Storage types in Data Warehousing

Tuesday, January 26, 2010

Overview

There are mainly three kinds of storage types based on where the data or facts are stored.
1. MOLAP (Multi-dimensional OLAP)
2. ROLAP (Relational OLAP)
3. HOLAP (Hybrid OLAP)

MOLAP
• In this storage mode the data and aggregated data are stored in multi dimensional cube.
• It processes data which is already stored in multidimensional array with all possible combinations of data reflected, each in a cell that can be accessed directly.
• The main advantage is it can retrieve data very fast.

ROLAP
• The data which is stored in the relational database. Aggregated or summary data is also stored in the relational database.
• 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.

HOLAP
• A combination of relational OLAP (ROLAP) and multidimensional OLAP.
• Data will be stored in relational format (ROLAP). Aggregations, on the other hand, are stored in multi dimensional format(MOLAP) in the cube to give better query performance.