Preface

This book is a major revision—in fact, a total rewrite—of an earlier one by the same authors. That earlier book was published by Morgan Kaufmann in 2003, under the title Temporal Data and the Relational Model. But the present book is so different from the previous one (except, to some extent, in its overall structure) that it doesn’t seem reasonable to refer to it as just a second edition. (We’ve even given it a different title, partly for that reason.) As a consequence, we won’t make any attempt in the text to call out specific points of difference with respect to that earlier book. Nor will we do what’s usually done with the preface to a new edition, which is first to repeat the preface from the previous edition and then to explain what’s changed in the new one. In other words, what you’re reading right now is, to all intents and purposes, the preface to a brand new book.

So what exactly is “temporal data”? Well, a temporal database system is one that includes special support for the time dimension; in other words, it’s a system that provides special facilities for storing, querying, and updating historical and/or future data. Database management systems (DBMSs for short) as classically understood aren’t temporal in this sense—they provide little or no special support for temporal data at all. However, this situation is beginning to change, for the following reasons among others:

Here are some examples of application scenarios where such temporal support is very definitely needed. The examples are taken, with permission, from “A Matter of Time: Temporal Data Management in DB2 10,” by Cynthia Saracco, Matthias Nicola, and Lenisha Gandhi (see Appendix F for further details):

That same paper by Saracco, Nicola, and Gandhi also gives some indication of the development benefits to be obtained from the availability of “native” temporal support in the DBMS (IBM’s DB2 product, in the case at hand):

Now, research into temporal databases isn’t new—technical papers on the subject have been appearing in the literature ever since the beginning of the 1980s, if not earlier. However, much of that research ultimately proved unproductive: It turned out to be excessively complicated, or it led to logical inconsistencies, or it failed to solve certain aspects of the problem, or it was unsatisfactory for some other reason. So we’ll have little to say about that research in this book (apart from a few remarks in Chapter 19 and in the annotation to some of the references in Appendix F). Instead, we’ll focus on what we regard as a much more promising approach, one that’s firmly rooted in the relational model of data, which those others mostly aren’t, or weren’t. Of course, it’s precisely because of its strong relational foundations that we believe the approach we favor will stand the test of time (as it were!). And since the approach in question is directly and primarily due to one of the present authors (Lorentzos), the book can be regarded as authoritative.

The book also includes much original material resulting from continuing investigations by all three authors, material that’s currently documented nowhere else at all. Examples include new database design techniques; a new normal form; new relational operators; new update operators; a new approach to the problem of temporal “granularity”; and support for “cyclic point types.”1 Overall, therefore, the book can be seen as, among other things, an abstract blueprint for the design of a temporal DBMS and the language interface to such a DBMS. In other words, it’s forward looking, in the sense that it describes not only how temporal DBMSs might and do work today, but also, and more importantly, how we think they should and will work in the future.

One further point: Although this book concentrates on temporal data as such, many of the concepts are actually of much wider applicability. To be specific, the basic data construct involved is the interval, and intervals don’t necessarily have to be temporal in nature. (On the other hand, certain of the ideas discussed are indeed specifically temporal ones: for example, the notion sometimes referred to, informally, as “the moving point now.”)

Structure of the Book

The body of the book is divided into four major parts:

To elaborate:

In addition to the foregoing, there are six appendixes, covering (as appendixes are wont to do) a somewhat mixed bag of topics. Appendixes A-C discuss possible extensions to certain of the notions introduced in the body of the book. Appendix D provides an abbreviated grammar, for reference purposes, for the language Tutorial D. Appendix E discusses implementation and optimization issues. Finally, as already indicated, Appendix F gives an annotated and consolidated list of references for the entire book....