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Last Updated: 2023-09-11 14:24:23
The Developer Tools Panel for WordPress
License: GNU General Public License v2.0
Languages: JavaScript, PHP, SCSS, Shell
Query Monitor is the developer tools panel for WordPress. It enables debugging of database queries, PHP errors, hooks and actions, block editor blocks, enqueued scripts and stylesheets, HTTP API calls, and more.
It includes some advanced features such as debugging of Ajax calls, REST API calls, user capability checks, and full support for block themes and full site editing. It includes the ability to narrow down much of its output by plugin or theme, allowing you to quickly determine poorly performing plugins, themes, or functions.
Query Monitor focuses heavily on presenting its information in a useful manner. Here's an example showing aggregate database queries grouped by the components responsible for them:
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.
200
response codeswitch_to_blog()
and restore_current_blog()
on Multisite installationsX-QM-Redirect
HTTP header containing the call stack, so you can use your favourite HTTP inspector or browser developer tools to trace where a redirect has come fromThe 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.
The response from an authenticated WordPress REST API 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 overview information.
To see more detailed information about a REST API request you need to perform an enveloped request which means appending ?_envelope
to the requested URL. In this case, Query Monitor will include debugging data in a qm
property in the response. Currently this includes database queries (including information about duplicates and errors), HTTP API requests, and transient updates. More information may be added in a future version.
By using the combination of the HTTP headers and the qm
property in the response to an enveloped request you'll get good insight into the aspects of a request which have the greatest impact on performance.
get_current_screen()
and a few global variablesDebugging messages can be sent to the Logs panel in Query Monitor using actions. This works as a good replacement for var_dump()
:
do_action( 'qm/debug', 'This happened!' );
The logger is PSR-3 compatible, so you can use any of the following actions which correspond to PSR-3 log levels:
qm/emergency
qm/alert
qm/critical
qm/error
qm/warning
qm/notice
qm/info
qm/debug
A log level of warning
or higher will trigger a notification in Query Monitor's admin toolbar.
Contextual interpolation can be used via the curly brace syntax:
do_action( 'qm/warning', 'Unexpected value of {foo} encountered', [
'foo' => $foo,
] );
A WP_Error
, Exception
, or Throwable
object can be passed directly into the logger:
if ( is_wp_error( $response ) ) {
do_action( 'qm/error', $response );
}
try {
// your code
} catch ( Exception $e ) {
do_action( 'qm/error', $e );
}
A non-scalar value can be passed to the logger and its value will be formatted and output in the same panel.
do_action( 'qm/debug', get_queried_object() );
Finally, the static logging methods on the QM
class can be used instead of calling do_action()
.
QM::error( 'Everything is broken' );
Basic performance profiling can be displayed in the Timings panel in Query Monitor using actions in your code:
// Start the 'foo' timer:
do_action( 'qm/start', 'foo' );
// Run some code
my_potentially_slow_function();
// Stop the 'foo' timer:
do_action( 'qm/stop', 'foo' );
The time taken and approximate memory usage used between the qm/start
and qm/stop
actions for the given function name will be recorded and shown in the Timings panel. Timers can be nested, although be aware that this reduces the accuracy of the memory usage calculations.
Timers can also make use of laps with the qm/lap
action:
// Start the 'bar' timer:
do_action( 'qm/start', 'bar' );
// Iterate over some data:
foreach ( range( 1, 10 ) as $i ) {
my_potentially_slow_function( $i );
do_action( 'qm/lap', 'bar' );
}
// Stop the 'bar' timer:
do_action( 'qm/stop', 'bar' );
Note that the times and memory usage displayed in the Timings panel should be treated as approximations, because they are recorded at the PHP level and can be skewed by your environment and by other code. If you require highly accurate timings, you'll need to use a low level profiling tool such as XHProf. See the Related Tools section for more information.
By default, Query Monitor's output is only shown to Administrators on single-site installations, and Super Admins on Multisite installations.
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 when you're logged in as a user who cannot usually see Query Monitor's output. See the Settings panel for details.
In order to do a few clever things, Query Monitor symlinks a custom db.php
into your WP_CONTENT_DIR
which means it loads very early. This file gets included before the database driver is loaded, meaning this portion of Query Monitor loads before WordPress even engages its brain.
In this file is Query Monitor's extension to the wpdb
class which:
If your WP_CONTENT_DIR
isn't writable and therefore the symlink for db.php
can't be put in place, Query Monitor still functions, but this extended functionality won't be available. You can manually create the db.php symlink if you have permission.
Yes, it's actively tested and working up to PHP 8.2.
By default, Query Monitor's output is only shown to Administrators on single-site installations, and Super Admins on Multisite installations.
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 when you're logged in as a user who cannot usually see Query Monitor's output. See the Settings panel for details.
Short answer: Yes, but only a little.
Long answer: Query Monitor has a small impact on page generation time because it hooks into a few places in WordPress in the same way that other plugins do. The impact is negligible.
On pages that have an especially high number of database queries (in the hundreds), Query Monitor currently uses more memory than I would like it to. This is due to the amount of data that is captured in the stack trace for each query. I have been and will be working to continually reduce this.
Yes, if anything calls do_action( 'qm/cease' )
then Query Monitor will cease operating for the remainder of the page generation. It detaches itself from further data collection, discards any data it's collected so far, and skips the output of its information.
This is useful for long-running operations that perform a very high number of database queries, consume a lot of memory, or otherwise are of no concern to Query Monitor, for example:
A list of add-on plugins for Query Monitor can be found here.
In addition, Query Monitor transparently supports add-ons for the Debug Bar plugin. If you have any Debug Bar add-ons installed, deactivate Debug Bar and the add-ons will show up in Query Monitor's menu.
Please use the issue tracker on Query Monitor's GitHub repo as it's easier to keep track of issues there, rather than on the wordpress.org support forums.
Yes, the Altis Developer Tools are built on top of Query Monitor.
Yes, but a user needs to be granted the view_query_monitor
capability to see Query Monitor even if they're an administrator. See the WordPress.com VIP documentation for more details.
wpdb
. How do I get my additional instances to show up in Query Monitor?This feature was removed in version 3.12 as it was rarely used and considerably increased the maintenance burden of Query Monitor itself. Feel free to continue using version 3.11 if you need to make use of this feature.
Yes! You can enable this on the Settings panel.
I am accepting sponsorships via the GitHub Sponsors program. If you work at an agency that develops with WordPress, ask your company to provide sponsorship in order to invest in its supply chain. The tools that I maintain probably save your company time and money, and GitHub sponsorship can now be done at the organisation level.
In addition, if you like the plugin then I'd love for you to leave a review. Tell all your friends about it too!
Query Monitor is private by default and always will be. It does not persistently store any of the data that it collects. It does not send data to any third party, nor does it include any third party resources.
Query Monitor's full privacy statement can be found here.
Query Monitor aims to be fully accessible to all of its users. It implements best practices for web accessibility, outputs semantic and structured markup, uses the accessibility APIs provided by WordPress and web browsers where appropriate, and is fully accessible via keyboard.
That said, Query Monitor does not conform to the Web Content Accessibility Guidelines (WCAG) 2.0 at level AA like WordPress itself does. The main issue is that the user interface uses small font sizes to maintain a high information density for sighted users. Users with poor vision or poor motor skills may struggle to view or interact with some areas of Query Monitor because of this. This is something which I'm acutely aware of and which I work to gradually improve, but the underlying issue of small font sizes remains.
If you've experienced or identified another accessibility issue in Query Monitor, please open a thread in the Query Monitor plugin support forum and I'll try my best to address it swiftly.
Debugging is rarely done with just one tool. Along with Query Monitor you should be aware of other plugins and tools for debugging and profiling your website. Here are some recommendations:
Query Monitor also has several add-on plugins which extend its functionality, and transparently supports add-ons for the Debug Bar plugin (see the FAQ for more info).
See also my list of WordPress Developer Plugins.
Code contributions, feedback, and feature suggestions are very welcome. See CONTRIBUTING.md for more details.
Query Monitor's icon was designed by Tubagus Didin Asrori.
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.