Existing data warehouse tables can be identified to QAD Data Warehouse Designer. The tables are seen and can be managed to a degree. The main disadvantage is the increased difficulty in utilizing these tables when trying to extend the data warehouse. This option is however significantly quicker and easier than a rebuild. It is discussed in detail in the Integrate (see "Integrating" on page 978) section of this chapter.

The decision as to which option to choose will depend on the size and complexity of the existing data warehouse. Another important factor is the degree to which the existing data warehouse is to be extended. If future enhancements revolve around new analysis areas that have little overlap with the existing environment then a integrate may be the best answer. If the data warehouse is small and relatively simple than a rebuild may be worth considering. In any event the best plan may be to do a test integrate and then re-evaluate the situation.

The rebuild process essentially is a total re-creation of the data warehouse. One of the major impacts of such an approach is the 'end user layer', or rather the effect on the end user tools and saved queries and reports that are currently in use. The redesign or redeployment of this interface to the end users of the data warehouse may be too large a task to undertake. The problem can be circumvented to some degree though the use of views to make the new data warehouse environment look the same as the previous. But it is this impact and the subsequent re-testing process that must be considered when deciding to undertake a rebuild.

The advantages of a rebuild is the seamless integration of future analysis areas into the data warehouse and the single point of management that is provided. The major steps in the rebuild process will depend very much on the environment being replaced. As a guideline the following steps may be worth considering.