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From "Ethan Wang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PHOENIX-418) Support approximate COUNT DISTINCT
Date Mon, 31 Jul 2017 06:38:03 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16106864#comment-16106864
] 

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

Regarding the syntax of approximate discount.  Carry on from the discussion from PHOENIX-3390,
purposing the syntax to be

Original cardinality count function:
select count(distinct name) from person

With approximate:
select count(distinct name) from person APPROXIMATE 
select count(distinct name) from person APPROXIMATE 'hll' 
select count(distinct name) from person APPROXIMATE 'algorithm ABC' (WITHIN 10 PERCENT)


> 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
>
> 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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