If you’re developing WordPress stuff, and haven’t yet installed John Blackbourn’s Query Monitor plugin, stop what you’re doing and go install it.
There are a bunch of development plugins out there. I’ve used a few. The Debug Bar is a popular one, and I still use it some. Depending on what you’re developing, you’ll probably know about Simply Show IDs, Theme Test Drive, User Switchers, and others. But Query Monitor is the one WP Dev plugin to rule them all. I find that whatever other development tools I have installed on a development site, Query Monitor is still the hub.
It’s not just about which queries you’re running on any given page load (although that’s handy). But from a quick drop-down menu on the Admin Bar, you can quickly tell which
is_ functions return true, which scripts and styles have loaded, any PHP errors of any kind have happened during the page’s life cycle, a complete run-down of what was set on every hook from start to finish. You can know about nearly anything that happens. It’s a huge head start when you’re trying to track down any unexpected thing that’s happened. Combine that with a debugger running in your IDE, and you can kiss most bugs good-bye in half the time you might have otherwise spent.
Seriously, after you’ve been using it for 15 minutes, you’ll wonder how you ever got anything done without it.