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https://issues.apache.org/jira/browse/CALCITE-1787?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16023175#comment-16023175
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Zain Humayun commented on CALCITE-1787:
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While I agree that post aggregations make theta sketch aggregators more useful,
the theta sketch aggregators can still be used independently of post
aggregators as the example shows in the docs you linked. CALCITE-1803 is
currently open for post aggregations, so the thetaSketch post aggregation
functionality should be added there.
> thetaSketch Support for Druid Adapter
> -------------------------------------
>
> Key: CALCITE-1787
> URL: https://issues.apache.org/jira/browse/CALCITE-1787
> Project: Calcite
> Issue Type: New Feature
> Components: druid
> Affects Versions: 1.12.0
> Reporter: Zain Humayun
> Assignee: Julian Hyde
> Priority: Minor
>
> Currently, the Druid adapter does not support the
> [thetaSketch|http://druid.io/docs/latest/development/extensions-core/datasketches-aggregators.html]
> aggregate type, which is used to measure the cardinality of a column
> quickly. Many Druid instances support theta sketches, so I think it would be
> a nice feature to have.
> I've been looking at the Druid adapter, and propose we add a new DruidType
> called {{thetaSketch}} and then add logic in the {{getJsonAggregation}}
> method in class {{DruidQuery}} to generate the {{thetaSketch}} aggregate.
> This will require accessing information about the columns (what data type
> they are) so that the thetaSketch aggregate is only produced if the column's
> type is {{thetaSketch}}.
> Also, I've noticed that a {{hyperUnique}} DruidType is currently defined, but
> a {{hyperUnique}} aggregate is never produced. Since both are approximate
> aggregators, I could also couple in the logic for {{hyperUnique}}.
> I'd love to hear your thoughts on my approach, and any suggestions you have
> for this feature.
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