It is advisable that the DBA should be involved in all phases of Systems Development Life Cycle (SDLC) or Database Development Life Cycle (DDLC); hence, a DBA should start tuning as early as the design phase given that over 70% of performance problems are due to design and coding, and to make sure the logical data model (metadata) is as best as possible. And perhaps continue tuning as late as the development phase where you can analyze the queries. This prevents major performance problems when the application goes into production.

This does not mean that the database analysis phase is not important. However, at the inception of the database project it mainly deals with identifying database performance goals and objectives; analyzing business functions to determine database features and functions; and identifying resources and capacities that are required for the new system or system enhancement. These baseline parameters become very vital during the later stages for tuning the database.

DBA Database Tuning Responsibilities
Some of the Tasks a DBA should be responsible for when tuning a database include the following:
  • Be involved in all SDLC phases.
  • Lead and assist in capacity planning.
  • Assist the application team with resolving performance issues.
  • Recommend to developers SQL tuning tools and how to optimize SQL code.
  • Gather and analyze database statistics.
  • Monitor database performance.
  • Monitor database space storage.
  • Educate users on the best practices for working with database applications. 
  • and so forth…

References:
Afyouni, A. H. (2004). Oracle 9i Performance Tuning: Optimizing Database
Productivity. Thompson Course Tech.

Alapati, S. R. (2008). Expert Oracle Database 11g Administration. Apress.

Burleson, D. K. & Danchenkov, A. B. (2005). Oracle Tuning: The Definitive Reference.
Rampant Techpress

Connolly, M. T, & Begg, E. C, (2002). Database Systems: A Practical Approach to
Design, Implementation, and Management. Addison-Wesley.