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From "Gopal V (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-17114) HoS: Possible skew in shuffling when data is not really skewed
Date Tue, 18 Jul 2017 03:07:00 GMT

    [ https://issues.apache.org/jira/browse/HIVE-17114?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16091039#comment-16091039
] 

Gopal V commented on HIVE-17114:
--------------------------------

[~lirui]: Changing the ObjectInspectorUtils.hashCode() implementation breaks all existing
bucketed tables, breaks SMB joins and Bucket map-joins.

Specifically this is a problem with Hive-on-Spark, which doesn't have an underlying partitioner
implementation for the ReduceSinkDesc$ReducerTraits.UNIFORM trait.

> HoS: Possible skew in shuffling when data is not really skewed
> --------------------------------------------------------------
>
>                 Key: HIVE-17114
>                 URL: https://issues.apache.org/jira/browse/HIVE-17114
>             Project: Hive
>          Issue Type: Bug
>            Reporter: Rui Li
>            Assignee: Rui Li
>            Priority: Minor
>
> Observed in HoS and may apply to other engines as well.
> When we join 2 tables on a single int key, we use the key itself as hash code in {{ObjectInspectorUtils.hashCode}}:
> {code}
>       case INT:
>         return ((IntObjectInspector) poi).get(o);
> {code}
> Suppose the keys are different but are all some multiples of 10. And if we choose 10
as #reducers, the shuffle will be skewed.



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