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https://issues.apache.org/jira/browse/HBASE-22980?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16969786#comment-16969786
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Hudson commented on HBASE-22980:
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Results for branch branch-2
        [build #2347 on 
builds.a.o|https://builds.apache.org/job/HBase%20Nightly/job/branch-2/2347/]: 
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> HRegionPartioner getPartition() method incorrectly partitions the regions of 
> the table.
> ---------------------------------------------------------------------------------------
>
>                 Key: HBASE-22980
>                 URL: https://issues.apache.org/jira/browse/HBASE-22980
>             Project: HBase
>          Issue Type: Bug
>          Components: mapreduce
>            Reporter: Shardul Singh
>            Assignee: Shardul Singh
>            Priority: Major
>             Fix For: 3.0.0, 2.3.0, 2.1.8, 2.2.3
>
>
> *Problem:*
> Partitioner class HRegionPartitioner in a HBase MapReduce job has a method 
> getPartition(). In getPartition(), there is a scenario where we have check 
> for less number of reducers than region. This scenario seems incorrect 
> because for a rowKey present in last region(let us say nth region) , 
> getPartition() should return value (n-1). But it is not returning n-1 for the 
> last region as it is falling in the case where number of reducers < number of 
> regions and returning some random value. 
> So if a client uses this class as a partitioner class in HBase MapReduce 
> jobs, this method incorrectly partitions the regions because rowKeys present 
> in the last regions does not fall to the last region.
> [https://github.com/apache/hbase/blob/fbd5b5e32753104f88600b0f4c803ab5659bce64/hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/HRegionPartitioner.java#L92]
> Consider the following scenario:
> if there are 5 regions for the table, partitions = 5 and number of reducers 
> is also 5.
> So in this case above check for reducers < regions should not return true.
> But for the last region when i=4(last region, 5th region) , getPartition 
> should return 4 but it returns 2 because it falls in the case of when we have 
> less reduces than region and returns true for the above condition even though 
> we have reducers = regions. So the condition is incorrect.
>  
> *Solution:*
> Instead of
>       {code} if (i >= numPartitions-1) {code} 
> It should be
>        {code} if (i >= numPartitions){  {code}



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