Function top_k_by_frequency 

pub fn top_k_by_frequency(
    dataset: &DataSet,
    column: &str,
    k: usize,
) -> Option<Vec<(Value, i64)>>

Non-null value frequencies; returns the top k pairs by count (desc), breaking ties by [value_sort_key] ascending. k == 0 yields an empty vector.

Returns None if the column is not in the schema.