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Date: Wed, 30 Aug 2017 17:54:00 +0000 (UTC)
From: "James Taylor (JIRA)" <jira@apache.org>
To: dev@phoenix.apache.org
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Subject: [jira] [Reopened] (PHOENIX-418) Support approximate COUNT DISTINCT
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archived-at: Wed, 30 Aug 2017 17:54:08 -0000


     [ https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

James Taylor reopened PHOENIX-418:
----------------------------------

> 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
>             Fix For: 4.12.0
>
>         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).
> Update:
> Syntax of using approximate count distinct as:
> select APPROX_COUNT_DISTINCT(name) from person
> select APPROX_COUNT_DISTINCT(address||name) from person
> It is equivalent of  Select COUNT(DISTINCT ID) from person. Implemented using hyperloglog, see discuss below.
> Source code patch link below, co-authorred with [~swapna]
> https://git-wip-us.apache.org/repos/asf?p=phoenix.git;a=commitdiff;h=d6381afc3af976ccdbb874d4458ea17b1e8a1d32


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