MOLAP, ROLAP, HOLAP

ByDr. SubraMANI Paramasivam

MOLAP, ROLAP, HOLAP

As promised in my previous post please find the definitions, PLUS some more information on the characteristic nature of the post title.

MOLAP

This is the traditional mode in OLAP analysis. In MOLAP data is stored in form of multidimensional cubes and not in relational databases. The advantages of this mode is that it provides excellent query performance and the cubes are built for fast data retrieval. All calculations are pre-generated when the cube is created and can be easily applied while querying data. The disadvantages of this model are that it can handle only a limited amount of data. Since all calculations have been pre-built when the cube was created, the cube cannot be derived from a large volume of data. This deficiency can be bypassed by including only summary level calculations while constructing the cube. This model also requires huge additional investment as cube technology is proprietary and the knowledge base may not exist in the organization.

ROLAP

The underlying data in this model is stored in relational databases. Since the data is stored in relational databases this model gives the appearance of traditional OLAP’s slicing and dicing functionality. The advantages of this model is it can handle a large amount of data and can leverage all the functionalities of the relational database. The disadvantages are that the performance is slow and each ROLAP report is an SQL query with all the limitations of the genre. It is also limited by SQL functionalities. ROLAP vendors have tried to mitigate this problem by building into the tool out-of-the-box complex functions as well as providing the users with an ability to define their own functions.

HOLAP

HOLAP technology tries to combine the strengths of the above two models. For summary type information HOLAP leverages cube technology and for drilling down into details it uses the ROLAP model.

Comparison

Comparing the use of MOLAP, HOLAP and ROLAP

The type of storage medium impacts on cube processing time, cube storage and cube browsing speed. Some of the factors that affect MOLAP storage are:

Cube browsing is the fastest when using MOLAP. This is so even in cases where no aggregations have been done. The data is stored in a compressed multidimensional format and can be accessed quickly than in the relational database. Browsing is very slow in ROLAP about the same in HOLAP. Processing time is slower in ROLAP, especially at higher levels of aggregation.

MOLAP storage takes up more space than HOLAP as data is copied and at very low levels of aggregation it takes up more room than ROLAP. ROLAP takes almost no storage space as data is not duplicated. However ROALP aggregations take up more space than MOLAP or HOLAP aggregations.

All data is stored in the cube in MOLAP and data can be viewed even when the original data source is not available. In ROLAP data cannot be viewed unless connected to the data source.

MOLAP can handle very limited data only as all data is stored in the cube.

About the Author

Dr. SubraMANI Paramasivam administrator

Dr.SubraMANI Paramasivam is a Data Platform MVP, Power BI Group Leader, International Speaker, Microsoft Certified Trainer and Principal Solutions Architect at DAGEOP Ltd, having 20+ years of DB, BI & Analytics experience. Dr.SubraMANI Paramasivam is a frequent speaker in SQLBITS, PASS Summit, PASS SQL Saturdays, SQLRelay, Data Platform Summit, MCT Summit, SQLServerGeeks Summit, INSIDESQL UK, Data Awareness Programmes, Data Day Events. He likes to enlighten the education & data and started his own journey with FREE Data Awareness Programme (www.dataap.org), in remote areas who are in need. Mani speaks a lot about Visualization, Data Science, R Analytics, Azure concepts, Performance Tuning, DB & SQL Server Architecture, DBA, Partitioning, Resource Governor, Disaster Recovery, High Availability, Development, T-SQL Programming and Microsoft certifications.

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