FastCount

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 PostgreSQL table with over 450 million records, you can get a 99.82% accurate count within a fraction of the time. See the table below for an example dataset.

SQL	Result	Accuracy	Time
SELECT count(*) FROM small_table	2037104	100.000%	4.900s
SELECT fast_count('small_table')	2036407	99.965%	0.050s
SELECT count(*) FROM medium_table	81716243	100.000%	257.5s
SELECT fast_count('medium_table')	81600513	99.858%	0.048s
SELECT count(*) FROM large_table	455270802	100.000%	310.6s
SELECT fast_count('large_table')	454448393	99.819%	0.046s

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

For MySQL, this gem uses internal statistics to return the estimated table's size. And as per documentation, it may vary from the actual value by as much as 40% to 50%. But still is useful to get a rough idea of the number of rows in very large tables (where COUNT(*) can literally take hours).

Supports PostgreSQL, MySQL, MariaDB, and SQLite.

Requirements

• Ruby 2.7+
• ActiveRecord 6+

If you need support for older versions, open an issue.

Installation

Add this line to your application's Gemfile:

gem 'fast_count'

And then execute:

$ bundle

Or install it yourself as:

$ gem install fast_count

If you are using PostgreSQL, you need to create a database function, used internally:

$ rails generate migration install_fast_count

with the content:

class InstallFastCount < ActiveRecord::Migration[7.0]
  def up
    FastCount.install
  end

  def down
    FastCount.uninstall
  end
end

Usage

Estimated table count

To quickly get an estimated count of the rows in a table:

User.fast_count # => 1_254_312_219

Result set size estimation

If you want to quickly get an estimation of how many rows will the query return, without actually executing it, yo can run:

User.where.missing(:avatar).estimated_count # => 324_200

Note: estimated_count relies on the database query planner estimations (basically on the output of EXPLAIN) to get its results and can be very imprecise. It is better be used to get an idea of the order of magnitude of the future result.

Exact distinct values count

To quickly get an exact number of distinct values in a column, you can run:

User.fast_distinct_count(column: :company_id) # => 243

It is suited for cases when there is a small amount of distinct values in a column compared to a total number of values (for example, 10M rows total and 200 distinct values).

Runs orders of magnitude faster than SELECT COUNT(DISTINCT column) FROM table.

Note: You need to have an index starting with the specified column for this to work.

Uses a "Loose Index Scan" technique.

Configuration

You can override the following default options:

# Determines for how large tables this gem should get the exact row count using SELECT COUNT.
# If the approximate row count is smaller than this value, SELECT COUNT will be used,
# otherwise the approximate count will be used.
FastCount.threshold = 100_000

Credits

Thanks to quick_count gem for the original idea.

Development

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 the created tag, and push the .gem file to rubygems.org.

Contributing

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

License

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