[GitHub] [incubator-iotdb] qiaojialin commented on a change in pull request #938: add system design eng

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qiaojialin commented on a change in pull request #938: add system design eng
URL: https://github.com/apache/incubator-iotdb/pull/938#discussion_r397572109
 
 

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 File path: docs/Documentation/SystemDesign/5-DataQuery/6-LastQuery.md
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+
+# Last query
+
+The main logic of Last query is in LastQueryExecutor
+
+* org.apache.iotdb.db.query.executor.LastQueryExecutor
+
+The Last query executes the `calculateLastPairForOneSeries` method for each specified time series.
+
+## Read MNode cache data
+
+We add a Last data cache to the MNode structure corresponding to the time series that needs to be queried.
+
+`calculateLastPairForOneSeries` method For the last query of a certain time series, first try to read the cached data in the MNode.
+
+```
+try {
+  node = MManager.getInstance().getDeviceNodeWithAutoCreateStorageGroup(seriesPath.toString());
+} catch (MetadataException e) {
+  throw new QueryProcessException(e);
+}
+if (((LeafMNode) node).getCachedLast() != null) {
+  return ((LeafMNode) node).getCachedLast();
+}
+```
+If it is found that the cache has not been written, execute the following standard query process to read the TsFile data.
+
+## Last standard query process
+
+Last standard query process needs to traverse all sequential files and out-of-order files to get query results, and finally write the query results back to the MNode cache.  In the algorithm, sequential files and out-of-order files are processed separately.
 
 Review comment:
   ```suggestion
   Last standard query process needs to traverse all sequential files and unsequential files to get query results, and finally write the query results back to the MNode cache.  In the algorithm, sequential files and out-of-order files are processed separately.
   ```

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