What is Denormalization of Database?

De-normalization is the process of attempting to optimize the performance of a database by adding redundant data. It is sometimes necessary because current DBMSs implement the relational model poorly.

A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De-normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access.


Denormalization of Database! Why?

Only one valid reason exists for denormalizing a relational design - to enhance performance. However, there are several indicators which will help to identify systems and tables which are potential denormalization candidates. These are:



* Many critical queries and reports exist which rely upon data from more than one table. Often times these requests need to be processed in an on-line environment.

* Repeating groups exist which need to be processed in a group instead of individually.

* Many calculations need to be applied to one or many columns before queries can be successfully answered.

* Tables need to be accessed in different ways by different users during the same timeframe.

* Many large primary keys exist which are clumsy to query and consume a large amount of DASD when carried as foreign key columns in related tables.

* Certain columns are queried a large percentage of the time. Consider 60% or greater to be a cautionary number flagging denormalization as an option.


Be aware that each new RDBMS release usually brings enhanced performance and improved access options that may reduce the need for denormalization. However, most of the popular RDBMS products on occasion will require denormalized data structures. There are many different types of denormalized tables which can resolve the performance problems caused when accessing fully normalized data. The following topics will detail the different types and give advice on when to implement each of the denormalization types.

its good

since performance is also a high needed factor factor i hope denormalisation must be put in place to create some data redundancy . thanx

Yes i got here satsifaction.this is proper explanation of Denormalization. Because i am always say about normalization but not familiar with Denormalization after reading above explanation i will give proper answer if in future some interviwer will aske me about both.......Thanks its realy good

The intentional introduce of redundancy in a table in order to improve performance is called “Denormalization”. Denormalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access. Denormalization is usually done to decrease the time required to execute complex queries. Drawbacks of a normalized database are mostly in performance. In a normalized database, more joins are required to gather all the information from multiple entities, as data is divided and stored in multiple entities rather than in one large table. Queries that have a lot of complex joins will require more CPU usage and will adversely affect performance. Sometimes, it is good to denormalize parts of the database. Examples of design changes to denormalize the database and improve performance are...
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