[jira] [Commented] (SPARK-22208) Improve percentile_approx by not rounding up targetError and starting from index 0

["Felix Cheung (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SPARK-22208?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16333659#comment-16333659 ] 

Felix Cheung commented on SPARK-22208:
--------------------------------------

Is this documented in the SQL programming guide/ migration guide?

[~ZenWzh]

[~smilegator]

 

> Improve percentile_approx by not rounding up targetError and starting from index 0
> ----------------------------------------------------------------------------------
>
>                 Key: SPARK-22208
>                 URL: https://issues.apache.org/jira/browse/SPARK-22208
>             Project: Spark
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 2.3.0
>            Reporter: Zhenhua Wang
>            Assignee: Zhenhua Wang
>            Priority: Major
>             Fix For: 2.3.0
>
>
> percentile_approx never returns the first element when percentile is in (relativeError, 1/N], where relativeError default is 1/10000, and N is the total number of elements. But ideally, percentiles in [0, 1/N] should all return the first element as the answer.
> For example, given input data 1 to 10, if a user queries 10% (or even less) percentile, it should return 1, because the first value 1 already reaches 10%. Currently it returns 2.
> Based on the paper, targetError is not rounded up, and searching index should start from 0 instead of 1. By following the paper, we should be able to fix the cases mentioned above.



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