The data is integrated and transformed before being loaded into the data warehouse. Users access the data warehouse by using a front-end tool or application. Data is made readily accessible without interrupting on-line operational workloads. System design is much simpler with one data source. There is no need to perform query optimization over heterogeneous sources, and since query execution does not involve data translation and communication with remote sources, complex queries can be executed easily and more efficiently.
Analytical applications are much easier to implement and use when data is consolidated.
A warehouse takes snapshots of operational data over and over again at specific intervals. These are then prepared for comparison, evaluation, projection, pattern recognition and related decision-support system activities. While the operational data could hold all the active accounts say, the warehouse data representing this activity over the last 24 months may well be at least 24 times that size. In operational applications, online querying is usually limited to very few rows at a time. The operator knows the primary key of the row of interest and the DBMS can locate, read and retrieve the required rows quickly.