Query Monitor

Query Monitor is the Developer Tools panel for WordPress. It includes some advanced features not available in other debugging plugins, including Ajax and REST API debugging, and the ability to narrow down its output by plugin or theme.

Query Monitor focuses heavily on presenting its information in a useful manner. It adds an admin toolbar menu showing an overview of the current page, with complete data shown in a panel once you select an item.

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

Here’s an overview of some of what’s shown:

Database Queries

• Shows all database queries performed on the current request
• Shows notifications for slow queries, duplicate 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 are making the most (or the slowest) database queries.

Hooks

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

Theme

• Shows the template filename for the current request
• Shows the complete template hierarchy for the current request (WordPress 4.7+)
• Shows all template parts used on the current request

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 query vars for the current request, and highlights custom query vars
• Shows all matched rewrite rules and associated query strings

Scripts & Styles

• Shows all enqueued scripts and styles on the current request, along with their URL and version
• Shows their dependencies and dependents, and displays an alert for any broken dependencies

Languages

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

HTTP Requests

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

User Capability Checks

• Shows every user capability check that is performed on the page, along with the result and any parameters passed along with the capability check.

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.

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.