could the data warehouse be kept in an existing database?

[shuijian-xu]

Usually, the warehouse is developed separately from any other application, in a database of its own. One obvious reason for this is, as stated above, due to the disparate nature of the source systems. There are other reasons, however:

1. There is a conflict between operational systems and data warehouse systems in that operational systems are organized around operational requirements and are not subject oriented.

2. Operational systems are constantly changing, whereas decision support systems are “quiet” (nonvolatile).

3. Operational systems schemas are often very large and complex, where any two tables might have multiple join paths. These complex relationships exist because the business processes require them. It follows that a knowledge worker writing queries has more than one option when attempting to join two tables. Each possible join path will produce a different result. The decision support schema allows only one join path

[shuijian-xu]

Another reason for keeping the systems separate is that operational systems do not normally hold historical data. While this is a design issue and, clearly, these systems could hold such data, the effect on performance is not likely to be acceptable to the operational system users. It is usually acceptable for a strategic question to take several minutes, or even hours, to be answered. It would not usually be acceptable for operational system updates to take more than a few seconds.