Chapter 15 Flashcards
What are additive facts
Additive facts are easy to work with summing, quantity, total etc.
What are non-additive facts
They are averages, percentages, ratios etc
What are classes of aggregates
- Distributive
- Holistic
- Algebraic
What are distributive aggregates
for example take minimum of all values and then take minimum of minimum to get global minimum. They are easy to use
What are algebraic aggregates
They are hard to handle. they are complex. They deal with ratio, percentages etc
What are holistic aggregates
They make all the data set strong. e.g. medium, count
What is fact-less fact table
A fact table that do not store facts. It stores relationships. It uses a dummy fact column that have value 1.
How to handle multi-valued dimension
One of the following approaches is adopted.
- Drop the dimension
- Use a primary value as a single value (e.g. maintenance operation at 5000 km)
- Add multiple values in a dimension table.
- User ‘Helpers’ table
What is helper table
Use an additional table between flat table and dimension table
What are slowly changing dimensions
These are dimensions that changes but slowly (e.g. inheritance of property)
Does OLTP system good at track history
No
In how much days OLTP system purge data from them
90 days or 180 days
How to handle slowly changing dimensions
- Overwrite
- Add new row - Preserve history (make versions in history)
- Add New field
What is the most used option for handling slowly changing dimensions
Add new row
