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https://issues.apache.org/jira/browse/CASSANDRA-8883?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14346325#comment-14346325
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Chris Lohfink commented on CASSANDRA-8883:
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+1 lgtm, thanks!
> Percentile computation should use ceil not floor in EstimatedHistogram
> ----------------------------------------------------------------------
>
> Key: CASSANDRA-8883
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8883
> Project: Cassandra
> Issue Type: Bug
> Components: Core
> Reporter: Chris Lohfink
> Assignee: Carl Yeksigian
> Priority: Minor
> Fix For: 2.1.4
>
> Attachments: 8883-2.1.txt
>
>
> When computing the pcount Cassandra uses floor and the comparison with
> elements is >= so given a simple example of there being a total of five
> elements
> {code}
> // data
> [1, 1, 1, 1, 1]
> // offsets
> [1, 2, 3, 4, 5]
> {code}
> Cassandra would report the 50th percentile as 2. While 3 is the more
> expected value. As a comparison using numpy
> {code}
> import numpy as np
> np.percentile(np.array([1, 2, 3, 4, 5]), 50)
> ==> 3.0
> {code}
> The percentiles was added in CASSANDRA-4022 but is now used a lot in metrics
> Cassandra reports. I think it should error on the side on overestimating
> instead of underestimating.
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