Really to Scale you need to know when your application will break. I have a tool set to help determine what application is producing what SQL and use that to figure out which SQL is producing the most load on the system. Some common tricks I do is put the execution path automatically as a SQL comment, then sample the FULL Processlist to build a graph on what application, function, SQL pattern is the top load.
Content reproduced on this site is the property of the respective copyright holders. It is not reviewed in advance by Oracle and does not necessarily represent the opinion of Oracle or any other party.