Database performance is essential and the most common bottleneck of web application is the database. Often the DBA is concerned about the performance but not only he should be, it's also the application developers that should keep an eye on database performance.
Burak Guzel shows in his article on net.tutsplus 20+ best practises for MySQL application developers.
Even this is specific to MySQL databases I found his tips are mostly applicable for any other database (for example PostgreSQL). The article should be read by anyone that's interested in database performance. ■