Types of OLAP Servers

• Relational OLAP
• Multi-Dimensional OLAP
• Hybrid OLAP
• Transparent OLAP

Relational OLAP (ROLAP): Star Schema Based

The ROLAP is based on the premise that data need not be stored multi-dimensionally to be viewed multi-dimensionally, and that it is possible to exploit the well-proven relational database technology to handle the multidimensionality of data. In ROLAP data is stored in a relational database. In essence, each action of slicing and dicing is equivalent to adding a “WHERE” clause in the SQL statement. ROLAP can handle large amounts of data. ROLAP can leverage functionalities inherent in the relational database.

Multidimensional OLAP (MOLAP): Cube-Based

MOLAP stores data on disks in a specialized multidimensional array structure. OLAP is performed on it relying on the random access capability of the arrays. Arrays elements are determined by dimension instances, and the fact data or measured value associated with each cell is usually stored in the corresponding array element. In MOLAP, the multidimensional array is usually stored in a linear allocation according to nested traversal of the axes in some predetermined order. 

But unlike ROLAP, where only records with non-zero facts are stored, all array elements are defined in MOLAP and as a result, the arrays generally tend to be sparse, with empty elements occupying a greater part of it. Since both storage and retrieval costs are important while assessing online performance efficiency, MOLAP systems typically include provisions such as advanced indexing and hashing to locate data while performing queries for handling sparse arrays. MOLAP cubes are fast data retrieval, optimal for slicing and dicing, and can perform complex calculations. All calculations are pre-generated when the cube is created.

Hybrid OLAP (HOLAP)

HOLAP is a combination of ROLAP and MOLAP. HOLAP servers allow for storing large data volumes of detailed data. On the one hand, HOLAP leverages the greater scalability of ROLAP. On the other hand, HOLAP leverages cube technology for faster performance and summary-type information. Cubes are smaller than MOLAP since detailed data is kept in the relational database. The database is used to store data in the most functional way possible.

Transparent OLAP (TOLAP)

TOLAP systems are designed to work transparently with existing RDBMS systems, allowing users to access OLAP features without needing to transfer data to a separate OLAP system. This allows for more seamless integration between OLAP and traditional RDBMS systems.

OLAP is considered (Online Analytical Processing) which is a type of software that helps in analyzing information from multiple databases at a particular time. OLAP is simply a multidimensional data model and also applies querying to it.