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Building the Data Warehouse Third Edition phần 4

Các nhà phân tích DSS Bắt đầu với yêu cầu hiện tại, nhưng bao thanh toán trong yêu cầu mới gần như là một bất khả. Một chu kỳ cuộc sống rất khác nhau được kết hợp phát triển với Các kho dữ liệu. Có hai cấp độ của granularity cần thiết? Hình 4.2 Sử dụng đầu ra của các ước tính không gian. | Uttama Reddy The Data Warehouse and Design j07 selective use of redundancy Description is nonredundant and is used frequently but is seldom updated. Figure 3.29 Description is redundantly spread over the many places it is used. It must be updated in many places when it changes but it seldom if ever does. Occasionally the introduction of derived i.e. calculated data into the physical database design can reduce the amount of I O needed. Figure 3.31 shows such a case. A program accesses payroll data regularly in order to calculate the annual pay and taxes that have been paid. If the program is run regularly and at the year s end it makes sense to create fields of data to store the calculated data. The data has to be calculated only once. Then all future requirements can access the calculated field. This approach has another advantage in that once the field is calculated it will not have to be calculated again eliminating the risk of faulty algorithms from incorrect evaluations. Uttama Reddy CHAPTER 3 Figure 3.30 Further separation of data based on a wide disparity in the probability of access. introducing derived data week 1 pay taxes FICA other week 2 pay taxes FICA other week 3 pay taxes FICA other week 52 pay taxes FICA other program annual pay taxes FICA other annual pay taxes FICA other week 1 pay taxes FICA other week 2 pay taxes FICA other week 3 pay taxes FICA other week 52 pay taxes FICA other Figure 3.31 Derived data calculated once then forever available. One of the most innovative techniques in building a data warehouse is what can be termed a creative index or a creative profile a term coined by Les Moore . Figure 3.32 shows an example of a creative index. This type of creative index is created as data is passed from the operational environment to the data warehouse environment. Because each unit of data has to be handled in any case it requires very little overhead to calculate or create an index at this point. Uttama Reddy The Data Warehouse and Design