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What are the major differences between ROLAP and MOLAP?

  • August 01 1999, 1:00am EDT
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What are the major differences between ROLAP and MOLAP? Could you explain with examples?


Doug Hackney's Answer: This is a condensation of a full day's worth of seminar information into a few paragraphs. Please forgive the brevity.

MOLAP (multidimensional OLAP) tools utilize a pre-calculated data set, commonly referred to as a data cube, that contains all the possible answers to a given range of questions. MOLAP tools feature very fast response, and the ability to quickly write back data into the data set (budgeting and forecasting are common applications). Primary downsides of MOLAP tools are limited scalability (the cubes get very big, very fast when you start to add dimensions and more detailed data), inability to contain detailed data (you are forced to use summary data unless your data set is very small), and load time of the cubes. The most common examples of MOLAP tools are Hyperion (Arbor) Essbase and Oracle (IRI) Express. MOLAP tools are best used for users who have "bounded" problem sets (they want to ask the same range questions every day/week/month on an updated cube, e.g. finance).

ROLAP (relational OLAP) tools do not use pre-calculated data cubes. Instead, they intercept the query and pose the question to the standard relational database and its tables in order to bring back the data required to answer the question. ROLAP tools feature the ability to ask any question (you are not limited to the contents of a cube) and the ability to drill down to the lowest level of detail in the database. Primary downsides of ROLAP tools are slow response and some limitations on scalability (depending on the technology architecture that is utilized). The most common examples of ROLAP tools are MicroStrategy and Sterling (Information Advantage). ROLAP tools are best used for users who have "unbounded" problem set (they don't have any idea what they want to ask from day to day; e.g., marketing). It is very important to pay close attention to the underlying architecture of ROLAP tools, as some tools are very "scalability challenged."

HOLAP (hybrid OLAP) addresses the shortcomings of both of these technologies by combining the capabilities of both approaches. HOLAP tools can utilize both pre-calculated cubes and relational data sources. The most common example of HOLAP architecture is OLAP services in Microsoft SQL Server 7.0. OLAP vendors of all stripes are working to make their products marketable as "hybrid" as quickly as possible. It is critically important to closely examine the architectures of these "repackaged/repositioned" offerings, as their "HOLAP" claims may be more marketing hype than architectural reality.

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