JingsongLi opened a new pull request #7961: [FLINK-11882][table-runtime-blink] 
Introduce BytesHashMap to batch hash agg
URL: https://github.com/apache/flink/pull/7961
 
 
   ## What is the purpose of the change
   
   Introduce bytes based hash table.
   It can be used for performing aggregations where the aggregated values are 
fixed-width.
   Because the data is stored in continuous memory, AggBuffer of variable 
length cannot be applied to this HashMap. The KeyValue form in hash map is 
designed to reduce the cost of key fetching in lookup.
   
   Add a test to do a complete hash agg. When HashMap has enough memory, pure 
hash AGG is performed; when memory is insufficient, it degenerates into sort 
agg.
   
   ## Verifying this change
   
   ut & coverage
   
   ## Does this pull request potentially affect one of the following parts:
   
     - Dependencies (does it add or upgrade a dependency): (yes, just add test 
dependency)
     - The public API, i.e., is any changed class annotated with 
`@Public(Evolving)`: (no)
     - The serializers: (no)
     - The runtime per-record code paths (performance sensitive): (no)
     - Anything that affects deployment or recovery: JobManager (and its 
components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
     - The S3 file system connector: (no)
   
   ## Documentation
   
     - Does this pull request introduce a new feature? (yes)
     - If yes, how is the feature documented? (JavaDocs)
   

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