Folks, I wrote a one pager on subquery support in Pinot. Please see and comment: https://docs.google.com/document/d/1_dQkNLmadOQUq6wtTykgJgNQ3Y_plzSZYg_kd4QSYcE/edit?usp=sharing

The devil is in details here. It would be good to see some discussion along the lines of what is in https://github.com/apache/incubator-pinot/issues/5664 (specially correlated vs. not-correlated subqueries, how their underlying implementation will look like, and any overlap with joins).

Regarding correlated and uncorrelated subqueries, they are relevant when the planner is trying to pull up sublinks or flatten subqueries into outer joins. In the proposal stated here, subqueries are modeled as UDFs, thus not needing to "open" the black box and letting the subquery plan be executed independently on top of the inner query result. Thus correlations and joins become irrelevant.