Rollup fact tables are typically the most heavily used fact tables in a data warehouse implementation. They provide a smaller set of information at a higher level of granularity. These fact tables typically have a monthly granularity in the time dimension and reflect figures for the month or month-to-date. They often combine multiple analysis areas (detail fact tables) into one super set of information. This allows simpler comparisons or joining of otherwise isolated pools of information.
An example may be the combination of daily sales, weekly demand forecasts, and monthly budgets, into one rollup fact table. Such a fact table provides the ability to compare monthly or month-to-date sales against budget and forecast.
These rollup/combined fact tables may and often do contain different levels of granularity, as well as different sets of dimensions.
For example, it may be possible to look at sales and forecasts by promotion, but the budget information does not have a populated promotion dimension, so we will not get a budgeted value for the promotion.