How to get a fast, but approximate, row count in ActiveRecord + Postgresql

application_record.rb

# frozen_string_literal: true
class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
  class << self
    def approximate_row_count(table_name = arel_table.name)
      results = ActiveRecord::Base.connection_pool.with_connection { |c|
        c.execute("SELECT reltuples::BIGINT AS approximate_row_count FROM pg_class WHERE relname = '#{table_name}'")
      }
      results.present? ? results.first['approximate_row_count'].to_i : nil
    end
  end
end

e.g. MyClass.approximate_row_count or ApplicationRecord.approximate_row_count('my_classes')

Sourced from here: https://wiki.postgresql.org/wiki/Count_estimate