Building the Data Warehouse

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Redundancy


One of the issues of a global data warehouse and its supporting local data warehouses is redundancy or overlap of data. Figure 6.12 shows that, as a policy, only minimal redundant data exists between the local levels and the global levels of data (and in this regard, it matters not whether global data is stored locally in a staging area or locally). On occasion, some detailed data will pass through to the global data warehouse untouched by any transformation or conversion. In this case, a small overlap of data from the global data warehouse to the local data warehouse will occur. For example, suppose a transaction occurs in France for US$10,000. That transaction may pass through to the global data warehouse untouched.


On the other hand, most data passes through some form of conversion, transformation, reclassification, or summarization as it passes from the local data warehouse to the data warehouse. In this case, there is—strictly speaking—no redundancy of data between the two environments. For example, suppose that a HK$175,000 transaction is recorded in Hong Kong. The transaction may be broken apart into several smaller transactions, the dollar amount may be converted, the transaction may be combined with other transactions, and so forth. In this case, there is certainly a relationship between the detailed data found in the local data warehouse and the data found in the global data warehouse. But there is no redundancy of data between the two environments.


A massive amount of redundancy of data between the local and the global data warehouse environments indicates that the scopes of the different warehouses probably have not been defined properly. When massive redundancy of data exists between the local and the global data warehouse environments, it is only a matter of time before spider web systems start to appear. With the appearance of such systems come many problems—reconciliation of inconsistent results, inability to create new systems easily, costs of operation, and so forth. For this reason, it should be a matter of policy that global data and local data be mutually exclusive with the exception of very small amounts of data that incidentally overlap between the two environments.

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