Virtual Column Aggregations

[winsmith]

We want to be able to support things like multiplication of count with the float value, but an arithmetic post aggregator can only multiply things after they've already been summed up. As far as I can tell, we'd need to support Druid's virtual column feature for this to work.

Example SQL query:

SELECT
  feature,
  SUM(count * floatValue) AS computed_value_sum
FROM "telemetry-signals"
WHERE appID=myAppId
GROUP BY feature;