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docs/Documentation-CHN/SystemDesign/5-DataQuery/6-LastQuery.md
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| <!-- | ||
| 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. | ||
| --> | ||
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| # 最近时间戳 Last 查询 | ||
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| Last 查询的主要逻辑在 LastQueryExecutor | ||
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| * org.apache.iotdb.db.query.executor.LastQueryExecutor | ||
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| Last查询对每个指定的时间序列执行`calculateLastPairForOneSeries`方法。 | ||
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| ## 读取MNode缓存数据 | ||
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| 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。`calculateLastPairForOneSeries`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。 | ||
| ``` | ||
| try { | ||
| node = MManager.getInstance().getDeviceNodeWithAutoCreateStorageGroup(seriesPath.toString()); | ||
| } catch (MetadataException e) { | ||
| throw new QueryProcessException(e); | ||
| } | ||
| if (((LeafMNode) node).getCachedLast() != null) { | ||
| return ((LeafMNode) node).getCachedLast(); | ||
| } | ||
| ``` | ||
| 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。 | ||
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| ## Last标准查询流程 | ||
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| Last标准查询流程需要遍历所有的顺序文件和乱序文件得到查询结果，最后将查询结果写回到MNode缓存。算法中对顺序文件和乱序文件分别进行处理。 | ||
| - 顺序文件由于是对其写入时间已经排好序，因此直接使用`loadChunkMetadataFromTsFileResource`方法取出最后一个`ChunkMetadata`，通过`ChunkMetadata`的统计数据得到最大时间戳和对应的值。 | ||
| ``` | ||
| if (!seqFileResources.isEmpty()) { | ||
| List<ChunkMetaData> chunkMetadata = | ||
| FileLoaderUtils.loadChunkMetadataFromTsFileResource( | ||
| seqFileResources.get(seqFileResources.size() - 1), seriesPath, context); | ||
| if (!chunkMetadata.isEmpty()) { | ||
| ChunkMetaData lastChunkMetaData = chunkMetadata.get(chunkMetadata.size() - 1); | ||
| Statistics chunkStatistics = lastChunkMetaData.getStatistics(); | ||
| resultPair = | ||
| constructLastPair( | ||
| chunkStatistics.getEndTime(), chunkStatistics.getLastValue(), tsDataType); | ||
| } | ||
| } | ||
| ``` | ||
| - 乱序文件则需要遍历所有的`ChunkMetadata`结构得到最大时间戳数据。需要注意的是当多个`ChunkMetadata`拥有相同的时间戳时，我们取`version`值最大的`ChunkMatadata`中的数据作为Last的结果。 | ||
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| ``` | ||
| long version = 0; | ||
| for (TsFileResource resource : unseqFileResources) { | ||
| if (resource.getEndTimeMap().get(seriesPath.getDevice()) < resultPair.getTimestamp()) { | ||
| break; | ||
| } | ||
| List<ChunkMetaData> chunkMetadata = | ||
| FileLoaderUtils.loadChunkMetadataFromTsFileResource(resource, seriesPath, context); | ||
| for (ChunkMetaData chunkMetaData : chunkMetadata) { | ||
| if (chunkMetaData.getEndTime() == resultPair.getTimestamp() | ||
| && chunkMetaData.getVersion() > version) { | ||
| Statistics chunkStatistics = chunkMetaData.getStatistics(); | ||
| resultPair = | ||
| constructLastPair( | ||
| chunkStatistics.getEndTime(), chunkStatistics.getLastValue(), tsDataType); | ||
| version = chunkMetaData.getVersion(); | ||
| } | ||
| } | ||
| } | ||
| ``` | ||
| - 最后将查询结果写入到MNode的Last缓存 | ||
| ``` | ||
| ((LeafMNode) node).updateCachedLast(resultPair, false, Long.MIN_VALUE); | ||
| ``` | ||
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| ## Last 缓存更新策略 | ||
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| Last缓存更新的逻辑位于`LeafMNode`的`updateCachedLast`方法内，这里引入两个额外的参数`highPriorityUpdate`和`latestFlushTime`。`highPriorityUpdate`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。`latestFlushTime`用来记录当前已被写回到磁盘的数据的最大时间戳。 | ||
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| 缓存更新的策略如下： | ||
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| 1. 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。 | ||
| 2. 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`latestFlushTime`，则将写入的数据写入到缓存中。 | ||
| 3. 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。 | ||
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| 具体代码如下 | ||
| ``` | ||
| 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 <= timeValuePair.getTimestamp()) { | ||
| cachedLastValuePair = | ||
| new TimeValuePair(timeValuePair.getTimestamp(), timeValuePair.getValue()); | ||
| } | ||
| } else if (timeValuePair.getTimestamp() > cachedLastValuePair.getTimestamp() | ||
| || (timeValuePair.getTimestamp() == cachedLastValuePair.getTimestamp() | ||
| && highPriorityUpdate)) { | ||
| cachedLastValuePair.setTimestamp(timeValuePair.getTimestamp()); | ||
| cachedLastValuePair.setValue(timeValuePair.getValue()); | ||
| } | ||
| } | ||
| ``` |
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