[jira] [Commented] (KYLIN-4083) Fact Distinct Column Step maybe failed or value lost when hashcode of the UHC column value is Integer.MIN_VALUE

["ASF GitHub Bot (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KYLIN-4083?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16885187#comment-16885187 ] 

ASF GitHub Bot commented on KYLIN-4083:
---------------------------------------

ZhengshuaiPENG commented on pull request #742: #KYLIN-4083, Fact Distinct Column Step maybe failed or value lost whe…
URL: https://github.com/apache/kylin/pull/742
 
 
   …n hashcode of the UHC column value is Integer.MIN_VALUE
 
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> Fact Distinct Column Step maybe failed or value lost when hashcode of the UHC column value is Integer.MIN_VALUE
> ---------------------------------------------------------------------------------------------------------------
>
>                 Key: KYLIN-4083
>                 URL: https://issues.apache.org/jira/browse/KYLIN-4083
>             Project: Kylin
>          Issue Type: Bug
>            Reporter: PENG Zhengshuai
>            Assignee: PENG Zhengshuai
>            Priority: Major
>
> In the Fact Distinct Column Step, kylin uses MR to de-dup the values of columns.
> If the column is UHC (ultra high cardinality) column and the value of the property *kylin.engine.mr.uhc-reducer-count* has been set greater than *1*, the Mapper task will write the output of UHC column values to different reducers by *FactDistinctColumnPartitioner* according to the reducer id 
> The reducer id will be calculated by hash, the implementation in *FactDistinctColumnsReducerMapping#getReducerIdForCol()*,  in this method, *the reducer id = reducerBeginIndex + Math.abs(value.hashCode()) % uhcReducerCount*
> When the value.hashCode() is Integer.MIN_VALUE, the Math.abs(value.hashCode()) return also Integer.MIN_VALUE. Thus the reducer id may return a negative value. This may cause the FactDistinctColumn step failed, or the UHC column value may be redirected to another reducer which not belongs to UHC column



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