Function evaluate_group_by 

pub fn evaluate_group_by(
    group_by: &PhysicalGroupBy,
    batch: &RecordBatch,
) -> Result<Vec<Vec<ArrayRef>>>

Evaluate a group by expression against a RecordBatch

Arguments:

• group_by: the expression to evaluate
• batch: the RecordBatch to evaluate against

Returns: A Vec of Vecs of Array of results The outer Vec appears to be for grouping sets The inner Vec contains the results per expression The inner-inner Array contains the results per row