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https://issues.apache.org/jira/browse/CALCITE-1787?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16032985#comment-16032985
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slim bouguerra commented on CALCITE-1787:
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[~zhumayun] please read the sketch docs.
1 - Don't agree with the claim that queries like {code} SELECT COUNT(DISTINCT
"user_unique") FROM "foodmart" WHERE "the_month" = 'April' AND "store_city" =
'Seattle'; {code} works fine.
Pushing only filters to druid will produce the wrong results, you need post
aggregation and filtered aggregator to do the intersection between sketches,
without intersection the result you get is the union which means you have
counted duplication thus you are not getting unique counts.
2 - for the filters on metrics or the more general case when we can not push
filter/query to druid in fact calcite can not do much, sketch is a binary blob
that needs that needs ser/desr library, not sure what is the perfect path to
take, i don't know calcite well to provide a question.
> 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: Zain Humayun
> 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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