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https://issues.apache.org/jira/browse/HIVE-14797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15565084#comment-15565084
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Rui Li commented on HIVE-14797:
-------------------------------

I did some tests locally. It turns out {{hconf.getInt(JobContext.NUM_REDUCES, 
-1)}} may not give us the number of reducers, i.e. just get -1. In my test, the 
#reducers is automatically determined and stored in ReduceSinkDesc.
We need to find out which way is more reliable to get #reducers.

> reducer number estimating may lead to data skew
> -----------------------------------------------
>
>                 Key: HIVE-14797
>                 URL: https://issues.apache.org/jira/browse/HIVE-14797
>             Project: Hive
>          Issue Type: Improvement
>          Components: Query Processor
>            Reporter: roncenzhao
>            Assignee: roncenzhao
>         Attachments: HIVE-14797.2.patch, HIVE-14797.3.patch, HIVE-14797.patch
>
>
> HiveKey's hash code is generated by multipling by 31 key by key which is 
> implemented in method `ObjectInspectorUtils.getBucketHashCode()`:
> for (int i = 0; i < bucketFields.length; i++) {
>       int fieldHash = ObjectInspectorUtils.hashCode(bucketFields[i], 
> bucketFieldInspectors[i]);
>       hashCode = 31 * hashCode + fieldHash;
>     }
> The follow example will lead to data skew:
> I hava two table called tbl1 and tbl2 and they have the same column: a int, b 
> string. The values of column 'a' in both two tables are not skew, but values 
> of column 'b' in both two tables are skew.
> When my sql is "select * from tbl1 join tbl2 on tbl1.a=tbl2.a and 
> tbl1.b=tbl2.b" and the estimated reducer number is 31, it will lead to data 
> skew.
> As we know, the HiveKey's hash code is generated by `hash(a)*31 + hash(b)`. 
> When reducer number is 31 the reducer No. of each row is `hash(b)%31`. In the 
> result, the job will be skew.



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