[jira] [Commented] (CALCITE-1803) Add post aggregation support in Druid to optimize druid queries.

["Julian Hyde (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CALCITE-1803?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16020492#comment-16020492 ] 

Julian Hyde commented on CALCITE-1803:
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Do you mind if we change the terminology? "Post aggregation" suggests aggregation that happens after something. But I think you mean "Post-aggregation projects". Or in simpler English, "Projects after aggregation".

To answer your question: You will need to have a DruidQuery that contains a Scan followed by an Aggregate followed by a Project.

Currently DruidProjectRule will not allow the Project to be pushed in, because "sap" (scan, aggregate, project) is not a valid signature according to DruidQuery.VALID_SIG. But you should make it valid.

I'm curious:
* Does Druid allow filters after aggregation? (I.e. HAVING)
* I know that Druid allows sort after aggregation. But is this before or after the post-aggregation projects?

> 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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