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From "LuGuangMing (Jira)" <j...@apache.org>
Subject [jira] [Updated] (HIVE-20983) Vectorization: Scale up small hashtables, when collisions are detected
Date Tue, 26 May 2020 10:03:00 GMT

     [ https://issues.apache.org/jira/browse/HIVE-20983?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

LuGuangMing updated HIVE-20983:
-------------------------------
    Issue Type: Improvement  (was: Bug)

> 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: Improvement
>            Reporter: Gopal Vijayaraghavan
>            Assignee: Mustafa Iman
>            Priority: Major
>              Labels: pull-request-available
>             Fix For: 4.0.0
>
>         Attachments: HIVE-20983.1.patch, HIVE-20983.2.patch, HIVE-20983.3.patch, HIVE-20983.4.patch,
HIVE-20983.5.patch
>
>          Time Spent: 20m
>  Remaining Estimate: 0h
>
> 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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