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https://issues.apache.org/jira/browse/CALCITE-1803?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16021547#comment-16021547
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Junxian Wu commented on CALCITE-1803:
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Yes, "Post Aggregation" means doing something after aggregation. Here, I am
referring to the "post-aggregator" in druid
query.(http://druid.io/docs/0.10.0/querying/post-aggregations.html) By
implementing this, Calcite can push more things into the Druid query.
The post aggregation in Druid is similar to the idea of "Projects after
aggregation" you mentioned in Calcite, but it also has consequences when I try
to implement it.
If I change the DruidProjectRule and allow the project to be pushed into Druid
Query as a post-aggregator, the result of the Druid query will contain the
result of the project (Minus operation project). Then the project should point
to the result of the post-aggregator in the Druid query, but for now, it seems
the project can only point to the result of an aggregator in a druid query.
For your questions:
1. Filter cannot be done after aggregation because "fields" in druid filter
cannot refer to the aggregated columns.
2. The "ordering" should be the implementation of sort in Druid, and both
aggregator and post aggregator have "ordering" field. I think the sort could be
done in post-aggregation because that's what the "ordering" field in
post-aggregator does.
> 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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