The document discusses different types of multidimensional schemas used in data warehousing, including star schemas, snowflake schemas, and fact constellation schemas. A star schema has a central fact table linked to dimension tables, while a snowflake schema normalizes dimensions into multiple tables. A fact constellation schema combines multiple fact tables sharing dimension tables. The schemas vary in their structure, data integrity, flexibility, and complexity. Choosing a schema depends on the analytical needs, data relationships, and tradeoffs between simplicity and normalization.

1. Multidimensional Schema
Chaand Chopra (17MCMC34), Mridul Gain (17MCMC40)

2. What is Multidimensional schemas?
● Schema is a logical description of the entire database.
● Designed to model data warehouse systems
● It includes the name and description of records of all record types including all
associated data-items and aggregates.
● A database uses relational model, while a data warehouse uses Star,
Snowflake, and Fact Constellation schema.

3. Types of Multidimensional Schemas
● Star Schema
● Snowflake Schema
● Fact Constellation Schema or Galaxy Schema

4. What is a Star Schema?
● The simplest type of Data Warehouse schema
● Structure resembles a star.
● Each dimension in a star schema is represented with only one-dimension
table.
● This dimension table contains the set of attributes.
● There is a fact table at the center. It contains the keys to each of four
dimensions.

5. Structure of Star Schema

6. Advantage of Star Schema
● Easy for Users to Understand
● Optimizes Navigation
● Most Suitable for Query Processing(against an OLTP system)

7. Disadvantages of Star Schema
● Data integrity is not enforced.
● Not as flexible in terms of analytical needs.
● Star schemas don’t reinforce many-to-many relationships within business
entities – at least not frequently.
● Uses large disk space

8. What is Snowflake Schema?
● Represented by centralized fact tables which are connected to multiple
dimensions tables.
● The principle behind snowflaking is normalization of the dimension tables by
removing low cardinality attributes and forming separate tables.
● It normalizes dimensions to eliminate redundancy

9. Snowflake Schema

10. Advantages of Snowflake Schema
● Reduces the the problem of data integrity
● Uses small disk space
● Improvement in query performance
● Easy to understand.

11. Disadvantages of Snowflakes Schema
● Adds complexity to source query joins
● Additional maintenance efforts needed due to the increase number of lookup
tables.

12. What is Galaxy or Fact constellation Schema?
● Combination other two schemas
● Constitutes of multiple fact tables sharing dimension tables.

13. Structure of Fact Constellation Schema

14. When to choose ?
● while introducing hierarchies in dimension.
● In order to deals with bigger dimension tables.
● helpful for aggregating fact tables.

15. Advantages of Fact Constellation Schema
● Provides a flexible schema.
○ Improved data retrival
○ Simplified business logic
○ Better understanding
○ Fast aggregation
○ Extensibility

16. Disadvantages of Fact Constellation Schema
● difficult to maintain
● more complex than star and snowflake schemas

17. Real World Scenario
Consider a database for a retailer that has many stores, with each store selling
many products in many product categories and of various brands. A data
warehouse or data mart for such a retailer would need to provide analysts the
ability to run sales reports grouped by store, date (or month, quarter or year), or
product category or brand.

18. When Using Star Schema

19. Sample Query
SQL query to get number of products sold by country and brand, when the database uses a
snowflake schema.

20. When Modeling with SnowFlake Schema

21. Query To Get Same Report

22. References
Oracle® OLAP Application Developer's Guide
https://docs.oracle.com/cd/B13789_01/olap.101/b10333/multimodel.htm
Multidimensional Data Model
http://www.lastnightstudy.com/Show?id=49/Multidimensional-Data-Model
Snowflake Schema vs. Star Schema
https://www.diffen.com/difference/Snowflake_Schema_vs_Star_Schema

23. Thank You
&
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