>
<li><p>因为 hash 索引创建消耗的时间很长，可以预计当表达到 1G 以上，创建 HASH 索引<br>将会变得非常艰难，保守估计，创建 hash 索引花费的时间至少是 创建btree 所花费<br>的时间 20 倍以上。 </p>
</li>
<li><p>尽管 Hash 索引适用于特定场合，但创建很慢，推测创建 HASH索引后，此索引维护<br>代价(指删入操作)非常大，所以不建议使用到生产库中。</p>
</li>
<li><p>至于在特定场合 Hash 索引是否比 btree 索引高效，还有侍验证。</p>
</li>
</ol>
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