What is De-normalization?

De-normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access.
De-normalization is an approach to speed up (optimizing) read performance (data retrieval) in which the administrator selectively adds back specific instances of redundant data after the data structure has been normalized.
De-normalization is the process of introducing redundancy into a table by incorporating data from a related table. Tables are usually de-normalized to prevent expensive SQL join operations between them. One should always normalize to Third Normal Form (3NF) and only apply de-normalization selectively as a last resort if performance problems are experienced.

Note: De-normalizations are not free and introduces the following problem into the design:
  • More disk space is used as the same data is duplicated in more than one table
  • DML operations are more expensive as the same data must be maintained in more than one table
  • Risk of  "out of sync" data increases 

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