[GitHub] [incubator-iotdb] little-emotion opened a new pull request #262: [IOTDB-144]meta data cache for query

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little-emotion opened a new pull request #262: [IOTDB-144]meta data cache for query
URL: https://github.com/apache/incubator-iotdb/pull/262
 
 
   To increase query speed, metadata needs to be cached, including TsFileMetaData and TsDeviceMetaData.
   The files in IOTDB are organized according to time, so the query frequency of different files varies greatly. Cache metadatas of frequently queried files can reduce the time of reading metaData from disk. Besides, all sensors' metadata of a device is in TsDeviceMetaData. In order to read the metadata of a sensor, we need to read the entire TsDeviceMetaData. In the current version, querying multiple sensors in a device simultaneously need to read TsDeviceMetaData multiple times, so consider caching TsDeviceMetaData.

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