XHProf was developed by facebook as a tool to diagnose heavy or slow responce areas of a website. This tool has now been used on several of our websites to diagnose and finetune the effectiveness of various queries in mySQL and PHP. The tool is free to use, and quite good. If you wish to know more about it, here are links to the xhprof documentation and also a link to where you can download the install files for xhprof.