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From "Ethan Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PHOENIX-418) Support approximate COUNT DISTINCT
Date Mon, 28 Aug 2017 21:32:02 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16144380#comment-16144380
] 

Ethan Wang commented on PHOENIX-418:
------------------------------------

Changes has been made to pom for using 2.9.5. The flaky IT tests went away with this batch
(v6).  
[~jamestaylor]

During the investigation, one thing observed is that both success runs happens when the Jenkins
node "ubuntu-eu2" pick it up (versus "H4" during the weekend). I'm not sure if this is related.
[~samarthjain]

> Support approximate COUNT DISTINCT
> ----------------------------------
>
>                 Key: PHOENIX-418
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-418
>             Project: Phoenix
>          Issue Type: Task
>            Reporter: James Taylor
>            Assignee: Ethan Wang
>              Labels: gsoc2016
>         Attachments: PHOENIX-418-v1.patch, PHOENIX-418-v2.patch, PHOENIX-418-v3.patch,
PHOENIX-418-v4.patch, PHOENIX-418-v5.patch, PHOENIX-418-v6.patch
>
>
> Support an "approximation" of count distinct to prevent having to hold on to all distinct
values (since this will not scale well when the number of distinct values is huge). The Apache
Drill folks have had some interesting discussions on this [here](http://mail-archives.apache.org/mod_mbox/incubator-drill-dev/201306.mbox/%3CJIRA.12650169.1369931282407.88049.1370645900553%40arcas%3E).
They recommend using  [Welford's method](http://en.wikipedia.org/wiki/Algorithms_for_calculating_variance_Online_algorithm).
I'm open to having a config option that uses exact versus approximate. I don't have experience
implementing an approximate implementation, so I'm not sure how much state is required to
keep on the server and return to the client (other than realizing it'd be much less that returning
all distinct values and their counts).



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