Architecture of Query Execution—Retrieving Data from Storage - Microsoft SQL Server 2008 Analysis Services - page 549

CHAPTER 30

IN THIS CHAPTER

. Query Execution Stages

Architecture of Query
Execution—Retrieving
Data from Storage

. Querying Different Types of

Measure Groups

E fficient execution of Multidimensional Expressions
(MDX) requests is the main responsibility of the formula
engine and the storage engine subsystems. In the preceding
chapter, we discussed the stages a query passes through the
formula engine and the data structures Analysis Services
uses to calculate the results of MDX queries. In this chapter,
we discuss how Analysis Services retrieves data from multi-
dimensional storage and how it handles requests for data
stored in different types of partitions and dimensions.

When the formula engine needs to retrieve data stored in
the multidimensional store, it forms an internal request to
the cache system, which returns data immediately if it
exists in the cache; otherwise, the cache system requests
data from the storage engine. In this chapter, we delve
deeper in our discussion about what happens when the
storage engine receives an internal request for data.

The storage engine subsystem supports two types of internal
requests for data: requests for dimension data and requests
for measure group data. The formula engine subsystem
detects what data is requested by a user, forms a dimension
or measure group subcube, and passes it to the cache
subsystem, which passes the request to the measure group
or dimension object that contains the requested data. The
difference between measure group and dimension subcubes
is only in the attributes that each references: A measure
group subcube references attributes of all the dimensions
included into a measure group, whereas a dimension
subcube references attributes of the dimension.

Analysis Services executes a request for measure group data
similarly to the way it executes a request for dimension
data. Therefore, we discuss the execution of a request for
measure group data and provide you with information that
you can use to deduce how Analysis Services retrieves
dimension data.