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https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16108353#comment-16108353
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James Taylor commented on PHOENIX-418:
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Let's just keep it simple and support APPROX_COUNT_DISTINCT. That way we don't
need any grammar changes.
> 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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