Function unproject_agg_exprs 

pub(crate) fn unproject_agg_exprs(
    expr: Expr,
    agg: &Aggregate,
    windows: Option<&[&Window]>,
) -> Result<Expr>

Recursively identify all Column expressions and transform them into the appropriate aggregate expression contained in agg.

For example, if expr contains the column expr “COUNT()” it will be transformed into an actual aggregate expression COUNT() as identified in the aggregate node.