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Posted Mar 14, 2005

Introduction to MSSQL Server Analysis Services: Point-and-Click Cube Schema Simplification - Page 2

By William Pearson

Simplify Schemas for Faster Cube Processing

Overview and Discussion

Cube processing performance is often a high visibility subject to our clients and employees. There are many means and considerations in optimizing build times, but few are as straightforward as the use of Optimize Schema, which quickly and "automatically" makes modifications that can provide palpable improvements, in many cases. Manual intervention can become necessary, in some instances, to achieve the intended objectives of the option for all dimensions, but the concepts behind the steps are identical.

The way this all works is, simply, that the Member Key Column property, aligned with the lowest level of a shared dimension in the cube, is pointed to a column in the fact table instead of a column in a dimension table. When this can be accomplished, the need for joins is eliminated in the SQL query that MSAS executes against the relational data source to populate the cube. As we might expect, such a circumstance imparts multiple benefits, including reductions in query complexity, reductions in the amount of data accessed in the relational data source, and reductions in network traffic between the Analysis Server and the relational data source.

Several conditions must exist for schema optimization (via Optimize Schema, or its manual equivalent) to be effective. For any dimension under consideration, these conditions are as follows:

  • The dimension must be a shared dimension;
  • The dimension must have been processed at least once prior to selecting the Optimize Schema option;
  • The lowest dimension level must be physically represented in the cube (it can be hidden, but not disabled);
  • The Member Key Column property for the lowest dimension level must contain the key that relates the fact table to the dimension table;
  • The Member Key Column property for the lowest dimension level must be the only key that is needed to relate the fact and dimension tables;
  • The Member Key Column property for the lowest dimension level must be unique.

Multiple considerations can arise in altering the schema of a cube through the Optimize Schema option. These include scenarios where our cube design is dependent upon joins between the dimension and fact tables, as a mechanism for forcing the exclusion of rows of the fact table from the cube that is to be generated. As is no doubt apparent to most of us, removal of all joins through Optimize Schema, or any other process will mean a corresponding removal of the filtering mechanism, and the resulting selection of all rows in the fact table.

Moreover, it is important to keep in mind that the Optimize Schema process, in its elimination of existing joins, can alter our selection of available tables in subsequent attempts to specify drillthrough options. This, like all the other considerations, needs to be evaluated as a part of planning to use the Optimize Schema option, or to pursue other steps to replicate its action, to enhance cube processing performance.

NOTE: When manually optimizing a cube schema, it is important to remember that no warning or error dialogs will appear if the incorrect column is selected in the respective Member Key Column property. If incorrect choices are made, incorrect values may be returned, or cube processing may fail altogether. If the optimized dimension is used as a slicer within a cube partitioning strategy, processing failures are likely, as well.

Within the "manual" scenario, consideration needs to be given to examining several variables before attempting to perform schema optimization. Consult the Microsoft Developer Network (MSDN), the Books Online, and other reliable documentation before attempting this on any but development systems - and with a backup in place before beginning in every case.

To confirm our understanding of the steps we will undertake, together with the impact of these steps, we will perform "before and after" examinations of the cube of a hypothetical client, particularly focusing on the settings of the Member Key Column property at the lowest level that we have discussed. The changes that occur to this property as a result of our Optimize Schema efforts will make the process more transparent. For purposes of our practice session, we will create a copy of the targeted cube, as we have in various articles within our Database Journal series'. We will then process the clone cube to "register" it with Analysis Services, before beginning our examination of Optimize Schema within our practice exercise.

Considerations and Comments

For purposes of this exercise, we will be working with the Warehouse cube, within the FoodMart 2000 MSAS database; these working samples accompany a typical installation of MSAS. If the samples are not installed in, or have been removed from, your environment, they can be obtained from the installation CD, from the Analysis Services section of the Microsoft website, and perhaps elsewhere. If you prefer not to alter the structure of your sample cubes, as they currently exist, make copies of the cube we reference in the article before beginning the practice exercises. For instructions on copying cubes, see the Preparation section of Introduction to MSSQL Server 2000 Analysis Services: Semi-Additive Measures and Periodic Balances.



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