24-07-2012, 03:41 PM
Online Analytical Processing
Online Analytical Processing by Hweichao Lu.ppt (Size: 193.5 KB / Downloads: 157)
What is OLAP
Basic idea: converting data into information that decision makers need
Concept to analyze data by multiple dimension in a structure called data cube
Why need OLAP over Relational Database II
Metadata-based queries
provide analysis functions that are difficult or impossible to express in SQL
SQL was developed primarily for transaction systems, not for reporting applications
Star Schema
Cubes are easily stored in relational databases, using a denormalized data structure called the star schema, developed by Ralph Kimball
starts with a central fact table
Each row in the central fact table contains some combination of keys that makes it unique. These keys are called dimensions.
MOLAP
The database is stored in a special, usually proprietary, structure that is optimized for multidimensional analysis.
+ : very fast query response time because data is mostly pre-calculated
-: practical limit on the size because the time taken to calculate the database and the space required to hold these pre-calculated values