Building the Data Warehouse

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Figure 8.6 There is only a faint resemblance of external data/unstructured data to a data model. Furthermore, nothing can be done about reshaping external data and unstructured data.

mistake. The most that can be done is to create subsets of the data that are compatible with the existing internal data.

Secondary Reports

Not only can primary data be put in the data warehouse, but when data is repetitive in nature, secondary reports can be created from the detailed data over time. For example, take the month-end Dow Jones average report shown in Figure 8.7.

In the figure, Dow Jones information comes into the data warehouse environment daily. The daily information is useful, but of even more interest are the long-term trends that are formed. At the end of the month, the Dow Jones average is shuffled off into a secondary report. The secondary report then becomes part of the store of external data contained in the data warehouse.

Figure 8.7 Creating a summary report from daily or monthly recurring information.

Archiving External Data

Every piece of information—external or otherwise—has a useful lifetime. Once past that lifetime, it is not economical to keep the information. An essential part of managing external data is deciding what the useful lifetime of the data is. Even after this is determined, there remains the issue of whether the data should be discarded or put into archives. As a rule, external data may be removed from the data warehouse and placed on less expensive storage. The meta data reference to the external data is updated to reflect the new storage place and is left in the meta data store. The cost of an entry into the meta data store is so low that once put there, it is best left there.

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