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Don Burleson Blog 







Oracle design third normal form

Oracle Database Tips by Donald BurlesonJuly 9, 2015

Question:  I have a discussion where a colleague says that we must start by designing our Oracle tables in third normal form (3NF).  I was told that 3NF is only a starting point, and that oracle suggests that the DBA deliberately add redundant data to reduce table joins.  Who is correct?

Answer:  It's been proven that the intelligent introduction of redundancy can improve performance and Oracle has many tools that help introduce redundant, non third-normal form data (materialized views, replication, VARRAY tables). 

But the degree of database normalization is tied directly to hardware costs.  Back in the 1980's, a gigabyte of disk was the size of a refrigerator and cost over $2000,000.  Back when disk was super-expensive DBA's did not have the luxury of de-normalizing table for performance, and virtually all relational databases were in third normal form, or even beyond 3NF to Boyce-Codd Normal Form (BCNF).

But now that disk is cheap, the rules have changed.  In addition to replication (Streams, Data Guard, and multi-master) and materialized views, Oracle offers several popular denormalization tools, some that create non first-normal form structures (0NF):

  • Object tables - Oracle has 0NF tables nested tables and varray table columns whereby repeating groups are stored within a row, violating 1NF.
  • Materialized Views - Tables are pre-joined together, queries are re-written to access the MV, and a method (Oracle snapshots) keeps the denormalization in-sync with the normalized representation of the data.

Violating third normal form

Even today, the Oracle designer start with a third normal form model and then intelligently introduces redundant data items to reduce unnecessary SQL joins.  The decision to introduce redundancy is based in three issues:

The amount of SQL that will experience less table joins:  Pre-joining table together is best when lots of SQL statements will benefit from it.

The size of the redundant item:  Large items consume more disk than small items

The volatility of the data item:  Remember, redundant items must be keep synchronized in many tables.  Seldom updated data items are better for adding redundancy than frequently-updated items.

Related database design notes:

See my important notes on:



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