Answer Posted / raja

06.

MOLAP ROLAP HOLAP

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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