Requirements are often unknown to begin with. Requirements emerge as the data warehouse is used to support decision making. The application must provide efficient performance which includes fast real time updates, high availability and data integrity. The application must provide access to a wide array of standardized and well integrated data. The architecture must be flexible so that data can be adjusted and new tools used.
Few organizations undertake the building of a data warehouse all at once.
The effort of building a complete, consolidated data warehouse would require an enormous effort and would take too long. A data mart can be created essentially by copying an operational database while retaining historical information. The problem with the standalone data mart approach is that it fails to integrate data between applications and across the organization. Creating data marts that can grow into an integrated data warehouse requires up front architectural effort. An architecture must be put into place so that there are clear guidelines for data mart builders.