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Posted to dev@hive.apache.org by "Gopal V (JIRA)" <ji...@apache.org> on 2018/11/29 08:03:00 UTC

[jira] [Created] (HIVE-20983) Vectorization: Scale up small hashtables, when collisions are detected

Gopal V created HIVE-20983:
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             Summary: Vectorization: Scale up small hashtables, when collisions are detected
                 Key: HIVE-20983
                 URL: https://issues.apache.org/jira/browse/HIVE-20983
             Project: Hive
          Issue Type: Bug
            Reporter: Gopal V


Hive's hashtable estimates are getting better with HyperLogLog stats in place, but an accurate estimate does not always result in a low number of collisions.

The hashtables which contain a very small number of items tend to lose their O(1) lookup performance where there are collisions. Since collisions are easy to detect within the fast hashtable implementation, a rehashing to a higher size will help these small hashtables avoid collisions and go back to O(1) perf.



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