[jira] [Commented] (HIVE-14797) reducer number estimating may lead to data skew

["Xuefu Zhang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HIVE-14797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15506967#comment-15506967 ] 

Xuefu Zhang commented on HIVE-14797:
------------------------------------

This seems making sense, but can we not hard code the number (31)?

> 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.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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