MOLAP Aggregate (SAP Library - Performance Optimization)

MOLAP Aggregate

Definition

A MOLAP aggregate is an aggregate of an InfoCube, whose data is physically stored in a MOLAP store.

Contrasting model: ROLAP.

Use

Similarities between MOLAP and ROLAP aggregates

MOLAP aggregates have significant properties in common with ROLAP aggregates.

Aggregates improve performance when executing queries, but do not improve the reporting functions. A user who constructs a query in an InfoCube with aggregates sees functionally no difference between MOLAP and ROLAP aggregates.
Data is stored redundantly and persistently in the aggregates.
The aggregates are used when a query is executed. The aggregates cannot be directly accessed in the BW system.

Differences between MOLAP and ROLAP aggregates

MOLAP aggregates differ from ROLAP aggregates with regard to the following features:

MOLAP aggregates are stored in a MOLAP store. For more information, see MOLAP Store

MOLAP aggregates do not allow for filter conditions for characteristics. The definition of a MOLAP aggregate only relates to the selection of characteristics, in which the system does not summarize. (The system summarizes in characteristics that are not selected).

Recommendation

You can find additional information about special features of ROLAP aggregates in Aggregation Levels.

Recommendations for using MOLAP aggregates.

Before making a decision about whether it is better to create MOLAP- or ROLAP aggregates for an InfoCube, you need to check the prerequisites for both options.

MOLAP uses numerous structural master data properties to improve efficiency. When changes are made to master data, MOLAP objects must be reconstructed in parallel, either partially or completely: Changing a BW hierarchy has the effect that all MOLAP Cubes that contain this hierarchy are forced to be reconstructed. On the other hand, changing attribute values requires less effort. Nevertheless, both lead to increased run time for the hierarchy- or attribute change run.

Structure

You can derive as many ROLAP aggregates as required directly from an InfoCube. However, the basis structure in a MOLAP store has two stages.

This graphic is explained in the accompanying text

Exactly one MOLAP Cube (or a MOLAP root aggregate) is derived from an InfoCube. This usually contains the same information as the InfoCube.

Note

Further information about a MOLAP Cube can be found at MOLAP Cube.

You can derive as many MOLAP aggregates from this MOLAP Cube (or MOLAP root aggregates) as required.

When you have created a MOLAP Cube, you can create, save and activate MOLAP aggregates in the same way as with ROLAP aggregates for any particular BW Cube.

You can find additional information about (ROLAP-) aggregates in Aggregates .

The most crucial difference is that MOLAP aggregates are created on the Microsoft Analysis Server. MOLAP aggregates can easily be converted to ROLAP aggregates, since the aggregate definition is, in contrast, stored in the BW.

Integration

The relationship of an InfoCube in the store differs from that of an aggregate, as below:

Aggregates	Basis InfoCube
ROLAP	ROLAP
MOLAP	ROLAP

It is possible to toggle between ROLAP and MOLAP as a store for aggregates of one and the same InfoCube.

Example

If the system determined that ROLAP aggregates could be derived from a particular InfoCube, this can be converted to MOLAP aggregates. During this process, the system deletes the existing ROLAP aggregates and builds them in MOLAP.

You can change from ROLAP to MOLAP and vice versa.