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https://issues.apache.org/jira/browse/CALCITE-2035?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16244596#comment-16244596
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Julian Hyde commented on CALCITE-2035:
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Thanks for your review, [~gian]; I incorporated your suggestions.
> Add APPROX_COUNT_DISTINCT aggregate function
> --------------------------------------------
>
> Key: CALCITE-2035
> URL: https://issues.apache.org/jira/browse/CALCITE-2035
> Project: Calcite
> Issue Type: Bug
> Reporter: Julian Hyde
> Assignee: Julian Hyde
> Fix For: 1.15.0
>
>
> Add {{APPROX_COUNT_DISTINCT}} aggregate function. The effect of
> {{APPROX_COUNT_DISTINCT(args)}} is the same as {{COUNT(DISTINCT args)}} but
> the planner may generate approximate results (e.g. by using HyperLogLog).
> Note "may" not "must", above: the planner may choose a plan that returns
> exact results.
> This is a step towards CALCITE-1588, which would allow an {{APPROXIMATE}}
> clause and specify in more detail the degree of approximation allowed.
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