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Posted to issues@hive.apache.org by "Gopal V (JIRA)" <ji...@apache.org> on 2018/11/29 08:07:00 UTC
[jira] [Updated] (HIVE-20983) Vectorization: Scale up small
hashtables, when collisions are detected
[ https://issues.apache.org/jira/browse/HIVE-20983?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Gopal V updated HIVE-20983:
---------------------------
Attachment: HIVE-20983.1.patch
> 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
> Assignee: Gopal V
> Priority: Major
> Attachments: HIVE-20983.1.patch
>
>
> 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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