Architecture of Query Execution—Calculating MDX Expressions - Microsoft SQL Server 2008 Analysis Services - page 545

The result of the request is a data structure called a data cache . A data cache is a
compressed data store. Its rows correspond to the nonempty coordinates of the multidi-
mensional space returned by the request, and its columns correspond to the attributes
requested by the subcube and, in the case of a request for measure group data, all the
members in that measure group. See Chapter 20, “The Physical Data Model,” for more
information about compressed stores.

The cache system pairs each request subcube and its corresponding data cache into a
CacheItem object. The data cache registry is a collection of the CacheItem s and an index
that enables fast access to the data stored in the cache. Figure 29.16 shows the scheme by
which the data cache registry functions.

Measure Group

Cache Registry

Granularity
Index

Slice
Index

Cache Item1

Subcube DataCache

gKey1 sKey1

gKey2 sKey2

Cache Item2

Subcube DataCache

… …

…

Cache ItemN

gKeyN sKeyN

Subcube DataCache

FIGURE 29.16

Structure of the data cache registry.

Requests come to the cache registry as subcubes, and the registry uses indexes to find the
subcube and the data cache that corresponds to the subcube. If Analysis Services finds a
subcube that completely matches the requested one, the data cache is returned to the
caller. If Analysis Services finds the subcube in the registry that represents a superset of the
data requested in the request subcube, it generates a new data cache and filters records
corresponding to the requested subcube into it. Then, Analysis Services registers a new
subcube and data cache in the registry. For example, assume that the data cache registry
already contains a subcube representing all members of the Month attribute, and the next
request to the cache registry uses a subcube for the months of the year 1998. Analysis
Services will find the CacheItem with data for all the months, and then filter out the data
cache records corresponding to the months of 1998 into another data cache object.