HOLAP is a newer form of on-line analytical processing that combines some of the best features of ROLAP and MOLAP.
MOLAP, while it can be quick to access and fast with precalculated values, can take time to build and have size limitations. ROLAP, while easier to use and virtually unlimited in size, can be slower to query and tougher to work with calculations. Overtime, a hybrid form emerged that uses a form of both of these technologies.
By using this approach, a client can get some of the aggregated speed of data in multidimensional structures, with further detail stored in relational form. How this relationship is determined will really depend on the type of data that is most frequently accessed. Executives may want to see high level summary data first, so that information would be in a MOLAP structure, with the detailed information stored in a ROLAP form. Or, daily sales information may be multidimensional, with the other time dimensions stored relationally. Most HOLAP technologies use a form of virtual drilling to get from one to another. There is usually a lag but the tradeoff is normally acceptable since the drilling is not as frequent. Most of the time the users will be working in a quicker MOLAP rollup.
Hybrid OLAP has been a little slower to catch on from the bigger vendors, but several of them have been able to build in the technology to their main stacks. Microsoft has a pretty straightforward option built into SQL server, where the BI developers can easily select the option to be used in the development studio. Oracle also has drilldown available through its OLAP to 11g databases. Microstrategy has it as a combination feature with its OLAP cubes. Drilling outside the cubes in a report hits the database, so users get a mix of performance and flexibility.
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