MOLAP, ROLAP, and HOLAP in DWH

What is OLAP
OLAP, On Line Analytical Processing

It is of Mainly three types
1)MOLAP
2)ROLAP
3)HOLAP
and DOLAP 

MOLAP-
Multidimensional OLAP
The MOLAP environment consists of a multi-dimensional cube with all possible perspectives precalculated. Therefore MOLAP applications do have a high performance. At the same time the need a lot of disk capacity to store all the precalculated figures.

Advantages:

• Excellent performance: MOLAP cubes are built for fast data retrieval, and is optimal for slicing and dicing operations.
• Can perform complex calculations: All calculations have been pre-generated when the cube is created. Hence, complex calculations are not only doable, but they return quickly.

Disadvantages:

• Limited in the amount of data it can handle: Because all calculations are performed when the cube is built, it is not possible to include a large amount of data in the cube itself. This is not to say that the data in the cube cannot be derived from a large amount of data. Indeed, this is possible. But in this case, only summary-level information will be included in the cube itself.
• Requires additional investment: Cube technology are often proprietary and do not already exist in the organization. Therefore, to adopt MOLAP technology, chances are additional investments in human and capital resources are needed.

ROLAP

The ROLAP envirenment consist of a relational database with a OLAP query engine. Every information need is calculated on the fly by the OLAP query engine. therefore ROLAP mosttimes has a slower performance. Never the less, it needs less disk space to store the data.

This methodology relies on manipulating the data stored in the relational database to give the appearance of traditional OLAP's slicing and dicing functionality. In essence, each action of slicing and dicing is equivalent to adding a "WHERE" clause in the SQL statement.

Advantages:

• Can handle large amounts of data: The data size limitation of ROLAP technology is the limitation on data size of the underlying relational database. In other words, ROLAP itself places no limitation on data amount.
• Can leverage functionalities inherent in the relational database: Often, relational database already comes with a host of functionalities. ROLAP technologies, since they sit on top of the relational database, can therefore leverage these functionalities.

Disadvantages:

• Performance can be slow: Because each ROLAP report is essentially a SQL query (or multiple SQL queries) in the relational database, the query time can be long if the underlying data size is large.
• Limited by SQL functionalities: Because ROLAP technology mainly relies on generating SQL statements to query the relational database, and SQL statements do not fit all needs (for example, it is difficult to perform complex calculations using SQL), ROLAP technologies are therefore traditionally limited by what SQL can do. ROLAP vendors have mitigated this risk by building into the tool out-of-the-box complex functions as well as the ability to allow users to define their own functions.

HOLAP

The HOLAP environment consists of both a relation database and a preconfigured multi-dimension cube. Most times you can be flexible in keeping information in the relational database and/or the preconfigured multi-dimensional cube.

HOLAP technologies attempt to combine the advantages of MOLAP and ROLAP. For summary-type information, HOLAP leverages cube technology for faster performance. When detail information is needed, HOLAP can "drill through" from the cube into the underlying relational data.