"SELECT DISTINCT node.nid AS nid" killing MySQL performance? My database server's CPU has been pegged for the last two hours and the site is down as a result. Looking at the process list, I see dozens of queries that all begin "SELECT DISTINCT node.nid AS nid," and from my reading, it appears that this should not be a common occurrence  because these queries are very expensive and this could be the source of my problems.

Is it safe to say there is a view or module making a query incorrectly and if so, how can I can track that query down?
from what I see, you may be running views, and complex views, that perform complex queries. execute() is a database abstraction layer function. I am deducing that you are using views from the image you sent, which lists queries that look like generated by the views module.

Luckily, if my supposition is correct, then your problem can be lifted. When editing the view, you can enable caching. Unless you have a constant flow of new view items, you can make caching time 10, 30, or even 60 minutes. Or more. Making these expensive queries cached will lift the overhead. Also, if you are using panels, you can cache panel panes as well, or whole pages for anonymous users, keeping in mind, that caching on the larger level (page vs a single view) is more preferable.