Function aggregate_output_key 

pub fn aggregate_output_key(a: &AggregateExpr) -> String

The canonical output-column key an aggregate’s value is stored under in a finalized group row.

A computed argument is named by its source text and a FILTER clause is appended, so SUM(a * 2), SUM(b * 3) and COUNT(*) FILTER (WHERE ...) each land under their own key.