Chapter 4. The Implications of Time in Data Warehousing

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The principal subject of this book is the design of data warehouses. One of the least well understood issues surrounding data warehouse design is the treatment and representation of time. This chapter introduces the characteristics of time and the way that it is used in data warehousing applications. The chapter goes on to describe fully the problems encountered with time. We need to introduce some rigor into the way that time is treated in data warehousing, and this chapter lays out the groundwork to enable that to be achieved. We also examine the more prominent solutions that other major practitioners have proposed in the past and that have been used ubiquitously in first-generation data warehouses. We will see that there are some issues that arise when these methods are adopted.

The presence of time and the dependence upon it is one of the things that sets data warehouse applications apart from traditional operational systems. Most business applications are suited to operating in the present environment where time does not require special treatment. In many cases, dates are no more than descriptive attributes. In a data warehouse, time affects the very structure of the system. The temporal requirements of a data warehouse are very different from those of an operational system, yet it is the operational system that feeds information about changed data to the data warehouse. In a temporal database management system, support for time would be implicit within the DBMS and the query language would contain time-specific functions to simplify the manipulation of time. Until such systems are generally available, the data warehouse database has to be designed to take account of time. The support for time has to be explicitly built into the table structures and the queries.

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