An excellent development aid. Will run "EXPLAIN" before each of your select queries in development, and provides a display which makes them easy to browse. In this fork, gemified and compatible with Rails 2 + 3.
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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

Use This Fork

I use this utility for most of my MySQL-backed 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

As of August 2011, This has become the "official" version to use. David has deprecated the original repo and recommends that people looking for this utility use this fork instead. 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. If you run into issues, please file a bug report or send a pull request. I consider this new fork to be a group effort so if you want to be added as a contributor to help maintain this, please let me know!

Database Limitations

As of right now, this plugin supports analyzing queries only for MySQL. I consider this the primary limitation of this gem. I am now using PostgreSQL for a number of my projects and the "EXPLAIN" output is totally different. Several people have started a PostgreSQL adapter for this plugin, but none that are mature enough to include here.

If anyone is interested at taking a stab at implementing PostgreSQL support for this utility, that would be hugely appreciated! I hope this plugin can come to support all the major Rails adapters over time.


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"

or to use the latest from github:

# Gemfile
gem "query_reviewer", :github => "nesquena/query_reviewer"

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" version of the stack trace should include files in /vendor
  • trace_includes_lib: whether the "short" version 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 maintenance done by Nathan Esquenazi. Also @ajvargo helped with several fixes.

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