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Query Monitor is a debugging plugin for anyone developing with WordPress. It has some advanced features not available in other debugging plugins, including automatic AJAX debugging, REST API debugging, and the ability to narrow down things by plugin or theme.

For complete information, please see Query Monitor's GitHub repo.

Here's an overview of what's shown:

Database Queries

• Shows all database queries performed on the current page
• Shows affected rows and time for all queries
• Show notifications for slow queries and queries with errors
• Filter queries by query type (SELECT, UPDATE, DELETE, etc)
• Filter queries by component (WordPress core, Plugin X, Plugin Y, theme)
• Filter queries by calling function
• View aggregate query information grouped by component, calling function, and type
• Super advanced: Supports multiple instances of wpdb on one page (more info in the FAQ)

Filtering queries by component or calling function makes it easy to see which plugins, themes, or functions on your site are making the most (or the slowest) database queries.

Hooks

• Shows all hooks fired on the current page, along with hooked actions, their priorities, and their components
• Filter hooks by part of their name
• Filter actions by component (WordPress core, Plugin X, Plugin Y, theme)

Theme

• Shows the template filename for the current page
• Shows the available body classes for the current page
• Shows the active theme name

PHP Errors

• PHP errors (warnings, notices, stricts, and deprecated) are presented nicely along with their component and call stack
• Shows an easily visible warning in the admin toolbar

Request

• Shows matched rewrite rules and associated query strings
• Shows query vars for the current request, and highlights custom query vars
• Shows the queried object details
• Shows details of the current blog (multisite only) and current site (multi-network only)

Scripts & Styles

• Shows all enqueued scripts and styles on the current page, along with their URL and version
• Shows their dependencies and dependents, and alerts you to any broken dependencies

Languages

• Shows you language settings and text domains
• Shows you the MO files for each text domain and which ones were loaded or not

HTTP Requests

• Shows all HTTP requests performed on the current page (as long as they use WordPress' HTTP API)
• Shows the response code, call stack, transport, component, timeout, and time taken
• Highlights erroneous responses, such as failed requests and anything without a 200 response code

Redirects

• Whenever a redirect occurs, Query Monitor adds an X-QM-Redirect HTTP header containing the call stack, so you can use your favourite HTTP inspector or browser developer tools to easily trace where a redirect has come from

AJAX

The response from any jQuery AJAX request on the page will contain various debugging information in its headers. Any errors also get output to the developer console. No hooking required.

Currently this includes PHP errors and some overview information such as memory usage, but this will be built upon in future versions.

REST API

The response from an authenticated WordPress REST API (v2 or later) request will contain various debugging information in its headers, as long as the authenticated user has permission to view Query Monitor's output.

Currently this includes PHP errors and some overview information such as memory usage, but this will be built upon in future versions.

Admin Screen

• Shows the correct names for custom column filters and actions on all admin screens that have a listing table
• Shows the state of get_current_screen() and a few variables

Environment Information

• Shows various PHP information such as memory limit and error reporting levels
• Highlights the fact when any of these are overridden at runtime
• Shows various MySQL information, including caching and performance related configuration
• Highlights the fact when any performance related configurations are not optimal
• Shows various details about WordPress and the web server
• Shows version numbers for all the things

Everything Else

• Shows any transients that were set, along with their timeout, component, and call stack
• Shows all WordPress conditionals on the current page, highlighted nicely
• Shows an overview at the top, including page generation time and memory limit as absolute values and as % of their respective limits

Authentication

By default, Query Monitor's output is only shown to Administrators on single-site installs, and Super Admins on Multisite installs.

In addition to this, you can set an authentication cookie which allows you to view Query Monitor output when you're not logged in (or if you're logged in as a non-administrator). See the bottom of Query Monitor's output for details.