CS240A: Databases and Knowledge Bases A Taxonomy of Temporal Databases

1
CS240A Databases and Knowledge BasesA Taxonomy
of Temporal Databases

• Carlo Zaniolo
• Department of Computer Science University of
  California, Los Angeles Revised January 2003

2
Valid Time and Transaction Time

• Valid Time of a fact when the fact is true in
  the modeled reality
• Transaction Time of a fact when it was recorded
  in the database
• Thus we have four different kinds of tables
• Snapshot
• Valid-time
• Transaction-time
• Bitemporal

3
Example Tom's Employment History

• On January 1, 1984, Tom joined thefaculty as an
  Instructor.
• On December 1, 1984, Tom completed his doctorate,
  and so was promoted to Assistant Professor
  effective retroactively on July 1, 1984.
• On March 1, 1989, Tom was promoted to Associate
  Professor, effective July 1, 1989 (proactive
  update).

4
Queries and Updates

• A transaction time table is append only it keeps
  the history of the updates made on the database.
• Transaction time tables supports rollback
  queries, such as
• On October 1, what rank was our database showing
  for Tom?
• A valid time table can be updated e.g., Toms
  past record is changed once his rank is changed
  retroactively.
• Valid time tables support historical queries,
  such as
• What was Toms rank on October 1 (according to
  our current database)?

5
Bitemporal Tables

• Bitemporal Tables are appendonly and supports
  queries of both kinds (rollbackhistorical) such
  as
• On October 1, 1984, what did we think Tom's rank
  was at that date?
• TSQL3
• SELECT Rank FROM Faculty AS F WHERE Name
  'Tom AND VALID(F) OVERLAPS DATE
  '19841001 AND TRANSACTION(F) OVERLAPS DATE
  '19841001'

6
Overview of Temporal Data Models

• What is timestamped?
• Tuples
• Attributes
• Group of Attributes
• Granularity of time-stamps
• Single instant in time (point-based
  representation)
• Maximum continuous periods
• Set of periods
• Explicit (operators on time-stamps) vs. implicit
  data model (as TSQL2)

7
More Timestamps?

• The value of an individual attribute
• Groups of attribute values
• Individual tuple
• Set of tuples Generally used for transaction
  time, to timestamp a set of tuples inserted or
  modified by a transaction.
• Object O-O DBs, XML documents
• Object graph E.g., associate a connected set of
  modules (a configuration) with a particular
  version identifier.
• Schema Item support for schema versions
  represents a difficult and important problem
• Events and time series very important!

8
Desiderata for a Temporal Data Model

• Capture the semantics of timevarying information
• Retain simplicity of the relational model Strict
  superset of the relational model
• Present all the information concerning an object
  in a coherent fashion
• Ensure ease of implementation
• Ensure high performance

9
Temporal DatabasesState of the Art

• Over 40 temporal data models and associated
  temporal query languages have been defined.
• Design space has been fairly well covered.
• A single data model satisfying all desirable
  objectives appears to be unattainable.
• TSQL2 a consensus approach proposed for
  inclusion in SQL3 standards.
• TSQL2 supports valid time, transaction-time and
  bitemporal relations, and
• Uses set of periods as its basic representation
  for time---but it is an implicit temporal model.