Data Mining

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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