IBM Cognos Dynamic Cubes

34 IBM Cognos Dynamic Cubes

thought of as the primary cache of a dynamic cube, but not of virtual cubes, which do not

support aggregate caches.

If a cube is not assigned an aggregate cache, the data cache becomes the primary cache.

Otherwise, it plays a secondary role, caching the values that do not directly hit the aggregate

cache.

2.5 Using the database to evaluate set expressions

For particular MDX functions, given a sufficiently large input set, Cognos Dynamic Cubes

pushes the evaluation of the expression to the underlying database for execution. Without this

feature, those MDX functions, such as TOPCOUNT, retrieve large sets of data from the

database and perform the function in the MDX engine. The transfer of the large data sets,

which can be in the range of millions of rows of data, can degrade query performance.

The intent of

query pushdown is to exploit the proximity of RDBMS to the data, allowing the

operation to be performed without requiring the transfer of the data to the MDX engine. The

effect is similar to a cache, in that it provides faster response time and has lower and more

consistent resource requirements.

Query pushdown is supported for the FILTER, TOPCOUNT, and TOPSUM functions, but is

not supported in the following cases:

򐂰 Sets of members from different levels of the same hierarchy

򐂰 Parent-child hierarchies

򐂰 Semi-aggregate measures

򐂰 Measures with an UNKNOWN aggregation rule

򐂰 Complex numeric expression in the ‘count’ parameter of the TOPCOUNT function

򐂰 Calculated members/measures in the numeric expression

򐂰 Sets containing

both negative values and nulls

Pushdown is not supported for queries that are posed to virtual cubes.

2.6 Management of Cognos Dynamic Cubes

DQM accepts most of its commands from the report servers by using an XML protocol via a

single port open for all such requests, including report execution and metadata retrieval.

DQM, as a Cognos service, also accepts BI bus requests, notably those that are used to

manage dynamic cubes. In addition, DQM also supports JMX, which is used to implement

portions of Cognos Administration interface for managing requests.

When any of the cube management requests are received by DQM, they are directed

dynamic cubes, which then dispatches the request (command) to the specified cube. The

following commands, to name several, are included:

򐂰 Start cube

򐂰 Stop cube

򐂰 Restart cube

򐂰 Refresh metadata cache

򐂰 Refresh data cache

򐂰 Refresh security

򐂰 Clear workload log

򐂰 View messages