IBM Cognos Dynamic Cubes

Chapter 2. IBM Cognos Dynamic Cubes architecture 29

2.3.2 Expression cache

A set within the context of MDX contains one or more tuples. A tuple contains a member from

one or more dimensions, where one of those dimensions can be the measures dimension.

Tuples within a set in MDX each contain a member from the same set of dimensions and in

the same order.

The expression cache is used by the MDX engine to cache the result of set expressions that

operate on large sets and output a much smaller set for subsequent reuse, either within the

same query or within subsequent MDX queries processed by the MDX engine. The savings

from the use of cache expression results can be significant, given that in some cases a set

can contain millions of tuples and only output five or ten tuples.

As with the result set cache, the intermediate results that are stored in the expression cache

are security-aware and are flushed when the data cache is refreshed.

Many MDX functions can operate on large sets, which can be characterized as those that

process a large set of tuples as input and, do the following tasks:

򐂰 Perform a simple operation to return a smaller set of tuples (for example, HEAD).

򐂰 Fetch the corresponding values, and return a single value (for example, AGGREGATE).

򐂰 Fetch the corresponding values, and return a smaller set of tuples.

The expression cache feature focuses on those MDX function which process a large input set

of tuples and return a smaller set of tuples. Specifically, the following MDX functions are

supported by the expression cache:

򐂰 TOPCOUNT

򐂰 BOTTOMCOUNT

򐂰 TOPPERCENT

򐂰 BOTTOMPERCENT

򐂰 TOPSUM

򐂰 BOTTOMSUM

򐂰 FILTER

In all, but the following, circumstances are the results of the functions (in the previous list) that

are cached:

򐂰 If any of the inputs to a function contains a calculated member, named set or inline named

set.

򐂰 If the report from which the MDX statement was generated has a prompt variable defined

and it is referenced within the expression.

򐂰 Expressions cached by the engine with an internal,

default value. The use of a default

value is an optimization within the MDX engine that can conflict with the use of the

expression cache.

Each entry in the expression cache has three components that uniquely identifies it:

򐂰 The text of the MDX expression.

򐂰 A security key that identifies a combination of security views.

򐂰 The context in which the expression was evaluated, expressed as a tuple.

Because the expression cache is stored in memory and is so closely associated with the data

cache (see 2.3.5, “Data cache” on page 31), the expression cache is actually stored within

the space allotted to the data cache.