Couchbase rule based optimizer it doesn’t know selectivity. IntersectScan uses early termination vs pure intersect (explained here ). If IntersectScan not performing (it based on selectivity which doesn’t know), avoid Intersect Scan by providing USE INDEX hint.
When ever it uses IntersectScan, UnionScan query can’t be covered because indexes maintained asynchronously and each will have different snapshots.
Instead of offset 100K explore keyset pagination https://blog.couchbase.com/offset-keyset-pagination-n1ql-query-couchbase/