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From "Rui Li (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HIVE-14797) reducer number estimating may lead to data skew
Date Wed, 19 Oct 2016 02:40:59 GMT

    [ https://issues.apache.org/jira/browse/HIVE-14797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15587451#comment-15587451
] 

Rui Li commented on HIVE-14797:
-------------------------------

Hi [~xuefuz], for the example in the description, B is skewed but (A, B) shouldn't skew ideally.
Other than shuffling, bucketed table should also suffer from this if the number of buckets
happens to be 31, and we can't adjust the number of reducers in that case.
I think the problem is valid, but more research is needed to find out how the hash code is
used, and whether the solution here is correct.

> 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-14797.4.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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