Introduction to MSSQL Server 2000 Analysis Services: Partitioning a Cube in Analysis Services - An Introduction - Page 2
October 11, 2004
Partition a Cube in Analysis Services
Objective and Business Scenario
Partitions allow us to divide a logical cube into separate "storage containers" for our data and its aggregations. All MSAS cubes have at least one partition, which is created along with the cube automatically. Additional partitions are not a requirement, and, indeed, many cubes perform well enough to meet our business needs with only the partition with which they were originally created. Nevertheless, very large cubes can benefit dramatically from a well-conceived partitioning strategy, whose benefits include enlarged flexibility in data storage and data source location, as well as enhanced query performance.
The creation of separate physical partitions means that we can design cube storage around our expected uses, particularly our expected access frequency, of the data in the partitions we design. An example of a common partitioning strategy that I see on client engagements includes the creation of one partition for each quarter of the current operating year, together with a separate partition for each prior year.
Because cube querying, and reporting in general, is typically at its highest frequency in the current year and its constituent quarters, we would likely want to design the "current" partitions with MOLAP storage, along with the aggregations required to deliver rapid query results. Prior years' partitions would receive perhaps less aggregation, because the further in the past a given year from the current year, the less routine demand we would anticipate for more than summary information. The need for access to the partitions in general would decrease with age, so the storage design strategy for those years' data might become more focused upon size conservation, and less upon query performance.
A cube containing financial information can, therefore, contain a partition for the data of each past year, and also partitions for each quarter of the current year. At the end of the year, the four quarterly partitions can be merged into a single partition for the year.
The "separate container" concept means each partition can have different combinations of options, enabling us to design cube storage strategies appropriate to our unique business requirements. These combinations include:
Many variations with regard to partition data sources are possible; we will investigate some of the alternatives in subsequent articles. We can store partition aggregate data on the Analysis server computer where the partition is defined (the default) or on another Analysis server computer, as a remote partition. Storage mode determines whether the partition's aggregate data is stored in a database specified in the partition's data source (ROLAP), stored completely within a cube on an Analysis server computer (MOLAP) or stored within a custom combination of the two (HOLAP).
Aggregation design, which specifies the number and contents of the aggregations created for the partition, can be different for each partition. As we learned in Basic Storage Design, we can tailor a partition's aggregation design, defining the balance between query performance and storage utilization, with the Storage Design Wizard. Moreover, with the Usage-Based Optimization Wizard, we can perform these same actions, as well as optimizing the aggregation design based on queries (which we can select) that the organization's information consumers have previously executed against the partition's cube. As we noted in our articles surrounding Storage Design and Usage-Based Optimization, the aggregations that we design in either case are actually created as a part of the subsequent cube processing cycle.
Considerations and Comments
For purposes of this exercise, we will prepare a copy of Budget cube, one of the sample cubes within the FoodMart 2000 sample database that accompanies the installation of MSAS. The "clone" will allow us to leave the original sample cube in its existing condition, as we might have saved various settings, structures, and so forth, for referential or other reasons. There will therefore be no need to remember to return and remove settings that we modify for purposes of the lesson, or otherwise restore the original sample to its previous state.
While the cloning process is simple, ensure that you have the authority, access and privileges needed to accomplish the process, and that the copy of an existing MSAS cube (which we will, of course, rename) presents no other issues in your environment. After the session, the clone we have created can be deleted or used for another purpose, whatever is convenient.
If the sample database / Budget cube was not installed, or was removed prior to your beginning this article, please see the MSAS documentation, including the Books Online, for the straightforward procedure to restore the database from the archive (.cab) file containing the samples. As of this writing, a copy of the archive can be obtained from the installation CD or via download from the appropriate Microsoft site(s).