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From "Ethan Wang (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (PHOENIX-418) Support approximate COUNT DISTINCT
Date Fri, 16 Jun 2017 22:28:00 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16052446#comment-16052446
] 

Ethan Wang edited comment on PHOENIX-418 at 6/16/17 10:27 PM:
--------------------------------------------------------------

Reply to [~pctony]: also in Blink DB, they have similar approximate aggregate feature which
they use a sql clause. Example would be:
`select count(id) From table ERROR 0.1 CONFIDENCE 95%

http://blinkdb.org/#Performance


was (Author: aertoria):
Reply to [~pctony]: also in Blink DB, they have similar approximate aggregate feature which
they use a sql clause. Example would be:
`select count(id) From table ERROR 0.1 CONFIDENCE 95%

> 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: maghamravikiran
>              Labels: gsoc2016
>
> 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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