Github user huawei-flink commented on the issue:

    https://github.com/apache/flink/pull/3574
  
    @fhueske Thanks a lot of the clarification. I understand the issue better 
now, and see your attempt to make an average case that would work for both in 
memory as well as on external persistence. Considering RocksDB as the state of 
art, your choice sounds much more reasonable. We are well aware of the costs of 
serialization, and the impact is definitely important.  However, low latency 
systems with strict SLA will likely run just in memory. 
    
    The O(n) of the MapState is granted by the fact that time is monothonic and 
therefore the sequential reading is managed by the key timestamp. The cost of 
each O(1) in the hashmap increseas with the size of the window thou as you need 
to search through the map index. We definitely need better data access patterns 
for the state of "time series" types of data. 
    
    I will try to internalize it and provide the MapState implementation
    



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