Relational aggregates and SAP NetWeaver BW Accelerator allow you to improve the performance of BEx queries when reading data from an InfoCube. They are redundant data stores in a BW InfoCube: In relational aggregates, data is stored in an aggregated form. In an SAP NetWeaver BW Accelerator index though, data is compressed but not aggregated.
Relational aggregates are useful if you want to improve the performance of one or more specific BEx queries, or make specific improvements to reporting with characteristic hierarchies.
SAP NetWeaver BW Accelerator is particularly useful in cases where relational aggregates (or other BW-specific methods of improving performance such as database indexes) are not sufficient, are too complex, or have other disadvantages.
More information:Creating and Managing BWA Indexes in the BW System.
You can use relational aggregates and a BW accelerator index for the same InfoCube. A BEx query always tries to use performance-optimized sources by checking the sources from which it can draw the requested data. It checks the sources in the following order:
If an active BW accelerator index exists, the OLAP processor always accesses this BW accelerator index and not the relational aggregates. With regard to modeling, we therefore recommend creating either relational aggregates or a BW accelerator index for an InfoCube.