Prerequisites

Dependencies between the tasks result in a partial order on their completion, as shown in Figure . These dependencies take the following considerations into account.

• The design of the time-stamp representation and of the user-defined time domain are independent of extensions of the underlying data model to incorporate valid or transaction time.
• The constructs for historical selection should be consistent with those used in expressions involving user-defined time.
• The constructs for historical selection and projection, unlike those for user-defined time, require a specified data model.
• While constructs for schema specification do not require the operations of historical selection and projection to be elaborated, the constructs for schema evolution will require new tuples to be generated that are consistent with a modified schema.
• Schema versioning is only possible if transaction time is supported.

While white papers that address only one task are best, those that address several tasks in concert will certainly still be welcome. Also, these dependencies are only suggestive; work can certainly proceed on multiple tasks concurrently.