Typically, there is a vast quantity of cool historical data in the data warehouse and a lesser volume of hot and warm data.
Cool data, though accessed lightly on average, may have temporal hot spots, such as those that occur when performing comparative analyses of sales data between different time periods. If the relational schema is modified by adding new columns or indexes, or if column data types are changed, the affected data cannot be considered dormant. Also, data that is periodically accessed and recast to make historical data relevant within the current business context is not dormant.
Dormant data is typically retained for national and international regulatory reasons such as the Sarbanes-Oxley Act in the United States, Bill 198 in Canada, or the Eighth Company Law Directive 1984/253/EEC in the European Union rather than for operational reasons. Therefore, the data stored in the warehouse is frequently a mix of both important and unimportant data (Unimportant from the perspective of the day-to-day operation of running the enterprise, not from a legal perspective), and a flexible management system is required to allocate the appropriate availability, reliability, and privacy levels for the data as its usage changes across time.
Icy, or truly dormant, data is a good candidate for alternative storage such as tape or optical disk. The treatment of icy data is not the subject of this section.