Extracting Information
Opaque Partial Answers : No extraction possible.
Transparent Partial Answers : Can ask a “parachute” query which is related to the original query.
For example, a parachute query for the earlier example can be:
select x.name
from x in person0, y in person1
where x.name = y.name
Parachute query is evaluated by rewriting it over the materialized relations.