Denormalization is the process of introducing redundancy into a table by incorporating data from a related table. Tables are usually denormalized to prevent expensive SQL join operations between them.
One should always normalize to third-normal form and only apply denormalization selectively as a last resort if performance problems are experienced. Remember that denormalizations is 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
- You run the risk that the data can get "out of sync"
 Also see
- Normalization - the reverse process
- Materialized view - method of introducing redundancy which is maintained by the database
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