Use numeric key columns for attributes with 500,000 or more members
This rule analyzes dimension attributes with an estimated count of more than 500,000 members to determine whether the data type of the attribute's key column is numeric.
For best performance, you should typically bind the key attribute of large dimensions to a column that contains a numeric data type. Using a numeric key column instead of a string key column or a composite key will improve the performance of attributes that contain many members. This is the same as the idea of using surrogate keys in relational tables for better performance in indexing. To make attribute members appear the same to end users, you can specify the numeric surrogate column as the key column and use a string column as the name column.
For more information about binding attributes to data source view columns, see Defining and Configuring Dimension Attributes in SQL Server Books Online.
For more information about performance issues related to the key attribute, see the section, "Reducing attribute overhead," in the SQL Server 2005 Analysis Services Performance Guide.