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https://issues.apache.org/jira/browse/PHOENIX-3390?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16062445#comment-16062445
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Julian Hyde commented on PHOENIX-3390:
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[~gjacoby], Thanks for the heads up. Even more pertinent than my data profiling
work (which is about the problem of computing 2^n approximate distinct-counts
simultaneously, see CALCITE-1616) is people's requirement to have fast,
approximate distinct count. Druid supports various sketches, and we wish to
surface them in Calcite's Druid adapter (see CALCITE-1787 theta-sketch,
CALCITE-1587 top-N, CALCITE-1853 knowing when approximate count-distinct is
acceptable).
Today many databases have a syntax for approximate aggregates, and
unfortunately the syntaxes are rarely the same and are often too closely
coupled to a particular algorithm (e.g HyperLogLog). I have logged CALCITE-1588
to introduce an {{APPROXIMATE}} clause, e.g. {{COUNT(DISTINCT customerId)
APPROXIMATE (WITHIN 10 PERCENT))}}. It would be great if Phoenix wants to go
with that syntax.
> Custom UDAF for HyperLogLogPlus
> -------------------------------
>
> Key: PHOENIX-3390
> URL: https://issues.apache.org/jira/browse/PHOENIX-3390
> Project: Phoenix
> Issue Type: New Feature
> Reporter: Swapna Kasula
> Assignee: Ethan Wang
> Priority: Minor
>
> With ref # PHOENIX-2069
> Custome UDAF to aggregate/union of Hyperloglog's of a column and returns a
> Hyperloglog.
> select hllUnion(col1) from table; //returns a Hyperloglog, which is the
> union of all hyperloglog's from all rows for column 'col1'
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