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https://issues.apache.org/jira/browse/HIVE-14797?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15512290#comment-15512290
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Hive QA commented on HIVE-14797:
--------------------------------



Here are the results of testing the latest attachment:
https://issues.apache.org/jira/secure/attachment/12829740/HIVE-14797.3.patch

{color:red}ERROR:{color} -1 due to no test(s) being added or modified.

{color:red}ERROR:{color} -1 due to 6 failed/errored test(s), 10554 tests 
executed
*Failed tests:*
{noformat}
org.apache.hadoop.hive.cli.TestCliDriver.testCliDriver[acid_mapjoin]
org.apache.hadoop.hive.cli.TestCliDriver.testCliDriver[ctas]
org.apache.hadoop.hive.cli.TestCliDriver.testCliDriver[vector_join_part_col_char]
org.apache.hadoop.hive.cli.TestMiniTezCliDriver.testCliDriver[explainuser_3]
org.apache.hadoop.hive.metastore.TestMetaStoreMetrics.testMetaDataCounts
org.apache.hive.jdbc.TestJdbcWithMiniHS2.testAddJarConstructorUnCaching
{noformat}

Test results: https://builds.apache.org/job/PreCommit-HIVE-Build/1266/testReport
Console output: https://builds.apache.org/job/PreCommit-HIVE-Build/1266/console
Test logs: 
http://ec2-204-236-174-241.us-west-1.compute.amazonaws.com/logs/PreCommit-HIVE-Build-1266/

Messages:
{noformat}
Executing org.apache.hive.ptest.execution.TestCheckPhase
Executing org.apache.hive.ptest.execution.PrepPhase
Executing org.apache.hive.ptest.execution.ExecutionPhase
Executing org.apache.hive.ptest.execution.ReportingPhase
Tests exited with: TestsFailedException: 6 tests failed
{noformat}

This message is automatically generated.

ATTACHMENT ID: 12829740 - PreCommit-HIVE-Build

> 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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