Data Mining- Posted on August 29, 2008 at 18:00 PM by Amit Satpute
Explain the storage models of OLAP.
MOLAP Multidimensional Online Analytical processing
In MOLAP data is stored in form of multidimensional cubes and not in
relational databases.
Advantage
Excellent query performance as the cubes have all calculations pre-generated
during creation of the cube.
Disadvantages
It can handle only a limited amount of data. Since all calculations have been
pre-generated, the cube cannot be created from a large amount of data.
It requires huge investment as cube technology is proprietary and the knowledge
base may not exist in the organization.
ROLAP Relational Online Analytical processing
The data is stored in relational databases.
Advantages
It can handle a large amount of data and
It provides all the functionalities of the relational database.
Disadvantages
It is slow.
The limitations of the SQL apply to the ROLAP too.
HOLAP Hybrid Online Analytical processing
HOLAP is a combination of the above two models. It combines the
advantages in the following manner:
For summarized information it makes use of the cube.
For drill down operations, it uses ROLAP.
Define Rollup and cube.
Custom rollup operators provide a simple way of controlling the process of
rolling up a member to its parents values.The rollup uses the contents of the
column as custom rollup operator for each member and is used to evaluate the
value of the member’s parents.
If a cube has multiple custom rollup formulas and custom rollup members, then
the formulas are resolved in the order in which the dimensions have been added
to the cube.
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