Junxian Wu created CALCITE-1803:
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Summary: 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: Bug
Components: druid
Affects Versions: 1.11.0
Reporter: Junxian Wu
Assignee: Julian Hyde
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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