This plugin, once activated, spits out the information passed by each filter used in manipulating database queries, as a learning tool.
The code itself is heavily documented and demonstrates best practices for working with those filters and actions to view / manipulate the query.
It is very important to note that this plugin is not meant for a production environment as it will insert a lot of really ugly HTML throughout your page. You’ll get the most use out of this plugin by looking at its code, and maybe previewing one or two pages on a sandbox, vanilla WordPress site.