MOLAP, ROLAP and HOLAP

MOLAP ( Multi-dimensional Online Analytical Processing) is a type of OLAP. The other type is ROLAP (Relational Online Anaytical Processing).

BI Reporting and analysis tools in the old days implemented either MOLAP feature or ROLAP feature. The tools were also described as MOLAP Tool or ROLAP Tool. But now the BI Reporting and Analysis tools are equipped with both MOLAP and ROLAP features and hence they are also referred to as HOLAP ( Hybrid OLAP) Tool.

As these concepts are already extensively written about in various other places I will just highlight the major differences

MOLAP - All the possible combinations of calculations are pre-calculated and the calculated data is stored as cubes to enable faster/shorter refresh time at the time of query.

ROLAP - Data is stored in relational format ( RDBMS) but using dimensional modeling technique to make it look like a cube without a physical cube, no pre-calculations, this is to avoid unnecessary storage of calculated data that may not be used and to save the time required for calculations. Calculations are done at query run time and sometimes this can take longer than in MOLAP.

BI Reporting and analysis Tools like Microstrategy has both the features ROLAP and MOLAP. It can run reports directly on Databases or on Intelligent cubes. Some other tools create a micro cube after fetching data from Relational Databases. These tools claim they are HOLAP tools

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