LeiRui commented on a change in pull request #262: [IOTDB-144]meta data cache
for query
URL: https://github.com/apache/incubator-iotdb/pull/262#discussion_r305601873
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File path:
iotdb/src/main/java/org/apache/iotdb/db/engine/storagegroup/StorageGroupProcessor.java
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@@ -447,6 +456,9 @@ public void putAllWorkingTsFileProcessorIntoClosingList() {
// TODO need a read lock, please consider the concurrency with flush manager
threads.
public QueryDataSource query(String deviceId, String measurementId,
QueryContext context) {
insertLock.readLock().lock();
+ synchronized (lruForSensorUsedInQuery) {
+ lruForSensorUsedInQuery.add(measurementId);
Review comment:
Same as above, the current `lruForSensorUsedInQuery` is for `measurementId`
only. My questions are:
1. Why not use `deviceId.measurementId`?
For example, `root.vehicle.d0.s0` and `root.vehicle.d1.s0` are different
series. However, by the current code, only `s0` is used. The hottest sensor
doesn't equal the hottest series.
2. Take it further, why not use `deviceId`
and cache `TsDeviceMetadata` for every recently used device? (Or frequenly
used device, but I prefer recently used device here. )
For example, I query d0.s0, d0.s1, d0.s2, d0.s3 and d0 is the most recently
used device.
One benefit is that if the `TsDeviceMetadata` of `file1.d0` is cached and
`file1.d0.s4` is not found in this `TsDeviceMetadata`, then `file1` can be
skipped safely with knowing that `d0.s4` doesn't exist in this file. However,
in the current pr, suppose that `List<ChunkMetadata>` of
`file1.d0.s1`、`file1.d0.s2`、`file1.d0.s3` is cached separately and
`file1.d0.s4` is not found in the cache, then `file1` needs to be read again
for `d0.s4`.
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