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https://issues.apache.org/jira/browse/CALCITE-1803?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16023070#comment-16023070
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slim bouguerra commented on CALCITE-1803:
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[~axeisghost] druid support the equivalent of sql having as per the
[doc|http://druid.io/docs/latest/querying/having.html]
> Add post aggregation support in Druid to optimize druid queries.
> ----------------------------------------------------------------
>
> Key: CALCITE-1803
> URL: https://issues.apache.org/jira/browse/CALCITE-1803
> Project: Calcite
> Issue Type: New Feature
> Components: druid
> Affects Versions: 1.11.0
> Reporter: Junxian Wu
> Assignee: Julian Hyde
> Original Estimate: 336h
> Remaining Estimate: 336h
>
> Druid post aggregations are not supported when parsing SQL queries. By
> implementing post aggregations, we can offload some computation to the druid
> cluster rather than aggregate on the client side.
> Example usage:
> {{SELECT SUM("column1") - SUM("column2") FROM "table";}}
> This query will be parsed into two separate Druid aggregations according to
> current rules. Then the results will be subtracted in Calcite. By using the
> {{postAggregations}} field in the druid query, the subtraction could be done
> in Druid cluster. Although the previous example is simple, the difference
> will be obvious when the number of result rows are large. (Multiple rows
> result will happen when group by is used).
> Questions:
> After I push Post aggregation into Druid query, what should I change on the
> project relational correlation? In the case of the example above, the
> {{BindableProject}} will have the expression to representation the
> subtraction. If I push the post aggregation into druid query, the expression
> of subtraction should be replaced by the representation of the post
> aggregations result. For now, the project expression seems can only point to
> the aggregations results. Since post aggregations have to point to
> aggregations results too, it could not be placed in the parallel level as
> aggregation. Where should I put post aggregations?
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