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https://issues.apache.org/jira/browse/HIVE-3455?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13458422#comment-13458422
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Edward Capriolo commented on HIVE-3455:
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I will look into this more. You example uses a select * which does not have a
reduce phase. So I do not see how your example produces that result.
> ANSI CORR(X,Y) is incorrect
> ---------------------------
>
> Key: HIVE-3455
> URL: https://issues.apache.org/jira/browse/HIVE-3455
> Project: Hive
> Issue Type: Bug
> Components: UDF
> Affects Versions: 0.7.1, 0.8.0, 0.8.1, 0.9.0, 0.10.0
> Reporter: Maxim Bolotin
>
> A simple test with 2 collinear vectors returns a wrong result.
> The problem is the merge of variances, file:
> http://svn.apache.org/viewvc/hive/trunk/ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDAFCorrelation.java?revision=1157222&view=markup
> lines:
> 347: myagg.xvar += xvarB + (xavgA-xavgB) * (xavgA-xavgB) * myagg.count;
> 348: myagg.yvar += yvarB + (yavgA-yavgB) * (yavgA-yavgB) * myagg.count;
> the correct merge should be like this:
> 347: myagg.xvar += xvarB+(xavgA - xavgB)*(xavgA-xavgB)/myagg.count*nA*nB;
> 348: myagg.yvar += yvarB+(yavgA - yavgB)*(yavgA-yavgB)/myagg.count*nA*nB;
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