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https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ethan Wang updated PHOENIX-418:
-------------------------------
Description:
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. But with much
smaller memory foot print. Implemented using hyperloglog.
Merged patch link below, co-authorred with [~swapna]
https://git-wip-us.apache.org/repos/asf?p=phoenix.git;a=commitdiff;h=d6381afc3af976ccdbb874d4458ea17b1e8a1d32
was:
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).
Updated:
Syntax of using approximate count distinct as:
Select COUNT(DISTINCT ID) from person
Select APPROX_COUNT_DISTINCT(ID) from person
https://git-wip-us.apache.org/repos/asf?p=phoenix.git;a=commitdiff;h=d6381afc3af976ccdbb874d4458ea17b1e8a1d32
> 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. But with much
> smaller memory foot print. Implemented using hyperloglog.
> Merged 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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