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44.8h-44.8V588.8c0-17.067-14.933-32-32-32H281.6c-17.067 0-32 14.933-32 32v121.6h-44.8c-25.6 0-44.8-19.2-44.8-44.8v-192c0-25.6 19.2-44.8 44.8-44.8h614.4c25.6 0 44.8 19.2 44.8 44.8v192z"></path></svg></button><div class="arrow end"></div></div><div class="vp-toc-wrapper"><ul class="vp-toc-list"><!--[--><li class="vp-toc-item"><a class="route-link vp-toc-link level2" href="#编码方式">编码方式</a></li><li><ul class="vp-toc-list"><!--[--><li class="vp-toc-item"><a class="route-link vp-toc-link level3" href="#基本编码方式">基本编码方式</a></li><!----><!--]--><!--[--><li class="vp-toc-item"><a class="route-link vp-toc-link level3" href="#数据类型与编码的对应关系">数据类型与编码的对应关系</a></li><!----><!--]--></ul></li><!--]--></ul><div class="vp-toc-marker" style="top:-1.7rem;"></div></div><!--[--><!----><!--]--></aside></div><!--[--><!----><!--]--><div class="theme-hope-content"><h2 id="编码方式" tabindex="-1"><a class="header-anchor" href="#编码方式"><span>编码方式</span></a></h2><h3 id="基本编码方式" tabindex="-1"><a class="header-anchor" href="#基本编码方式"><span>基本编码方式</span></a></h3><p>为了提高数据的存储效率，需要在数据写入的过程中对数据进行编码，从而减少磁盘空间的使用量。在写数据以及读数据的过程中都能够减少 I/O 操作的数据量从而提高性能。IoTDB 支持多种针对不同类型的数据的编码方法：</p><ul><li>PLAIN 编码（PLAIN）</li></ul><p>PLAIN 编码，默认的编码方式，即不编码，支持多种数据类型，压缩和解压缩的时间效率较高，但空间存储效率较低。</p><ul><li>二阶差分编码（TS_2DIFF）</li></ul><p>二阶差分编码，比较适合编码单调递增或者递减的序列数据，不适合编码波动较大的数据。</p><ul><li>游程编码（RLE）</li></ul><p>游程编码，比较适合存储某些数值连续出现的序列，不适合编码大部分情况下前后值不一样的序列数据。</p><p>游程编码也可用于对浮点数进行编码，但在创建时间序列的时候需指定保留小数位数（MAX_POINT_NUMBER，具体指定方式参见本文 <a class="route-link" href="/zh/UserGuide/V1.1.x/Reference/SQL-Reference.html">SQL 参考文档</a>）。比较适合存储某些浮点数值连续出现的序列数据，不适合存储对小数点后精度要求较高以及前后波动较大的序列数据。</p><blockquote><p>游程编码（RLE）和二阶差分编码（TS_2DIFF）对 float 和 double 的编码是有精度限制的，默认保留 2 位小数。推荐使用 GORILLA。</p></blockquote><ul><li>GORILLA 编码（GORILLA）</li></ul><p>GORILLA 编码是一种无损编码，它比较适合编码前后值比较接近的数值序列，不适合编码前后波动较大的数据。</p><p>当前系统中存在两个版本的 GORILLA 编码实现，推荐使用<code>GORILLA</code>，不推荐使用<code>GORILLA_V1</code>（已过时）。</p><p>使用限制：使用 Gorilla 编码 INT32 数据时，需要保证序列中不存在值为<code>Integer.MIN_VALUE</code>的数据点；使用 Gorilla 编码 INT64 数据时，需要保证序列中不存在值为<code>Long.MIN_VALUE</code>的数据点。</p><ul><li>字典编码 （DICTIONARY）</li></ul><p>字典编码是一种无损编码。它适合编码基数小的数据（即数据去重后唯一值数量小）。不推荐用于基数大的数据。</p><ul><li>频域编码 （FREQ）</li></ul><p>频域编码是一种有损编码，它基于变换编码的思想，将时序数据变换为频域，仅保留部分高能量的频域分量，以少许的精度损失为代价大幅提高空间效率。该编码适合于频域能量分布较为集中的数据（特别是具有明显周期性的数据），不适合能量分布均匀的数据（如白噪声）。</p><blockquote><p>频域编码在配置文件中包括两个参数：<code>freq_snr</code>指定了编码的信噪比（与标准均方根误差的关系为$NRMSE=10^{-SNR/20}$），该参数增大会同时降低压缩比和精度损失，请根据实际应用的需要进行设置；<code>freq_block_size</code>指定了编码进行时频域变换的分组大小，推荐不对默认值进行修改。参数影响的实验结果和分析详见设计文档。</p></blockquote><ul><li>ZIGZAG 编码</li></ul><p>ZigZag编码将有符号整型映射到无符号整型，适合比较小的整数。</p><ul><li>CHIMP</li></ul><p>CHIMP 是一种无损编码。它是一种新的流式浮点数据压缩算法，可以节省存储空间。这个编码适用于前后值比较接近的数值序列，对波动小和随机噪声少的序列数据更加友好。</p><p>使用限制：如果对 INT32 类型数据使用 CHIMP 编码，需要确保数据点中没有 <code>Integer.MIN_VALUE</code>。 如果对 INT64 类型数据使用 CHIMP 编码，需要确保数据点中没有 <code>Long.MIN_VALUE</code>。</p><h3 id="数据类型与编码的对应关系" tabindex="-1"><a class="header-anchor" href="#数据类型与编码的对应关系"><span>数据类型与编码的对应关系</span></a></h3><p>前文介绍的五种编码适用于不同的数据类型，若对应关系错误，则无法正确创建时间序列。数据类型与支持其编码的编码方式对应关系总结如下表所示。</p><table><thead><tr><th style="text-align:left;">数据类型</th><th style="text-align:left;">支持的编码</th></tr></thead><tbody><tr><td style="text-align:left;">BOOLEAN</td><td style="text-align:left;">PLAIN, RLE</td></tr><tr><td style="text-align:left;">INT32</td><td style="text-align:left;">PLAIN, RLE, TS_2DIFF, GORILLA, FREQ, ZIGZAG</td></tr><tr><td style="text-align:left;">INT64</td><td style="text-align:left;">PLAIN, RLE, TS_2DIFF, GORILLA, FREQ, ZIGZAG</td></tr><tr><td style="text-align:left;">FLOAT</td><td style="text-align:left;">PLAIN, RLE, TS_2DIFF, GORILLA, FREQ</td></tr><tr><td style="text-align:left;">DOUBLE</td><td style="text-align:left;">PLAIN, RLE, TS_2DIFF, GORILLA, FREQ</td></tr><tr><td style="text-align:left;">TEXT</td><td style="text-align:left;">PLAIN, DICTIONARY</td></tr></tbody></table><p>当用户输入的数据类型与编码方式不对应时，系统会提示错误。如下所示，二阶差分编码不支持布尔类型：</p><div class="language-text line-numbers-mode" data-ext="text" data-title="text"><pre class="language-text"><code>IoTDB&gt; create timeseries root.ln.wf02.wt02.status WITH DATATYPE=BOOLEAN, ENCODING=TS_2DIFF
Msg: 507: encoding TS_2DIFF does not support BOOLEAN
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