ROLAP

ROLAP (relational online analytical processing) is an alternative to the MOLAP (Multidimensional OLAP) technology. While both ROLAP and MOLAP analytic tools are designed to allow analysis of data through the use of a multidimensional data model, ROLAP differs significantly in that it does not require the pre-computation and storage of information. Instead, ROLAP tools access the data in a relational database and generate SQL queries to calculate information at the appropriate level when an end user requests it. With ROLAP, it is possible to create additional database tables (summary tables or aggregations) which summarize the data at any desired combination of dimensions.

While ROLAP uses a relational database source, generally the database must be carefully designed for ROLAP use. A database which was designed for OLTP will not function well as a ROLAP database. Therefore, ROLAP still involves creating an additional copy of the data. However, since it is a database, a variety of technologies can be used to populate the database.

ROLAP vs. MOLAP

The discussion of the advantages and disadvantages of ROLAP below, focus on those things that are true of the most widely used ROLAP and MOLAP tools available today. In some cases there will be tools which are exceptions to any generalization made.[1]

Advantages of ROLAP

Disadvantages of ROLAP

Performance of ROLAP

OLAP Survey

In the OLAP industry ROLAP is usually perceived as being able to scale for large data volumes, but suffering from slower query performance as opposed to MOLAP. The OLAP Survey, the largest independent survey across all major OLAP products, being conducted for 6 years (2001 to 2006) have consistently found that companies using ROLAP report slower performance than those using MOLAP even when data volumes were taken into consideration.

However, as with any survey there are a number of subtle issues that must be taken into account when interpreting the results.

Downside of flexibility

Some companies select ROLAP because they intend to re-use existing relational database tables—these tables will frequently not be optimally designed for OLAP use. The superior flexibility of ROLAP tools allows this less than optimal design to work, but performance suffers. MOLAP tools in contrast would force the data to be re-loaded into an optimal OLAP design.

Trends

The undesirable trade-off between additional ETL cost and slow query performance has ensured that most commercial OLAP tools now use a "Hybrid OLAP" (HOLAP) approach, which allows the model designer to decide which portion of the data will be stored in MOLAP and which portion in ROLAP.

Products

Examples of commercial products using ROLAP include Microsoft Analysis Services, MicroStrategy, SAP Business Objects, Oracle Business Intelligence Suite Enterprise Edition (the former Siebel Analytics), Tableau Software and Tagetik. There is also an open source ROLAP servers, Mondrian and Olaper.

References

  1. ↑ Bach Pedersen, Torben; S. Jensen, Christian (December 2001). "Multidimensional Database Technology". Distributed Systems Online (IEEE): 40–46. ISSN 0018-9162.
This article is issued from Wikipedia - version of the Friday, February 26, 2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.