[jira] [Updated] (ARROW-7400) [Java] Avoids the worst case for quick sort

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-7400?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

ASF GitHub Bot updated ARROW-7400:
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    Labels: pull-request-available  (was: )

> [Java] Avoids the worst case for quick sort
> -------------------------------------------
>
>                 Key: ARROW-7400
>                 URL: https://issues.apache.org/jira/browse/ARROW-7400
>             Project: Apache Arrow
>          Issue Type: Improvement
>          Components: Java
>            Reporter: Liya Fan
>            Assignee: Liya Fan
>            Priority: Major
>              Labels: pull-request-available
>
> This issue is in response of a discussion in: https://github.com/apache/arrow/pull/5540#discussion_r329487232.
> The quick sort algorithm can degenerate to an O(n^2) algorithm, if the pivot is selected poorly. This is an important problem, as the worst case can happen, if the input vector is alrady sorted, which is frequently encountered in practice.
> After some investigation, we solve the problem with a simple but effective approach: take 3 samples and choose the median (with at most 3 comparisons) as the pivot. This sorts the vector which is already sorted in O(nlogn) time. 



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