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https://issues.apache.org/jira/browse/CALCITE-1787?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16037588#comment-16037588
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Joshua Walters commented on CALCITE-1787:
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[~julianhyde] There is a problem with this approach due to how Druid schema is
usually designed. This link explains this:
http://druid.io/docs/latest/ingestion/schema-design.html (search for {{High
cardinality dimensions (e.g. unique IDs)}}).
In Druid, you don't want to store very high cardinality columns like
{{user_id}}, you want to store those as aggregates (sketches). This is because
for each distinct combination, there will be a rollup stored in Druid. If you
have a column with cardinality in the billions, then Druid will have to store
billions of rows. In practice, Druid dimensions are usually less than 100k, and
in most systems I have seen each dimension is less than a few hundred unique
values.
In summary, if you have a column like {{user_id}} in Druid, you store it only
as a metric, and never as a dimension. You can't filter on it, it can only be
an output metric.
> 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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