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Unformatted text preview: LAP systems are: 1. MOLAP (Multidimensional OLAP) and 2. ROLAP (Relational OLAP) The analytical ability and the interface of these two architectures are the same, what differs is the way data is physically stored. In MOLAP, data is stored in a proprietary multidimensional database (MDDB) to be viewed in a multidimensional manner. On the other hand, in ROLAP, data is stored in a conventional relational database and query languages (like SQL) are used to perform analysis on the data in the data warehouse. The relative advantages and limitations of the two architectures are as follows: 1. Pre-identified queries are easier and faster to process with MOLAP than with ROLAP because the multiple dimensions of the multidimensional database can be properly structured to handle such queries efficiently. 2. Since MOLAP architecture stores the same data in the form of multiple dimensions, the storage space requirement is more in case of MOLAP than ROLAP. Hence MOLAP architecture has a limit on the physical database size it can handle. ROLAP architecture is scalable up to several terabytes. 3. MOLAP architecture can handle a limited number of dimensions. ROLAP architecture does not have any limit on the number of dimensions it can handle. 4. A proprietary multidimensional database is essential for MOLAP. ROLAP can be implemented using conventional relational database technology. Star and Snowflake Schemas The two commonly used schema models for designing data warehouses are: 1. The star schema, and 2. The snowflake schema In the star schema model, the data warehouse consists of one fact (major) table and many dimension (minor) tables. The fact table stores the primary keys for all dimension tables. A simple example of a star schema is shown in Figure 16.17. Here sales is a fact table, whereas customer, product, time, and store are dimension tables. The design appears like a star, with the fact table in the center, and dimension tables connected to it by radial lines. The main advantages of star schema are that it is easy to define, reduces the numbe...
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This document was uploaded on 04/07/2014.

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