Databases Reference
In-Depth Information
Now() stands for a function, not a value. However, we will often use
Now()
to designate a specific point in time. For example, we may say
that a time period starts at Now() and continues on until 9999. This is a
shorthand way of emphasizing that, whenever that time period was
created, it was given as its begin date the value returned by Now() at that
moment. (From Chapter 3.)
Components:
clock tick, point in time.
object
Mechanics:
what is represented by the object identifier (oid) in an asserted
version table.
Semantics:
an instance of a type of thing which exists over time, has properties
and relationships, and can change over time.
Comments:
See also:
events
. Events, whether points in time or durations in time, are
not objects, because events, by definition, do not change.
Examples of objects include vendors, customers, employees, regulatory
agencies, products, services, bills of material, invoices, purchase orders,
claims, certifications, etc.
Components:
asserted version table, instance, object identifier, oid, represent,
type, thing.
object identifier
Mechanics:
the unique identifier of the persistent object represented by a row in
an asserted version table, used as part of the primary key of that row.
Comments:
The unique identifier of a row in an asserted version table is the
concatenation of an object identifier, an effective begin date, and an
assertion begin date.
Components:
asserted version table, persistent object.
occupied
Mechanics:
a series of one or more clock ticks is occupied by an object if and only
if those clock ticks are all included within the effective time period of a
version of that object.
Semantics:
a time period is occupied by an object if and only if the object is
represented in every clock tick in that time period.
Components:
clock tick, effective time period, include, object, represent,
version.
oid
See
object identifier
.
ontological time
Semantics:
the ontological time of a row in a bi-temporal table is the period of
time during which its referenced object exists.
Comments:
A neutral term referring to either the standard temporal model's valid
time or to Asserted Versioning's effective time.
Components:
bi-temporal table, object, referent, time period.
open episode
Mechanics:
An episode whose effective end date is 9999.
Semantics:
an episode whose effective end date is not known.
