work.rowanhick.com
How to avoid hanging
yourself with Rails
Using ActiveRecord right the first time
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Discussion tonight
• Intended for new Rails Developers
• People that think Rails is slow
• Focus on simple steps to improve
common :has_many performance problems
• Short - 15mins
• All links/references up on
http://work.rowanhick.com tomorrow
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About me
• New Zealander (not Australian)
• Product Development Mgr for a startup in Toronto
• Full time with Rails for 2 years
• Previously PHP/MySQL for 4 years
• 6 years Prior QA/BA/PM for Enterprise CAD/
CAM software dev company
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Disclaimer
• For sake of brevity and understanding, the SQL
shown here is cut down to “psuedo sql”
• This is not an exhaustive in-depth analysis, just
meant as a heads up
• Times were done using ApacheBench through
mongrel in production mode
• ab -n 1000 http://127.0.0.1/orders/test_xxxx
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ActiveRecord lets you get in
trouble far to quick.
• Super easy syntax comes at a cost.
@orders = Order.find(:all)
@orders.each do |order|
puts order.customer.name
puts order.customer.country.name
end
✴Congratulations, you just overloaded your DB
with (total number of Orders x 2) unnecessary
SQL calls
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What happened there?
• One query to get the orders
@orders = Order.find(:all)
“SELECT * FROM orders”
• For every item in the orders collection
customer.name:
“SELECT * FROM customers WHERE id = x”
customer.country.name:
“SELECT * FROM customers WHERE id = y”
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Systemic Problem in
Web development
I’ve seen:
- 15 Second page reloads
- 10000 queries per page
“<insert name here> language performs
really poorly, we’re going to get it
redeveloped in <insert new language
here>”
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Atypical root cause
• Failure to build application with *real* data
• ie “It worked fine on my machine” but the
developer never loaded up 100’000 records
to see what would happen
• Using Rake tasks to build realistic data sets
• Test, test, test
• tail -f log/development.log
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Faker to the rescue
• in lib/xchain.rake
namespace :xchain do
desc \"Load fake customers\"
task :load_customers => :environment do
require 'Faker'
Customer.find(:all, :conditions => \"email LIKE('%XCHAIN_
%')\").each { |c| c.destroy }
1..300.times do
c = Customer.new
c.status_id = rand(3) + 1
c.country_id = rand(243) + 1
c.name = Faker::Company.name
c.alternate_name = Faker::Company.name
c.phone = Faker::PhoneNumber.phone_number
c.email = \"XCHAIN_\"+Faker::Internet.email
c.save
end
end
$ rake xchain:load_customers
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Eager loading
• By using :include in .finds you create sql joins
• Pull all required records in one query
find(:all, :include => [ :customer, :order_lines ])
✓ order.customer, order.order_lines
find(:all, :include => [ { :customer
=> :country }, :order_lines ])
✓ order.customer order.customer.country
order.order_lines
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Improvement
• Let’s start optimising ...
@orders = Order.find(:all, :include => {:customers => :country} )
• Resulting SQL ...
“SELECT orders.*, countries.* FROM orders LEFT JOIN
customers ON ( customers.id = orders.customers_id )
LEFT JOIN countries ON ( countries.id =
customers.country_id)
✓ 7.70 req/s 1.4x faster
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Select only what you
need
• Using the :select parameter in the find
options, you can limit the columns you are
requesting back from the database
• No point grabbing all columns, if you only
want :id and :name
Orders.find(:all, :select => ‘orders.id,
orders.name’)
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The last slide was very
important
• Not using selects is *okay* provided you
have very small columns, and never any
binary, or large text data
• You can suddenly saturate your DB
connection.
• Imagine our Orders table had an Invoice
column on it storing a pdf of the invoice...
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Oops
• Can’t show a benchmark
• :select and :include don’t work together !,
reverts back to selecting all columns
• Core team for a long time have not
included patches to make it work
• One little sentence in ActiveRecord rdoc
“Because eager loading generates the SELECT
statement too, the :select option is ignored.”
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‘mrj’ to the rescue
• http://dev.rubyonrails.org/attachment/ticket/
7147/init.5.rb
• Monkey patch to fix select/include problem
• Produces much more efficient SQL
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Updated finder
• Now :select and :include playing nice:
@orders = Order.find(:all,
:select => 'orders.id, orders.created_at, customers.name,
countries.name, order_statuses.name',
:include => [{:customer[:name]
=> :country[:name]}, :order_status[:name]],
:conditions => conditions,
:order => 'order_statuses.sort_order ASC,order_statuses.id ASC,
orders.id DESC')
✓15.15 req/s 2.88x faster
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r8672 change
• http://blog.codefront.net/2008/01/30/living-on-the-
edge-of-rails-5-better-eager-loading-and-more/
• The following uses new improved association load
(12 req/s)
@orders = Order.find(:all, :include => [{:customer
=> :country}, :order_status] )
• The following does not
@orders = Order.find(:all, :include => [{:customer
=> :country}, :order_status], :order =>
‘order_statuses.sort_order’ )
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r8672 output...
• Here’s the SQL
Order Load (0.000837) SELECT * FROM `orders` WHERE (order_status_id <
100) LIMIT 10
Customer Load (0.000439) SELECT * FROM `customers` WHERE
(customers.id IN (2106,2018,1920,2025,2394,2075,2334,2159,1983,2017))
Country Load (0.000324) SELECT * FROM `countries` WHERE (countries.id
IN (33,17,56,150,194,90,91,113,80,54))
OrderStatus Load (0.000291) SELECT * FROM `order_statuses` WHERE
(order_statuses.id IN (10))
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But I want more
• Okay, this still isn’t blazing fast. I’m building
the next killr web2.0 app
• Forgetabout associations, just load it via
SQL, depending on application, makes a
huge difference
• Concentrate on commonly used pages
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Catch 22
• Hard coding SQL is the fastest solution
• No construction of SQL, no generation of
ActiveRecord associated classes
• If your DB changes, you have to update
SQL
‣ Keep SQL with models where possible
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It ain’t pretty.. but it’s
fast
• Find by SQL
class order
def self.find_current_orders
find_by_sql(\"SELECT orders.id, orders.created_at, customers.name
as customer_name, countries.name as country_name, order_statuses.name
as status_name FROM orders LEFT OUTER JOIN `customers` ON
`customers`.id = `orders`.customer_id LEFT OUTER JOIN `countries` ON
`countries`.id = `customers`.country_id LEFT OUTER JOIN
`order_statuses` ON `order_statuses`.id = `orders`.order_status_id
WHERE order_status_id < 100 ORDER BY order_statuses.sort_order
ASC,order_statuses.id ASC, orders.id DESC\")
end
end
• 28.90 req/s ( 5.49x faster )
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And the results
find(:all) 5.26 req/s
find(:all, :include) 7.70 req/s 1.4x
find(:all, :select, :in
15.15 req/s 2.88x
clude)
find_by_sql() 28.90 req/s 5.49x
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Don’t forget indexes
• 64000 orders
OrderStatus.find(:all).each { |os| puts
os.orders.count }
• Avg 0.61 req/s no indexes
• EXPLAIN your SQL
ALTER TABLE `xchain_test`.`orders` ADD INDEX
order_status_idx(`order_status_id`);
• Avg 23 req/s after index (37x improvment)
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Avoid .count
• It’s damned slow
OrderStatus.find(:all).each { |os| puts
os.orders.count }
• Add column orders_count + update code
OrderStatus.find(:all).each { |os| puts
os.orders_count }
✓34 req/s vs 108 req/s (3x faster)
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For the speed freaks
• Merb - http://merbivore.com
• 38.56 req/s - 7x performance improvement
• Nearly identical code
• Blazingly fast
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The End
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