Aggregation Design and Usage-Based Optimization - Microsoft SQL Server 2008 Analysis Services - page 419

dropped from the partition as a result of an update of the dimension; rigid aggregations
remain.

The logic Analysis Services applies to determine that an aggregation is rigid is this: For
every attribute that participates in the aggregation, the relationship chain to the granular-
ity attribute of the dimension is rigid; that is, all the relationships in the chain are rigid.
On the other hand, if any relationship is flexible, the aggregation itself is flexible. For
example, for the aggregation shown in Figure 23.2, the attribute relationship chains are
as follows:

. In the Product dimension — Product Subcategory to Brand , Brand to Product

. In the Time dimension — Month to Date

. In the Store dimension — Store City to Store

. In the Warehouse dimension — City to Warehouse

If any of these relationships is flexible, the aggregation is considered flexible. If all the
relationships are rigid, the aggregation is a rigid one. As it happens, this aggregation is a
flexible one.

With a flexible aggregation, members can move from one parent to another during a
dimension update. The aggregate totals based on one of the parent attributes would be
invalid. And the flexible aggregation would be invalid. Analysis Services drops flexible
aggregations during dimension updates to make sure that the aggregate numbers are
correct.

NOTE
You can force the rebuilding of flexible aggregations during an incremental update of a
dimension. Specify the ProcessAffectedObjects option in the dimension processing
command. If you didn't specify this option, you can use the ProcessIndex command
for your dimension; or you can rely on lazy processing to build flexible aggregations in
the background. Any of these is an expensive solution.

However, you can use the ProcessAdd command to add only new members to a dimen-
sion. Using ProcessAdd eliminates the need to drop any aggregations. Along with that,
you can use the ProcessAdd command to modify the properties of existing members.

Aggregation Objects and Aggregation Design Objects

In Analysis Services, physical aggregations belong to individual partitions. However, you
logically define aggregation objects as part of an aggregation design object that belongs to
a measure group. Figure 23.8 contains a diagram of these relationships.