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

["nichunen (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/KYLIN-4083?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

nichunen closed KYLIN-4083.
---------------------------
    Resolution: Fixed

> 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
> For example:
> If a UHC column value is "539019926", its hashcode is Integer.MIN_VALUE.
> "539019926".hashCode() == Integer.MIN_VALUE == -2147483648.  The Math.abs(-2147483648) returns -2147483648. 
> so the reducerId = beginIndex + (-2147483648) % uhcReducerCount.
> If the beginIndex is 8, uhcReducerCount is 35.The method *FactDistinctColumnsReducerMapping#getReducerIdForCol()* will return -15.
> To Fix it: convert hashCode() value to *long* when calculating Math.abs() to avoid Integer.MIN_VALUE instead of *int*.
> Because the hashCode() method return an int value,  Math.abs(longValue) will never meet the Long.MIN_VALUE, so it's safe
> After fix, *FactDistinctColumnsReducerMapping#getReducerIdForCol()* will return 31



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