extremely slow csv exporting - ruby-on-rails

Question

We are trying to generate a CSV file for users to download. However, it's extremely slow, about 5 minutes for 10k lines of CSV.
Any good idea on improving? Code below:
def download_data
start_date, end_date = get_start_end_date
report_lines = #report.report_lines.where("report_time between (?) and (?)", start_date, end_date)
csv_string = CSV.generate do |csv|
report_lines.each do |report_data|
csv << [report_data.time, report_data.name, report_data.value]
end
end
respond_to do |format|
format.csv { send_data(csv_string, :filename => "#{Time.now}.csv", :type => "text/csv") }
end
end

Answer 1

I would start by checking if report_time is indexed, unindexed report_time is certainly going to contribute to the slowness. Refer to Active Record Migration for details on adding index.
Second, you could trim down the result to only what you need, i.e. instead of selecting all columns, select only time, name, and value:
report_lines = #report.report_lines
.where("report_time between (?) and (?)", start_date, end_date)
.select('time, name, value')
Try with:
def download_data
start_date, end_date = get_start_end_date
report_lines = #report.report_lines.where("report_time between (?) and (?)", start_date, end_date).select('time, name, value')
csv_string = CSV.generate do |csv|
report_lines.map { |row| csv << row }
end
respond_to do |format|
format.csv { send_data(csv_string, :filename => "#{Time.now}.csv", :type => "text/csv") }
end
end

Answer 2

Check which part takes so much time.
If it's SQL query, then try to optimize it: check if you have indexes, maybe you need to force another index.
If query is fast, but Ruby is slow, then you can try 2 things:
either try some faster CSV lib (like fastest-csv)
generate as much of CSV as possible in SQL. What I mean is: instead of creating ActiveRecord objects there, fetch just concatenated string for each row. If it's better, but still too slow, you can end up generating just one big string in database and just rendering it for ust (this solution might not seem good, and it looks quite ugly, but in one project I had to use it, because it was the fastest way)