Function choose_index

src/transform/src/dataflow.rs:846–861

Pick an index from a given Vec of index keys. Currently, we pick as follows: - If there is an index on a unique key, then we pick that. (It might be better distributed, and is less likely to get dropped than other indexes.) - Otherwise, we pick an arbitrary index. TODO: There are various edge cases where a better choice would be possible: - Some indexes might be less skewed than others. (Althoug

(
    source_keys: &BTreeMap<GlobalId, BTreeSet<Vec<MirScalarExpr>>>,
    id: &GlobalId,
    indexes: &Vec<(GlobalId, Vec<MirScalarExpr>)>,
)

/// - Some indexes might have more extra data in their keys (because of being on more complicated
///   expressions than just column references), which won't be used in a full scan.
fn choose_index(
    source_keys: &BTreeMap<GlobalId, BTreeSet<Vec<MirScalarExpr>>>,
    id: &GlobalId,
    indexes: &Vec<(GlobalId, Vec<MirScalarExpr>)>,
) -> Option<(GlobalId, Vec<MirScalarExpr>)> {
    match source_keys.get(id) {
        None => indexes.iter().next().cloned(), // pick an arbitrary index
        Some(coll_keys) => match indexes
            .iter()
            .find(|(_idx_id, key)| coll_keys.contains(&*key))
        {
            Some((idx_id, key)) => Some((*idx_id, key.clone())),
            None => indexes.iter().next().cloned(), // pick an arbitrary index
        },
    }
}

#[derive(Debug)]
struct CollectIndexRequests<'a> {