Gavin Powell, "Oracle High Performance Tuning for 9i and 10g"(repost)

There are three parts to tuning an Oracle database: data modeling, SQL code tuning and physical database configuration.
A data model contains tables and relationships between tables. Tuning a data model involves normalization and de-normalization. Different approaches are required depending on the application, such as OLTP or a Data Warehouse. Inappropriate database design can make SQL code impossible to tune. Poor data modeling can have a most profound effect on database performance since all SQL code is constructed from the data model.