Ethan Wang updated PHOENIX-418:
    Attachment: PHOENIX-418-v4.patch

Make DistinctCountHyperLogLogAggregateFunction extends 
DistinctCountAggregateFunction to inherit many benefits instead of reimplement 

Thanks [~jamestaylor] I should've done in this way the first place.

> 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
> 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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