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https://issues.apache.org/jira/browse/PHOENIX-418?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16140468#comment-16140468
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Ethan Wang edited comment on PHOENIX-418 at 8/24/17 8:34 PM:
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Thanks for suggestions [~jamestaylor].
{quote}Make sure to call TestUtil.analyzeTable(connection, fullTableName) prior
to running your TABLESAMPLE queries. You'll get more rows back, since you'll
have guideposts in addition to region boundaries.{quote}
Correct me if I'm mistaken. Within the test for approximate count distinct,
table sampling technique was not used since HyperLogLog doesn't rely on
sampling (hence no run time speed gain during counting). So updating guidepost
may have less impact on approximate count distinct.
was (Author: aertoria):
Thanks for suggestions [~jamestaylor].
bq. Make sure to call TestUtil.analyzeTable(connection, fullTableName) prior to
running your TABLESAMPLE queries. You'll get more rows back, since you'll have
guideposts in addition to region boundaries.
If I understand this part right, within the test for approximate count
distinct, table sampling technique was not used since HyperLogLog doesn't rely
on sampling (hence no run time speed gain during counting).
> 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
>
>
> 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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