Mike Willbanks has posted another part in his "PHP performance" series today. In his previous article, he talked about caching techniques including things like opcode caching and database memory tables. This time he talks about getting the most out of your MySQL database.
Application level SQL performance is much different than the performance of the SQL query itself but rather how it has been designed to work in the application. Many of the items I will be addressing in this area is designing your application to make less queries thus improving scalability and likely performance. However, performance does not always equal scalability as the same with scalability does not always equal performance.
He looks at a few different topics like lazy connections, iterating queries, need-based selects, normalization and a few simple things that you can apply to every SQL your application uses to optimize it as much as possible.