Question

The following query runs fairly quickly, but the series processing that needs to take place afterwards is really slowing this method down. I could use some help in refactoring.

def self.sum_amount_chart_series(start_time)
  orders_by_day = Widget.archived.not_void.
                  where(:print_datetime => start_time.beginning_of_day..Time.zone.now.end_of_day).
                  group(pg_print_date_group).
                  select("#{pg_print_date_group} as print_date, sum(amount) as total_amount")


  # THIS IS WHAT IS SLOWING THE METHOD DOWN!
  (start_time.to_date..Date.today).map do |date|
    order = orders_by_day.detect { |order| order.print_date.to_date == date }
    order && order.total_amount.to_f.round(2) || 0.0
  end

end

def self.pg_print_date_group
  "CAST((print_datetime + interval '#{tz_offset_hours} hours') AS date)"
end

I have benchmarked this method and the offending code is the series loop where it generates a series of dates and then maps out a new array with an amount for each date. This way I get a series back with amounts for every date, regardless if it has an amount or not.

When the query only returns a few dates, it runs fairly quickly. But set the start date back a year or two and it becomes impossibly slow. The real offender is the .detect method. It’s very slow at scanning the array of activerecord objects.

Is there a faster method to generates this series?

Answer 1

orders_by_day is grouped by “pg_print_date_group” so it should be a hash of “date” to objects. so why don’t you just do

(start_time.to_date..Date.today).map do |date|
  order = orders_by_day[date.to_s(:db)]
  order && order.total_amount.to_f.round(2) || 0.0
end

That should seriously reduce the Big O of your run. And if I’m misunderstanding and your orders_by_day isn’t a hash, preprocess it into a hash and then run the map, you definitely don’t want to detect for every date.