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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