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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></nav>  <ul class="sidebar-links"><li><section class="sidebar-group depth-0"><p class="sidebar-heading"><span>系统设计</span> <!----></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>应用概览</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>TsFile</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>查询引擎</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>元数据管理</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>存储引擎</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading open"><span>数据查询</span> <span class="arrow down"></span></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/zh/SystemDesign/DataQuery/DataQuery.html" class="sidebar-link">数据查询</a></li><li><a href="/zh/SystemDesign/DataQuery/QueryFundamentals.html" class="sidebar-link">查询基础介绍</a></li><li><a href="/zh/SystemDesign/DataQuery/SeriesReader.html" class="sidebar-link">查询基础组件</a></li><li><a href="/zh/SystemDesign/DataQuery/RawDataQuery.html" class="sidebar-link">原始数据查询</a></li><li><a href="/zh/SystemDesign/DataQuery/AggregationQuery.html" class="sidebar-link">聚合查询</a></li><li><a href="/zh/SystemDesign/DataQuery/GroupByQuery.html" class="sidebar-link">降采样查询</a></li><li><a href="/zh/SystemDesign/DataQuery/LastQuery.html" aria-current="page" class="active sidebar-link">最近时间戳 Last 查询</a></li><li><a href="/zh/SystemDesign/DataQuery/AlignByDeviceQuery.html" class="sidebar-link">按设备对齐查询</a></li><li><a href="/zh/SystemDesign/DataQuery/FillFunction.html" class="sidebar-link">空值填充</a></li><li><a href="/zh/SystemDesign/DataQuery/GroupByFillQuery.html" class="sidebar-link">降采样补空值查询</a></li><li><a href="/zh/SystemDesign/DataQuery/OrderByTimeQuery.html" class="sidebar-link">按时间倒序查询</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>工具</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>连接器</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>客户端</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="content content__default"><h1 id="最近时间戳-last-查询"><a href="#最近时间戳-last-查询" class="header-anchor">#</a> 最近时间戳 Last 查询</h1> <p>Last 查询的主要逻辑在 LastQueryExecutor</p> <ul><li>org.apache.iotdb.db.query.executor.LastQueryExecutor</li></ul> <p>Last查询对每个指定的时间序列执行<code>calculateLastPairForOneSeries</code>方法。</p> <h2 id="读取mnode缓存数据"><a href="#读取mnode缓存数据" class="header-anchor">#</a> 读取MNode缓存数据</h2> <p>我们在需要查询的时间序列所对应的MNode结构中添加Last数据缓存。<code>calculateLastPairForOneSeries</code>方法对于某个时间序列的Last查询，首先尝试读取MNode中的缓存数据。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>try {
  node = IoTDB.metaManager.getDeviceNodeWithAutoCreateStorageGroup(seriesPath.toString());
} catch (MetadataException e) {
  throw new QueryProcessException(e);
}
if (((LeafMNode) node).getCachedLast() != null) {
  return ((LeafMNode) node).getCachedLast();
}
</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></div></div><p>如果发现缓存没有被写入过，则执行下面的标准查询流程读取TsFile数据。</p> <h2 id="last标准查询流程"><a href="#last标准查询流程" class="header-anchor">#</a> Last标准查询流程</h2> <p>Last标准查询流程需要以倒序方式扫描顺序文件和乱序文件，一旦得到满足要求的Last查询结果就将其写回到MNode缓存中并返回。算法中对顺序文件和乱序文件分别进行处理。</p> <ul><li><p>顺序文件由于是按照写入时间已经排好序，因此直接使用<code>loadTimeSeriesMetadata()</code>方法取出最后一个不为空的<code>TimeseriesMetadata</code>。若<code>TimeseriesMetadata</code>的统计数据可用即可直接得到Last时间戳和对应的值；如不可用则需要使用<code>loadChunkMetadataList()</code>方法得到下一层的最后一个<code>ChunkMetadata</code>，通过统计数据得到Last结果。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>for (int i = seqFileResources.size() - 1; i &gt;= 0; i--) {
    TimeseriesMetadata timeseriesMetadata = FileLoaderUtils.loadTimeSeriesMetadata(
            seqFileResources.get(i), seriesPath, context, null, sensors);
    if (timeseriesMetadata != null) {
      if (!timeseriesMetadata.isModified()) {
        Statistics timeseriesMetadataStats = timeseriesMetadata.getStatistics();
        resultPair = constructLastPair(
                timeseriesMetadataStats.getEndTime(),
                timeseriesMetadataStats.getLastValue(),
                tsDataType);
        break;
      } else {
        List&lt;ChunkMetadata&gt; chunkMetadataList = timeseriesMetadata.loadChunkMetadataList();
        if (!chunkMetadataList.isEmpty()) {
          ChunkMetadata lastChunkMetaData = chunkMetadataList.get(chunkMetadataList.size() - 1);
          Statistics chunkStatistics = lastChunkMetaData.getStatistics();
          resultPair =
              constructLastPair(
                  chunkStatistics.getEndTime(), chunkStatistics.getLastValue(), tsDataType);
          break;
        }
      }
    }
  }
</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></div></div></li> <li><p>对于乱序文件，需要遍历所有不为空的<code>TimeseriesMetadata</code>结构并更新当前最大时间戳的Last数据，直到扫描完所有乱序文件为止。需要注意的是当多个<code>ChunkMetadata</code>拥有相同的最大时间戳时，我们取<code>version</code>值最大的<code>ChunkMatadata</code>中的数据作为Last的结果。</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>long version = 0;
for (TsFileResource resource : unseqFileResources) {
  if (resource.getEndTime(seriesPath.getDevice()) &lt; resultPair.getTimestamp()) {
    continue;
  }
  TimeseriesMetadata timeseriesMetadata =
      FileLoaderUtils.loadTimeSeriesMetadata(resource, seriesPath, context, null, sensors);
  if (timeseriesMetadata != null) {
    for (ChunkMetadata chunkMetaData : timeseriesMetadata.loadChunkMetadataList()) {
      if (chunkMetaData.getEndTime() == resultPair.getTimestamp()
          &amp;&amp; chunkMetaData.getVersion() &gt; version) {
        Statistics chunkStatistics = chunkMetaData.getStatistics();
        resultPair =
            constructLastPair(
                chunkStatistics.getEndTime(), chunkStatistics.getLastValue(), tsDataType);
        version = chunkMetaData.getVersion();
      }
    }
  }
}
</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></div></div></li> <li><p>最后将查询结果写入到MNode的Last缓存</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>((LeafMNode) node).updateCachedLast(resultPair, false, Long.MIN_VALUE);
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br></div></div></li></ul> <h2 id="last-缓存更新策略"><a href="#last-缓存更新策略" class="header-anchor">#</a> Last 缓存更新策略</h2> <p>Last缓存更新的逻辑位于<code>LeafMNode</code>的<code>updateCachedLast</code>方法内，这里引入两个额外的参数<code>highPriorityUpdate</code>和<code>latestFlushTime</code>。<code>highPriorityUpdate</code>用来表示本次更新是否是高优先级的，新数据写入而导致的缓存更新都被认为是高优先级更新，而查询时更新缓存默认为低优先级更新。<code>latestFlushTime</code>用来记录当前已被写回到磁盘的数据的最大时间戳。</p> <p>缓存更新的策略如下：</p> <ol><li>当缓存中没有记录时，对于查询到的Last数据，将查询的结果直接写入到缓存中。</li> <li>当缓存中没有记录时，对于写入的最新数据如果时间戳大于或等于<code>latestFlushTime</code>，则将写入的数据写入到缓存中。</li> <li>当缓存中已有记录时，根据查询或写入的数据时间戳与当前缓存中时间戳作对比。写入的数据具有高优先级，时间戳不小于缓存记录则更新缓存；查询出的数据低优先级，必须大于缓存记录的时间戳才更新缓存。</li></ol> <p>具体代码如下</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>public synchronized void updateCachedLast(
  TimeValuePair timeValuePair, boolean highPriorityUpdate, Long latestFlushedTime) {
    if (timeValuePair == null || timeValuePair.getValue() == null) return;
    
    if (cachedLastValuePair == null) {
      // If no cached last, (1) a last query (2) an unseq insertion or (3) a seq insertion will update cache.
      if (!highPriorityUpdate || latestFlushedTime &lt;= timeValuePair.getTimestamp()) {
        cachedLastValuePair =
            new TimeValuePair(timeValuePair.getTimestamp(), timeValuePair.getValue());
      }
    } else if (timeValuePair.getTimestamp() &gt; cachedLastValuePair.getTimestamp()
        || (timeValuePair.getTimestamp() == cachedLastValuePair.getTimestamp()
            &amp;&amp; highPriorityUpdate)) {
      cachedLastValuePair.setTimestamp(timeValuePair.getTimestamp());
      cachedLastValuePair.setValue(timeValuePair.getValue());
    }
}
</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></div></div></div> <div class="page-nav"><p class="inner"><span class="prev">
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