QuickCount

Unfortunately, it's currently notoriously difficult and expensive to get an exact count on large tables.

Luckily, there are some tricks for quickly getting fairly accurate estimates. For example, on a table with over 450 million records, you can get a 99.82% accurate count within a fraction of of the time. See the table below for an example dataset.

Supports:

• PostgreSQL
  • Multi-table Inheritance
• MySQL

SQL	Result	Accuracy	Time
SELECT count(*) FROM small_table;	2037104	100.0000000%	4.900s
Post.quick_count	2036407	99.96578476%	0.050s
SELECT count(*) FROM medium_table;	81716243	100.0000000%	257.5s
Post.quick_count	81600513	99.85837577%	0.048s
SELECT count(*) FROM large_table;	455270802	100.0000000%	310.6s
Post.quick_count	454448393	99.81935828%	0.046s

These metrics were pulled from real databases being used in a production environment.

Installation

Add this line to your application's Gemfile:

gem 'quick_count'

And then execute:

$ bundle

That's it. QuickCount automatically integrates with ActiveRecord via a Railtie — no setup step required.

Usage

# Fast estimated count for large tables
User.quick_count

# Override the default threshold (500,000) on a case-by-case basis
User.quick_count(threshold: 1_000_000)

# Estimate row count for an arbitrary query
User.where(active: true).count_estimate

If the estimated count is below the threshold, quick_count falls back to an exact SELECT COUNT(*). For tables above the threshold, it returns the estimate directly.

License

The gem is available as open source under the terms of the MIT License.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/TwilightCoders/quick_count.