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https://issues.apache.org/jira/browse/CALCITE-1587?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15828599#comment-15828599
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Julian Hyde commented on CALCITE-1587:
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Agree with [~gian], we should enable approximation only if the user asks for
it. In the Calcite framework I will add a property whether to allow approximate
topN, and another whether to allow approximate distinct-count. I do not think
it wise to expose it via a system or connection property, until we also extend
our SQL to allow users to ask for approximations in the SQL: see CALCITE-1588.
To fix this issue, add the property, default false, and the only way to set it
at present will be via code (e.g. from the test suite).
> Druid adapter: topN returns approximate results
> -----------------------------------------------
>
> Key: CALCITE-1587
> URL: https://issues.apache.org/jira/browse/CALCITE-1587
> Project: Calcite
> Issue Type: Bug
> Components: druid
> Affects Versions: 1.11.0
> Reporter: Jesus Camacho Rodriguez
> Assignee: Julian Hyde
> Fix For: 1.12.0
>
>
> Currently, we convert to _topN_ queries. However, metrics returned by Druid
> will be approximate values. Thus, probably we should not convert to Druid
> topN queries and rather always use Druid groupBy.
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