Data Warehouse Design - Chapter 1.8 Flashcards
What are the three major approaches to implementing data warehouses?
- ROLAP
- MOLAP
- HOLAP
What is ROLAP?
Relational OLAP, an implementation based on relational DBMSs
What is MOLAP?
Multidimensional OLAP, an implementaiton based on multidimensional DBMSs
What is HOLAP?
Hybrid OLAP, an implementation using both relational and multidimensional techniques
Why does relational technology has a solid foundation in terms of storing data in a data warehouse?
- Huge amount of literature written about relational model.
- Broadly available corporate experience with relational database usage/management
- Top performance and flexibility standards
What is the problem with relational technology in data warehouses?
It does not include concepts such as dimension, measure and hierarchy.
–> specific schema’s such as the star schema need to be created to represent the multidimensional model in terms of basic relational elements such as attributes and relations.
What is the main problem with ROLAP implementations?
Performance hits caused by join operations between large tables.
How is the number of joins reduced in ROLAP?
Using denormalization
- breach in the third nominal form to improve efficiency
What is middleware or a multidimensional engine?
An engine in between the relational back-end-service and the front-end components that turn OLAP formulated queries from end-users into SQL instructutions for the relational back-end with the support of meta-data.
–> can be connected to any relational server.
What is an aggregate navigator?
Component in the multidimensional engine that selects a view from among all the alternatives to solve a specific query at the minimum access cost.
How does MOLAP work?
It is based on an ad hoc logical model that can be used to represent multidimensional data and operations directly. The underlying multidimensional database physically stores data as arrays and the access to it is positional.
What is the greatest advantage of MOLAP?
Multidimensional operations can be performed in an easy, natural way, without the need for complex join operations –> excellent system performance
What is the downside of MOLAP?
MOLAP system implementations have little in common as there is no multidimensional logical standard.
–> this lack of common standard is slowly but progressively solved.
How does HOLAP work?
Combines both ROLAP as MOLAP. Takes advantage from standardization and the ability to handle large data from ROLAP, but the query speed from MOLAP.
Largest amount of data stored in a relational datbase, and a multidimensional model for the most frequently used data.
