DWH Schemas

The star and snowflake schema are most commonly found in dimensional data warehouses and data marts where speed of data retrieval is more important than the efficiency of data manipulations. As such, the tables in these schema are not normalized much, and are frequently designed at a level of normalization short of third normal form.

The decision whether to employ a star schema or a snowflake schema should consider the relative strengths of the database platform in question and the query tool to be employed. Star schema should be favored with query tools that largely expose users to the underlying table structures, and in environments where most queries are simpler in nature. Snowflake schema are often better with more sophisticated query tools that isolate users from the raw table structures and for environments having numerous queries with complex criteria.

Star Schema:

The star schema is the simplest style of data warehouse schema. The star schema consists of a few fact tables (possibly only one, justifying the name) referencing any number of dimension tables.

star_schema

Another example for Star Schema – Sales fact Star Schema

Snowflake Schema:

A snowflake schema is a logical arrangement of tables in a relational database such that the entity relationship diagram resembles a snowflake in shape. Closely related to the star schema, the snowflake schema is represented by centralized fact tables which are connected to multiple dimensions. In the snowflake schema, however, dimensions are normalized into multiple related tables whereas the star schema’s dimensions are denormalized with each dimension being represented by a single table. When the dimensions of a snowflake schema are elaborate, having multiple levels of relationships, and where child tables have multiple parent tables (“forks in the road”), a complex snowflake shape starts to emerge. The “snowflaking” effect only affects the dimension tables and not the fact tables.

snowflake



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