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From "James Taylor (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PHOENIX-418) Support approximate COUNT DISTINCT
Date Mon, 28 Aug 2017 18:39:00 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16144178#comment-16144178
] 

James Taylor commented on PHOENIX-418:
--------------------------------------

[~aertoria]. Thanks for the updated patch. Please update the phoenix-core/pom.xml to use the
2.9.5 version of com.clearspring.analytics:stream instead of 2.7.0 if there's no reason not
to go with the latest.

Any idea why the test runs are so flaky? I'm going to try running them locally too, but it
seems like more than the usual suspects failing.

> 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
>
>
> 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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