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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (PHOENIX-418) Support approximate COUNT DISTINCT
Date Wed, 30 Aug 2017 06:32:01 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16146707#comment-16146707
] 

Lars Hofhansl edited comment on PHOENIX-418 at 8/30/17 6:31 AM:
----------------------------------------------------------------

Shouldn't the clearspring library be part of the server jar? I just did a clean build from
scratch*, and indeed APPROX_COUNT_DISTINCT fails with
{code}
...
Caused by: java.lang.NoClassDefFoundError: com/clearspring/analytics/stream/cardinality/HyperLogLogPlus
...
{code}
thrown at the server.

\* mvn -DskipTests package


was (Author: lhofhansl):
Shouldn't the clearspring library be part of the server jar? I just did a clean build from
scratch, and indeed APPROX_COUNT_DISTINCT fails with
{code}
...
Caused by: java.lang.NoClassDefFoundError: com/clearspring/analytics/stream/cardinality/HyperLogLogPlus
...
{code}
thrown at the server.


> 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).
> 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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