Introduction to SQL Server 2000 Analysis Services: Exploring Virtual Cubes - Page 2

December 11, 2002

Exploring Virtual Cubes

Much as a "view" can logically combine tables, as well as other views, within a relational database, a virtual cube is a "logical cube" that is created through the combination of multiple cubes. We select the dimensions and measures of the virtual cube from the "pooled" dimensions and measures that exist within the cubes that we are consolidating; we do not need to use all dimensions and measures but can select those that provide us the most ideal "view" of the data that we are attempting to produce from the disparate data sources. As a logical cube, the virtual cube appears as a single cube from the perspective of the information consumer.

While virtual cubes often consist of selected dimensions and measures from multiple cubes, we will likely encounter situations in our organizations where, rather than combining the data from multiple OLAP data sources, we wish to restrict the data we present to information consumers to a subset of the dimensions and measures within a single cube. This represents yet another scenario where a virtual cube might provide an excellent option.

Other scenarios exist that are well addressed by the flexible functionality that virtual cubes readily offer. These include the use of linked cubes in addition to normal cubes in their creation.

Strengths of virtual cubes include their flexibility, portability and ease of creation and modification. In addition, the storage space required by a virtual cube is modest: since virtual cubes store only their own definitions, leaving the storage of the data of the cubes that underlie them strictly to the cubes within which the data resides, physical storage space is almost negligible. The minimal storage requirement adds handsomely to the flexibility factor, as it enables us to create combinations and variations of existing cubes without the introduction of material overhead into the equation.

The capacity of a virtual cube to act as an adjunct to an overall security strategy is also a strength. The physical security provided by the virtual cube lies in the options it provides to restrict access for selected information consumer groups to sensitive or other selected information.

As we mentioned before, a virtual cube can be based upon a linked cube, but we need to keep in mind the limitations when this is the case, including the fact that the virtual cube does not support (for the linked cube):

  • cell calculations
  • custom member formulas
  • custom rollup operators
  • custom rollup formulas

Creating Virtual Cubes

We will create virtual cubes for a couple of the main reasons that they are typically created: to consolidate information that exists in more than one cube, and to create a subset of information in a cube/combined cubes as a means of controlling its presentation/accessibility to information consumers. We will point out the reasons that these options might be valuable to information consumers or management of the organization as we undertake the steps involved.

Let's get started with an example that mirrors the primary reason for creating a virtual cube: to consolidate information. One common reason we might want to do this could be to allow for comparison between measures that exist in two separate cubes.

Page 3: Creating a Virtual Cube

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