[jira] [Updated] (HIVE-7074) The reducer parallelism should be a prime number for better stride protection

["Gopal V (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HIVE-7074?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Gopal V updated HIVE-7074:
--------------------------

    Attachment: HIVE-7074.1.patch

First cut patch to set reducer-parallelism to a prime number in the optimizer.

> The reducer parallelism should be a prime number for better stride protection
> -----------------------------------------------------------------------------
>
>                 Key: HIVE-7074
>                 URL: https://issues.apache.org/jira/browse/HIVE-7074
>             Project: Hive
>          Issue Type: Improvement
>          Components: Statistics
>            Reporter: Gopal V
>            Assignee: Gopal V
>         Attachments: HIVE-7074.1.patch
>
>
> The current hive reducer parallelism results in stride issues with key distribution.
> a JOIN generating even numbers will get strided onto only some of the reducers.
> The probability of distribution skew is controlled by the number of common factors shared by the hashcode of the key and the number of buckets.
> Using a prime number within the reducer estimation will cut that probability down by a significant amount.



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