Function synth_rows 

pub fn synth_rows(
    desc: &RelationDesc,
    start: u64,
    n: u64,
    pad: usize,
) -> Vec<Row>

Generate n synthetic rows for desc, with row indices running start..start + n.

Successive batches over disjoint index ranges produce distinct rows that never consolidate, so a downstream count equals the total rows written (provided the schema carries a wide-enough column; see synth_cell).