messages_by_dates.rb

dates_messages = {}

Message.where(message_type: 'MESSAGE').select([:message_type, :created_at, :id]).find_in_batches(batch_size: 10000) do |messages|
  messages.each do |message|
    dates_messages[message.created_at.to_date.to_s] ||= 0
    dates_messages[message.created_at.to_date.to_s] += 1
  end
end; nil

dates_messages.each do |date, messages_count|
  puts "#{date}\t#{messages_count}"
end; nil