Function record_path_arrangements 

fn record_path_arrangements(
    arrangements: &mut [BTreeSet<Vec<LirScalarExpr>>],
    raw: &mut [bool],
    source_relation: usize,
    source_key: Option<&Vec<LirScalarExpr>>,
    stage_plans: &[DeltaStagePlan],
)

Records what a single delta path reads from each input: each stage’s lookup_key and, for the seed relation, either its source_key arrangement or, when source_key is None, its raw collection (flagged in raw).