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snowflake schema
In computing, a snowflake schema is a logical arrangement of tables in a multidimensional database such that the entity relationship diagram resembles a snowflake shape. The snowflake schema is represented by centralized fact tables which are connected to multiple dimensions.. "Snowflaking" is a method of normalising the dimension tables in a star schema. When it is completely normalised along all the dimension tables, the resultant structure resembles a snowflake with the fact table in the middle. The principle behind snowflaking is normalisation of the dimension tables by removing low cardinality attributes and forming separate tables.〔Paulraj Ponniah. ''Data Warehousing Fundamentals for IT Professionals.'' Wiley, 2010, pp. 29–32. ISBN 0470462078.〕
The snowflake schema is similar to the star schema. However, in the snowflake schema, dimensions are normalized into multiple related tables, whereas the star schema's dimensions are denormalized with each dimension represented by a single table. A complex snowflake shape emerges when the dimensions of a snowflake schema are elaborate, having multiple levels of relationships, and the child tables have multiple parent tables ("forks in the road").
== Common uses ==
Star and snowflake schemas 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 schemas are not normalized much, and are frequently designed at a level of normalization short of third normal form.
Deciding whether to deploy a star schema or a snowflake schema should involve considering the relative strengths of the database platform in question and the query tool to be employed. Star schemas 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 schemas are often better with more sophisticated query tools that create a layer of abstraction between the users and raw table structures for environments having numerous queries with complex criteria.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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