[jira] [Created] (CALCITE-1828) Push the FILTER clause into Druid as a Filtered Aggregator

["Zain Humayun (JIRA)" <ji...@apache.org>]

Zain Humayun created CALCITE-1828:
-------------------------------------

             Summary: Push the FILTER clause into Druid as a Filtered Aggregator 
                 Key: CALCITE-1828
                 URL: https://issues.apache.org/jira/browse/CALCITE-1828
             Project: Calcite
          Issue Type: Improvement
          Components: druid
    Affects Versions: 1.12.0
            Reporter: Zain Humayun
            Assignee: Zain Humayun


Druid has support for a special aggregator it calls the [Filtered Aggregator|http://druid.io/docs/latest/querying/aggregations.html] that allows aggregations to occur with filters independent to other filters in the Druid query.

An example where the filtered aggregator is useful:

{code:sql}
SELECT 
sum("col1") FILTER (WHERE <condition1>),
sum("col2") FILTER (WHERE <condition2>)
FROM "table"; 
{code} 

Currently, calcite will scan Druid, then do the filtering and aggregation itself. With filtered aggregators, both the filter and aggregation and be pushed into Druid. 

*A few comments/questions:*

1) If all conditions in the filter clause are the same, then instead of pushing filtered aggregators individually, it would make more sense to push 1 single filter into the Druid query. I.e the filters can be factored out into 1 filter. I don't see calcite currently do this, does it have such a rule in place already?

2) The filters can/should only be pushed if they are filtering on dimension columns

3) Currently, the above query would create the following relation: 
DruidQuery -> Project -> Aggregate. There is already a rule called {{DruidAggregateProjectRule}} which matches the previous relation. Is it better to add logic to that rule, or to create a new rule that also matches that relation?



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