Major Description
The db field of the Criterion specifies which database dialect is used for rendering this Criterion in SQL.
In my opinion the database dialect should be determined from a criteria field, not from any nested object.
Implications:
As a criterion is no standalone class, but an inner class of a Criteria, it always has a criteria attached and its SQL is only rendered in the context of this criteria. As the criteria has a field dbName by which the db adapter can be found out via Torque.getDB(criteria.getDbName()), I propose to remove the db field from Criterion together with its getters and setters and use the dbName field of the criteria instead to determine which Adapter should be used.