Function canonical_key_of_constraint 

pub fn canonical_key_of_constraint(c: &[(usize, OwnedValue)]) -> Vec<CanonVal>

Canonical key over a (ColId, OwnedValue) constraint’s values, in constraint order. For a join constraint built in to_key order this agrees with canonical_key(parent_row, to_key) for any parent the constraint matches — the two sides a flipped join compares to group each child under its parent’s IN-batch entry.