A list of valid configurations for the given database
Notes
The vschema and the database are coupled and passed to
the system as a VDB.
While it's possible to get the list of valid
configurations from the vschema presence condition, we're
not doing so for now!
The vquery is written in variational relational algebra
for now. We intend to introduce VSQL and add translations
from VSQL to variational relational algebra.
We don't have a separate data type for relational algebra
and the variational version of it. They're both encoded in
variational relational algebra except that queries written
in relational algebra does not allow for choices in
(sub)queries or parts of queries.
Trans-union-filter approach (unoptimized for now!)