thiagoa

-- DISTINCT ON: INNER JOIN vs LEFT JOIN
--
-- With LEFT JOIN, Postgres switches to a Hash Right Join that loses
-- the index's sort order. It hash-joins all 5 million rows, sorts
-- them on disk, and then deduplicates. LIMIT can't stop early
-- because the Unique node sits on top of the sort.
--
-- With INNER JOIN, Postgres can use a Merge Join that walks the covering
-- index in order. The output is already sorted, so LIMIT stops
-- after the first 15 unique users — no disk sort at all.

thiagoa

-- A common variation uses MAX(id) to find the latest status:
SELECT u.id, s.status
FROM users u
JOIN user_statuses s ON s.user_id = u.id
WHERE s.id = (
    SELECT MAX(id) FROM user_statuses s2 WHERE s2.user_id = u.id
);

-- This assumes the highest id is always the latest status.
-- That holds under normal operation, but breaks during backfills

thiagoa

-- DISTINCT ON with LIMIT is slow because Postgres can't push the LIMIT
-- through the Unique node. It hash-joins all status rows, sorts them on
-- disk, and only then returns 15.
--
-- Workaround: pre-filter users with a subquery so Postgres only sorts the
-- status rows belonging to those users.

-- Slow (~1,900 ms with 5M status rows):
SELECT DISTINCT ON (user_statuses.user_id)
  users.id, users.name, user_statuses.status

thiagoa

# Making has_one work efficiently with a lateral join
#
# The naive has_one with an ordering scope loads all matching rows
# and discards all but the most recent per user in Ruby. This hack
# makes the filtering happen at the SQL level.
#
# The trick: alias the users table as user_statuses so that Rails'
# preloader filters the users table instead of the user_statuses
# table before the lateral join runs. Without the aliasing trick,
# Rails would filter on the join table's user_id, so it would need

thiagoa

RSpec.shared_examples_for "an interface" do |objects, class_methods: false|
  let(:class_methods) { class_methods }

  it "classes have compatible interfaces" do
    objects.each_cons(2) do |left, right|
      left_params = params_for(left, method(:normalize_params))
      right_params = params_for(right, method(:normalize_params))

      diff = (left_params - right_params) + (right_params - left_params)

thiagoa

### Using TestOptions
#
# TestOptions is a Minitest extension that enables setup functionality
# at the test declaration level. This allows you to declaratively
# control test behavior through options passed at the class or
# individual test level.
#
# Use TestOptions for contextual/generic behavior that is not
# essential for understanding the test's purpose, such as VCR,
# freezing time, environment variables, etc.

thiagoa

require "json"
require "faraday"

# Configure before using or execute this code as-is
# and get a 401
module Configuration
  class << self
    attr_accessor :organization_id, :api_key
  end
end

thiagoa

If your running a x64 bit Ubuntu or other Linux and find USB transfers hang at the end apply this fix:

echo $((16*1024*1024)) > /proc/sys/vm/dirty_background_bytes
echo $((48*1024*1024)) > /proc/sys/vm/dirty_bytes

I suggest you edit your /etc/rc.local file to make this change persistant across reboots.

sudo nano /etc/rc.local

thiagoa

RSpec.shared_examples_for "check interface against canonical" do |canonical_interface, classes_to_check|
  classes_to_check.each do |class_to_check|
    example "#{class_to_check} conforms to the required interface", :aggregate_failures do
      interface_methods = canonical_interface.public_instance_methods(false)

      methods_to_check = class_to_check.public_instance_methods(false)
      diff = interface_methods - methods_to_check

      expect(diff).to be_empty, (<<~MESSAGE).split("\n").join(" ")
        Expected #{class_to_check} to be polymorphic with

thiagoa

RSpec.shared_examples_for "interface checker" do
  def diff_for(array_1, array_2)
    (array_1 - array_2) + (array_2 - array_1)
  end

  def arities_for(object)
    object.public_methods.map do |m|
      arity = object.method(m).arity
      [m, arity == -1 ? anything : arity]
    end