| import{_ as e,o as n,c as s,a,f as t}from"./app-5cdcb249.js";const i={},r=t(`<h1 id="最新点查询" tabindex="-1"><a class="header-anchor" href="#最新点查询" aria-hidden="true">#</a> 最新点查询</h1><p>最新点查询是时序数据库 Apache IoTDB 中提供的一种特殊查询。它返回指定时间序列中时间戳最大的数据点，即一条序列的最新状态。</p><p>在物联网数据分析场景中，此功能尤为重要。为了满足了用户对设备实时监控的需求，Apache IoTDB 对最新点查询进行了<strong>缓存优化</strong>，能够提供毫秒级的返回速度。</p><h3 id="相关配置项" tabindex="-1"><a class="header-anchor" href="#相关配置项" aria-hidden="true">#</a> 相关配置项</h3><p>IoTDB 在 <code>iotdb-common.properties</code> 中提供了 <code>enable_last_cache</code> 和 <code>schema_memory_proportion</code> 两个配置参数，分别用于开启/关闭最新点缓存，以及控制打开最新点缓存后的内存占用。</p><h4 id="enable-last-cache" tabindex="-1"><a class="header-anchor" href="#enable-last-cache" aria-hidden="true">#</a> enable_last_cache</h4><p><code>enable_last_cache</code> 为 <code>true</code> 时，开启最新点缓存；为 <code>false</code> 时，关闭最新点缓存。</p><h4 id="schema-memory-proportion" tabindex="-1"><a class="header-anchor" href="#schema-memory-proportion" aria-hidden="true">#</a> schema_memory_proportion</h4><p>指定了 SchemaRegion, SchemaCache 以及 PartitionCache的内存分配比例，最新点缓存在 SchemaCache 中，所以可以通过调整这个参数，达到调整最新点缓存内存占用的效果。<br> 默认为 <code>5:4:1</code>，即最新点缓存所在的 SchemaCache，占用元数据内存的 40%。</p><h3 id="sql-语法" tabindex="-1"><a class="header-anchor" href="#sql-语法" aria-hidden="true">#</a> SQL 语法：</h3><div class="language-sql line-numbers-mode" data-ext="sql"><pre class="language-sql"><code><span class="token keyword">select</span> <span class="token keyword">last</span> <span class="token operator"><</span>Path<span class="token operator">></span> <span class="token punctuation">[</span>COMMA <span class="token operator"><</span>Path<span class="token operator">></span><span class="token punctuation">]</span><span class="token operator">*</span> <span class="token keyword">from</span> <span class="token operator"><</span> PrefixPath <span class="token operator">></span> <span class="token punctuation">[</span>COMMA <span class="token operator"><</span> PrefixPath <span class="token operator">></span><span class="token punctuation">]</span><span class="token operator">*</span> <span class="token operator"><</span>whereClause<span class="token operator">></span> <span class="token punctuation">[</span><span class="token keyword">ORDER</span> <span class="token keyword">BY</span> TIMESERIES <span class="token punctuation">(</span><span class="token keyword">DESC</span> <span class="token operator">|</span> <span class="token keyword">ASC</span><span class="token punctuation">)</span>?<span class="token punctuation">]</span> |
| </code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>其含义是： 查询时间序列 prefixPath.path 中最近时间戳的数据。</p><ul><li><p><code>whereClause</code> 中当前只支持时间过滤条件，任何其他过滤条件都将会返回异常。当缓存的最新点不满足过滤条件时，IoTDB 需要从存储中获取结果，此时性能将会有所下降。</p></li><li><p>结果集为四列的结构：</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>+----+----------+-----+--------+ |
| |Time|timeseries|value|dataType| |
| +----+----------+-----+--------+ |
| </code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div></li><li><p>可以使用 <code>ORDER BY TIME/TIMESERIES/VALUE/DATATYPE (DESC | ASC)</code> 指定结果集按照某一列进行降序/升序排列。当值列包含多种类型的数据时，按照字符串类型来排序。</p></li></ul><p><strong>示例 1：</strong> 查询 root.ln.wf01.wt01.status 的最新数据点</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>IoTDB> select last status from root.ln.wf01.wt01 |
| +-----------------------------+------------------------+-----+--------+ |
| | Time| timeseries|value|dataType| |
| +-----------------------------+------------------------+-----+--------+ |
| |2017-11-07T23:59:00.000+08:00|root.ln.wf01.wt01.status|false| BOOLEAN| |
| +-----------------------------+------------------------+-----+--------+ |
| Total line number = 1 |
| It costs 0.000s |
| </code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><strong>示例 2：</strong> 查询 root.ln.wf01.wt01 下 status，temperature 时间戳大于等于 2017-11-07T23:50:00 的最新数据点。</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>IoTDB> select last status, temperature from root.ln.wf01.wt01 where time >= 2017-11-07T23:50:00 |
| +-----------------------------+-----------------------------+---------+--------+ |
| | Time| timeseries| value|dataType| |
| +-----------------------------+-----------------------------+---------+--------+ |
| |2017-11-07T23:59:00.000+08:00| root.ln.wf01.wt01.status| false| BOOLEAN| |
| |2017-11-07T23:59:00.000+08:00|root.ln.wf01.wt01.temperature|21.067368| DOUBLE| |
| +-----------------------------+-----------------------------+---------+--------+ |
| Total line number = 2 |
| It costs 0.002s |
| </code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><strong>示例 3：</strong> 查询 root.ln.wf01.wt01 下所有序列的最新数据点，并按照序列名降序排列。</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>IoTDB> select last * from root.ln.wf01.wt01 order by timeseries desc; |
| +-----------------------------+-----------------------------+---------+--------+ |
| | Time| timeseries| value|dataType| |
| +-----------------------------+-----------------------------+---------+--------+ |
| |2017-11-07T23:59:00.000+08:00|root.ln.wf01.wt01.temperature|21.067368| DOUBLE| |
| |2017-11-07T23:59:00.000+08:00| root.ln.wf01.wt01.status| false| BOOLEAN| |
| +-----------------------------+-----------------------------+---------+--------+ |
| Total line number = 2 |
| It costs 0.002s |
| </code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p><strong>示例 4：</strong> 查询 root.ln.wf01.wt01 下所有序列的最新数据点，并按照dataType降序排列。</p><div class="language-text line-numbers-mode" data-ext="text"><pre class="language-text"><code>IoTDB> select last * from root.ln.wf01.wt01 order by dataType desc; |
| +-----------------------------+-----------------------------+---------+--------+ |
| | Time| timeseries| value|dataType| |
| +-----------------------------+-----------------------------+---------+--------+ |
| |2017-11-07T23:59:00.000+08:00|root.ln.wf01.wt01.temperature|21.067368| DOUBLE| |
| |2017-11-07T23:59:00.000+08:00| root.ln.wf01.wt01.status| false| BOOLEAN| |
| +-----------------------------+-----------------------------+---------+--------+ |
| Total line number = 2 |
| It costs 0.002s |
| </code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div>`,21);function l(o,d){return n(),s("div",null,[a(` |
| |
| Licensed to the Apache Software Foundation (ASF) under one |
| or more contributor license agreements. See the NOTICE file |
| distributed with this work for additional information |
| regarding copyright ownership. The ASF licenses this file |
| to you under the Apache License, Version 2.0 (the |
| "License"); you may not use this file except in compliance |
| with the License. You may obtain a copy of the License at |
| |
| http://www.apache.org/licenses/LICENSE-2.0 |
| |
| Unless required by applicable law or agreed to in writing, |
| software distributed under the License is distributed on an |
| "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| KIND, either express or implied. See the License for the |
| specific language governing permissions and limitations |
| under the License. |
| |
| `),r])}const p=e(i,[["render",l],["__file","Last-Query.html.vue"]]);export{p as default}; |
