The first three chapters in Part III dealt with dimension extensions. This chapter, on the other hand, discusses two techniques for extending the fact.
Some users, especially managers, may require data for a particular time. In other words, they need snapshots of data. The two fact extension techniques that deals with snapshots are periodic and accumulating.
A periodic snapshot is a periodic summary of the fact at a certain time. For example, a monthly sales order periodic snapshot is the total sales order amount at the end of every month.
An accumulating snapshot tracks the changes of a fact. For example, the data warehouse may need to accumulate (store) the data of a sales order as it progresses through the order life cycle, starting from the time the order was placed to the time products are allocated to fulfill the order to packing, shipping, and receiving. The user can take a snapshot on the accumulated sales order progress status at certain times.
The following sections discuss periodic snapshots and accumulated snapshots in detail.