require 'benchmark/ips'

ActiveRecord::Base.logger = nil

def pagy_count(collection)
  (c = collection.count(:all)).is_a?(Hash) ? c.size : c
end

def pagy_count2(collection)
  result = collection.count(:all)
  collection.group_values.empty? ? result : result.size  
end

def pagy_count3(collection)
  if collection.group_values.empty? 
    collection.count(:all)
  else
    collection.count(:all).size  
  end
end

def pagy_arel_count(collection)
  # COUNT(*) OVER ()
  sql = Arel.star.count.over(Arel::Nodes::Grouping.new([]))
  collection.unscope(:order).limit(1).pluck(sql).first
end

def pagy_arel_conditional_count(collection)
  if collection.group_values.empty?
    # COUNT(*)
    collection.count(:all)
  else
    # COUNT(*) OVER ()
    sql = Arel.star.count.over(Arel::Nodes::Grouping.new([]))
    collection.unscope(:order).limit(1).pluck(sql).first
  end
end

collection = ::Movies::Rating.all
Benchmark.ips do |x|
  x.time = 30
  x.report("pagy_count:") { ApplicationRecord.uncached { pagy_count(collection) } }
  x.report("pagy_count2:") { ApplicationRecord.uncached { pagy_count2(collection) } }
  x.report("pagy_count3:") { ApplicationRecord.uncached { pagy_count3(collection) } }
  x.report("pagy_arel_count:") { ApplicationRecord.uncached { pagy_arel_count(collection) } }
  x.report("pagy_arel_conditional_count:") { ApplicationRecord.uncached { pagy_arel_conditional_count(collection) } }
  x.compare!
end

<<-OUTPUT

Warming up --------------------------------------
         pagy_count:     2.000  i/100ms
        pagy_count2:     1.000  i/100ms
        pagy_count3:     2.000  i/100ms
    pagy_arel_count:     1.000  i/100ms
pagy_arel_conditional_count:
                         2.000  i/100ms
Calculating -------------------------------------
         pagy_count:     26.297  (±19.0%) i/s -    758.000  in  30.060235s
        pagy_count2:     30.137  (±26.5%) i/s -    849.000  in  30.005502s
        pagy_count3:     31.145  (±16.1%) i/s -    912.000  in  30.055667s
    pagy_arel_count:      7.863  (±12.7%) i/s -    227.000  in  30.119870s
pagy_arel_conditional_count:
                         27.263  (±22.0%) i/s -    762.000  in  30.013175s

Comparison:
        pagy_count3::       31.1 i/s
        pagy_count2::       30.1 i/s - same-ish: difference falls within error
pagy_arel_conditional_count::       27.3 i/s - same-ish: difference falls within error
         pagy_count::       26.3 i/s - same-ish: difference falls within error
    pagy_arel_count::        7.9 i/s - 3.96x  slower


OUTPUT 

collection = ::Movies::Rating.group(:movie_id).select(:movie_id)
# 1682 movies
Benchmark.ips do |x|
  x.time = 30
  x.report("pagy_count:") { ApplicationRecord.uncached { pagy_count(collection) } }
  x.report("pagy_count2:") { ApplicationRecord.uncached { pagy_count2(collection) } }
  x.report("pagy_count3:") { ApplicationRecord.uncached { pagy_count3(collection) } }
  x.report("pagy_arel_count:") { ApplicationRecord.uncached { pagy_arel_count(collection) } }
  x.report("pagy_arel_conditional_count:") { ApplicationRecord.uncached { pagy_arel_conditional_count(collection) } }
  x.compare!
end

<<-OUTPUT
Warming up --------------------------------------
         pagy_count:     1.000  i/100ms
        pagy_count2:     1.000  i/100ms
        pagy_count3:     1.000  i/100ms
    pagy_arel_count:     1.000  i/100ms
pagy_arel_conditional_count:
                         1.000  i/100ms
Calculating -------------------------------------
         pagy_count:      5.874  (±34.0%) i/s -    166.000  in  30.288133s
        pagy_count2:      9.154  (±21.8%) i/s -    256.000  in  30.024894s
        pagy_count3:      9.811  (±20.4%) i/s -    278.000  in  30.101955s
    pagy_arel_count:      8.969  (±33.5%) i/s -    249.000  in  30.070671s
pagy_arel_conditional_count:
                         12.439  (±24.1%) i/s -    351.000  in  30.087875s

Comparison:
pagy_arel_conditional_count::       12.4 i/s
        pagy_count3::        9.8 i/s - same-ish: difference falls within error
        pagy_count2::        9.2 i/s - same-ish: difference falls within error
    pagy_arel_count::        9.0 i/s - same-ish: difference falls within error
         pagy_count::        5.9 i/s - 2.12x  slower
OUTPUT


collection = ::Movies::Rating.group(:rating).select(:rating)
# 5 ratings (1,2,3,4,5)
Benchmark.ips do |x|
  x.time = 30
  x.report("pagy_count:") { ApplicationRecord.uncached { pagy_count(collection) } }
  x.report("pagy_count2:") { ApplicationRecord.uncached { pagy_count2(collection) } }
  x.report("pagy_count3:") { ApplicationRecord.uncached { pagy_count3(collection) } }
  x.report("pagy_arel_count:") { ApplicationRecord.uncached { pagy_arel_count(collection) } }
  x.report("pagy_arel_conditional_count:") { ApplicationRecord.uncached { pagy_arel_conditional_count(collection) } }
  x.compare!
end

<<-OUTPUT

Warming up --------------------------------------
         pagy_count:     1.000  i/100ms
        pagy_count2:     1.000  i/100ms
        pagy_count3:     1.000  i/100ms
    pagy_arel_count:     1.000  i/100ms
pagy_arel_conditional_count:
                         1.000  i/100ms
Calculating -------------------------------------
         pagy_count:      7.855  (±25.5%) i/s -    224.000  in  30.015308s
        pagy_count2:      9.501  (±21.1%) i/s -    271.000  in  30.038578s
        pagy_count3:      9.978  (±20.0%) i/s -    288.000  in  30.018907s
    pagy_arel_count:     11.988  (±25.0%) i/s -    342.000  in  30.003528s

OUTPUT