Represents a constraint application on a single parent SubPlan.
Represents a base relation defined by the instance set of an enumerable PConstraint; there are no parent SubPlans.
Represents a natural join of two parent SubPlans.
Abstract superclass for representing a high-level query evaluation operation.
Represents a projection of a single parent SubPlan onto a limited set of variables.
No constraints, and no parent SubPlan, just a (possibly empty) set of a priori known (input) variables.