This rails plugin not only runs "EXPLAIN" before each of your select queries in development, but provides a small DIV in the rendered output of each page with the summary of query warnings that it analyzed.
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QueryReviewer is an advanced SQL query analyzer. It accomplishes the following goals:

  • View all EXPLAIN output for all SELECT queries to generate a page
  • Rate a page's SQL usage into one of three categories: OK, WARNING, CRITICAL
  • Attach meaningful warnings to individual queries, and collections of queries
  • Display interactive summary on page

This Fork

I use this utility for most of my rails projects. Still the best out there in my opinion for analyzing and understanding your generated SQL queries. I forked the original query_reviewer and applied a collection of patches that have been made since the plugin was created. A list of the biggest additions below:

  • Snazzed up the README into markdown for better readability
  • Full compatibility for Rails 3 (including Railtie)
  • Cleanup and move rake task to lib/tasks to fix deprecation warnings
  • Added gemspec for use with Bundler (as a gem)
  • Fixed missing tags and additional XHTML escaping
  • Fix SQL escaping for better XHTML compatibility
  • Fixes for deprecation warnings and for 1.9 compatiblity
  • Converts templates to more modern foo.html.erb naming

Last commit to the main repository was on March 30th, 2009. This fork compiles a variety of patches that were made since that time along with additional work to support compatibility with 1.9 and Rails 3. Also: If anyone else creates generally useful enhancements to this utility please start by forking this and then issue me a pull request.

Note: This plugin should work for Rails 2.X and Rails 3. Support for Rails 3 has been confirmed in the latest revision (with fixed deprecation warnings).


All you have to do is install it into your Rails 2 or 3 project.

gem install query_reviewer

Right now if you use bundler, simply add this to your Gemfile:

# Gemfile
gem "query_reviewer", :git => "git://"

If you are not using bundler, you might want to start using it. You can also install this as a plugin:

script/plugin install git://

In Rails 2, the rake tasks are not loaded automatically (as a gem), you’ll need to add the following to your Rakefile:

# Rakefile
  require 'query_reviewer/tasks'
rescue LoadError
  STDERR.puts "The query_reviewer gem could not be found!"

You can then run:

$ rake query_reviewer:setup

Which will create config/query_reviewer.yml in your application, see below for what these options mean. If you don't create a config file, the gem will use the default in vendor/plugins/query_reviewer.


The configuration file allows you to set configuration parameters shared across all rails environment, as well as overriding those shared parameteres with environment-specific parameters (such as disabling analysis on production!)

  • enabled: whether any output or query analysis is performed. Set this false in production!
  • inject_view: controls whether the output automatically is injected before the </body> in HTML output.
  • profiling: when enabled, runs the MySQL SET PROFILING=1 for queries longer than the warn_duration_threshold / 2.0
  • production_data: whether the duration of a query should be taken into account
  • stack_trace_lines: number of lines of call stack to include in the "short" version of the stack trace
  • trace_includes_vendor: whether the "short" verison of the stack trace should include files in /vendor
  • trace_includes_lib: whether the "short" verison of the stack trace should include files in /lib
  • warn_severity: the severity of problem that merits "WARNING" status
  • critical_severity: the severity of problem that merits "CRITICAL" status
  • warn_query_count: the number of queries in a single request that merits "WARNING" status
  • critical_query_count: the number of queries in a single request that merits "CRITICAL" status
  • warn_duration_threshold: how long a query must take in seconds (float) before it's considered "WARNING"
  • critical_duration_threshold: how long a query must take in seconds (float) before it's considered "CRITICIAL"


If you disable the inject_view option above, you'll need to manually put the analyzer's output into your view:

# view.html.haml
= query_review_output

and that will display the analyzer view!


Random collection of resources that might be interesting related to this utility:

Other related gems that prove useful for database optimization:


There have been other alternatives created since this was originally released. A few of the best are listed below. I for one still prefer this utility over the other options:

Know of a better alternative? Let me know!


Created by Kongregate & David Stevenson. Refactorings and compilations of all fixes since was done by Nathan Esquenazi. Also, ajvargo for helping with some fixes.

Copyright (c) 2007-2008 Kongregate & David Stevenson, released under the MIT license