1. CS240A: Databases and Knowledge Bases A Taxonomy of Temporal DBs Carlo Zaniolo Department of Computer Science University of California, Los Angeles

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: 1. Snapshot 2. Valid-time 3. Transaction-time 4. 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., Tom’s past record is changed once his rank is changed retroactively.  Valid time tables support historical queries, such as:  What was Tom’s rank on October 1 (according to our current database)?

5. Bitemporal Tables  Bitemporal Tables are append­only and supports queries of both kinds (rollback&historical) 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?  Tuple timestamping  Attribute­value timestamping

7. Tuple Timestamping and Coalescing  Time stamping the individual tuples: If we want the salary history, we have to coalesce the last three tuples into one: nameempnosalarytitle deptno startend Bob1000360000Engineerd011995-01-011995-05-31 Bob1000370000Engineerd011995-06-011995-09-30 Bob1000370000Sr Engineerd021995-10-011996-01-31 Bob1000370000Tech Leaderd021996-02-011996-12-31 nameempnosalarystartend Bob10003600001995-01-011995-05-31 Bob10003700001995-06-011996-12-31

8. Attribute Timestamping-  Time-stamped tuples in relations nameempnosalarytitle deptno startend Bob1000360000Engineerd011995-01-011995-05-31 Bob1000370000Engineerd011995-06-011995-09-30 Bob1000370000Sr Engineerd021995-10-011996-01-31 Bob1000370000Tech Leaderd021996-02-011996-12-31 nameempnosalarytitledeptno Bob 1995-01-01: 1996-12-31 10003 1995-01-01: 1996-12-31 60000 1995-01-01: 1996-05-31 Engineer 1995-01-01: 1995-09-30 d01 1995-01-01: 1995-09-30 70000 1995-06-01:1996-12-31 Sr Engineer 1995-10-01:1996-01-31 d02 1995-10-01: 1996-12-31 Tech Leader 1996-02-01: 1996-12-31  Time-stamped attributes: Temporal grouping

9. What Is Timestamped?  The value of an individual attributes: temporally grouped data models.  Individual tuples  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.  Granularity of time-stamps :  Maximum continuous periods  Set of periods  Single instant in time (point-based representation—avoids coalescing)  Plethora of data models and query language extensions thus proposed

10. 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

11. Temporal Databases: State 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.