Sunday, April 6, 2014

What is denormalization?

Problem

What is denormalization? Explain the pros and cons

Solution

We have already studied normalization here, but now just study opposite of it.

Denormalization is the process of attempting to optimize the performance of a database by adding redundant data or by grouping data. Denormalization is generally used to either :
  • Avoid a certain number of queries
  • Remove some joins
Lets take an example. Suppose you are working in a team, which is client facing (say in some investment bank), that serves the trade data to the client. Now this trade data can be stored in 2 ways :
  • Separate tables with proper normalization, such that column A in table 1, column B in table 2, and we have to return the product of these 2 columns to the UX.
  • Few tables storing redundant information, like column A, B and C, such that C is product of A and B.
Now, as they are client facing, they want there UX to be responsive, and hence to achieve this, they kept there DB with pre-computed data:
  • Keeping information A and B in same table, help them reduce join, hence more speed
  • Keeping C=A*B in the table save them from computation on later stage. (I know its a simple calculation, but think of a table with v.v. large number of records)

So, here, denormalization helped cover up the inefficiencies inherent in relational database software. A relational normalized database imposes a heavy access load over physical storage of data even if it is well tuned for high performance

A normalized design will often store different but related pieces of information in separate logical tables (called relations). If these relations are stored physically as separate disk files, completing a database query that draws information from several relations (a join operation) can be slow. If many relations are joined, it may be prohibitively slow. There are two strategies for dealing with this. The preferred method is to keep the logical design normalized, but allow the database management system (DBMS) to store additional redundant information on disk to optimize query response. In this case, it is the DBMS software’s responsibility to ensure that any redundant copies are kept consistent. This method is often implemented in SQL as indexed views (Microsoft SQL Server) or materialized views (Oracle). A view represents information in a format convenient for querying, and the index ensures that queries against the view are optimized.

The more usual approach is to denormalize the logical data design. With care, this can achieve a similar improvement in query response, but at a cost — it is now the database designer’s responsibility to ensure that the denormalized database does not become inconsistent. This is done by creating rules in the database called constraints, that specify how the redundant copies of information must be kept synchronized. It is the increase in logical complexity of the database design and the added complexity of the additional constraints that make this approach hazardous. Moreover, constraints introduce a trade-off, speeding up reads (SELECT in SQL) while slowing down writes (INSERT, UPDATE, and DELETE). This means a denormalized database under heavy write load may actually offer worse performance than its functionally equivalent normalized counterpart.

A denormalized data model is not the same as a data model that has not been normalized, and denormalization should only take place after a satisfactory level of normalization has taken place and that any required constraints and/or rules have been created to deal with the inherent anomalies in the design. For example, all the relations are in third normal form and any relations with join and multivalued dependencies are handled appropriately.

References
http://tianrunhe.wordpress.com/2012/04/17/what-is-denormalization/
http://stackoverflow.com/questions/2349270/in-what-way-does-denormalization-improve-database-performance

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