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_r305601378
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File path:
iotdb/src/main/java/org/apache/iotdb/db/engine/storagegroup/StorageGroupProcessor.java
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@@ -458,6 +470,24 @@ public QueryDataSource query(String deviceId, String
measurementId, QueryContext
}
}
+ /**
+ * returns the top k% measurements which are most frequently used in queries.
Review comment:
I don't think the current method behaves as this annotation reads.
The current `lruForSensorUsedInQuery` is `LinkedHashSet`, which only records
the access order of sensors used by query.
Even some kind of LruLinkedHashSet is used, it can only return the first
`num` oldeset sensors kept in the LRU, not the most frequently used ones.
For example, I query d0.s0, d0.s1, d1.s2, d0.s3, d1.s1, d2.s1, d3.s1, d4.s2.
The sensor are s0,s1,s2,s3,s1,s1,s1,s2. Suppose `num`=3
1) `LinkedHashSet`
returns s0,s1,s2
2) some kind of LruLinkedHashSet. Suppose memory is big enough.
returns s0,s3,s1. The top 3 of most recently used measurements.
But the top 3 of most frequently used measurements should be s1,s2,s3/s0.
Maybe you should think twice about the design, `most frequently used
measurements` or `most recently used measurements`? Or as I comment in the next
review, `most recently used device`?
**most frequenly used / most recently used + measurements / devices /
device.measurement**
So we have 6 possible plans:
1. most frequenly used measurements
2. most recently used measurements
3. most frequenly used devices
4. most recently used devices
5. most frequenly used device.measurement
6. most recently used device.measurement
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