Query Monitor is a powerful and modern debugging plugin for anyone developing with WordPress. It is free and open source software.
Query Monitor has some advanced features not often available in other debugging plugins, including debugging of:
* Ajax calls
* REST API requests
* Hooks and actions
* Template hierarchy
* Template parts
* Rewrite rules
* Redirects
* Broken script and style dependencies
* HTTP API requests
* Translation files
Query Monitor focuses on making its information useful, rather than dumping lots of information on the screen without thought. It includes the ability to narrow down its output by plugin or theme, to see which components are responsible for queries or actions, and to highlight problematic areas such as slow queries, suboptimal configuration, and PHP errors.