Aggregations

Aggregations in an OLAP cube are presummarized data sets. They are analogous to a SQL SELECT statement with a GROUP BY clause. SSAS can use these aggregations when it answers queries to reduce the amount of necessary calculations, returning the answers quickly to the user.

Built-in aggregations in the OLAP cube reduce the amount of aggregation SSAS has to perform at query time. Building the correct aggregations can drastically improve query performance. This is often an evolving process throughout the lifetime of the OLAP cube as its queries and usage change.

A base set of aggregations is usually created that will be useful for most of the queries against the OLAP cube. Aggregations are built for each partition of an OLAP cube within a measure group. When an aggregation is built, certain attributes of dimensions are included in the presummarized data set.

Users can quickly query the data based on these aggregations when they browse the OLAP cube. Aggregations must be designed carefully because the number of potential aggregations is so large that building all of them would take an unreasonable amount of time and storage space.

Service Manager uses the following two options when it builds and designs aggregations in Service Manager OLAP cubes: 

• Performance Gain Reaches
• Usage-based optimization

The Performance Gain Reaches option defines what percentage of aggregations is built. For example, setting this option to the default and recommended value of 30 percent means that aggregations will be built to give the OLAP cube a 30-percent estimated performance gain. However, this does not mean that 30 percent of the possible aggregations will be built.

Usage-based optimization makes it possible for SSAS to log the requests for data so that when a query is run, the information is fed into the aggregation design process. SSAS then reviews the data and recommends which aggregations should be built to give the best estimated performance gain.

Displays the aggregation mode and the storage mode of the partition. The storage mode is displayed first: relational online analytical processing (ROLAP), multidimensional online analytical processing (MOLAP), or hybrid online analytical processing (HOLAP). 

The aggregation mode is displayed as a percentage of optimization requested, as a measure of space requested or used, or as the number of aggregations created. Click the ... button to display the Aggregation Design Wizard and define the aggregation design for the specified partition.

Description

Type the optional description of the partition.

New Partition...

Click to display the Partition Wizard and create a new partition in the selected measure group.

Storage settings...

Click to display the Storage Settings dialog box and specify storage mode, proactive caching, and notification settings for the selected partition.