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https://issues.apache.org/jira/browse/CALCITE-1803?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16033455#comment-16033455
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Julian Hyde commented on CALCITE-1803:
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Since Calcite rewrites {{AVG(x)}} to {{SUM(x) / COUNT(x)}}, this change should
enable
{code}SELECT "store_state", AVG("unit_sales") FROM "foodmart" GROUP BY
"store_state"{code}
to be pushed down to Druid in its entirety.
> Push Project that follows Aggregate down to Druid
> -------------------------------------------------
>
> 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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