discourse/db/migrate/20170524182846_add_unread_tracking_columns.rb

class AddUnreadTrackingColumns < ActiveRecord::Migration
  def up
    # no op, no need to create all data, next migration will delete it
  end

  def down
    raise "Can not be reverted"
  end
end
PERF: improve performance of unread queries Figuring out what unread topics a user has is a very expensive operation over time. Users can easily accumulate 10s of thousands of tracking state rows (1 for every topic they ever visit) When figuring out what a user has that is unread we need to join the tracking state records to the topic table. This can very quickly lead to cases where you need to scan through the entire topic table. This commit optimises it so we always keep track of the "first" date a user has unread topics. Then we can easily filter out all earlier topics from the join. We use pg functions, instead of nested queries here to assist the planner. 2017-05-26 03:07:12 +08:00			`class AddUnreadTrackingColumns < ActiveRecord::Migration`
			`def up`
Revert unread optimisation, has too many edge cases 2017-05-26 21:04:13 +08:00			`# no op, no need to create all data, next migration will delete it`
PERF: improve performance of unread queries Figuring out what unread topics a user has is a very expensive operation over time. Users can easily accumulate 10s of thousands of tracking state rows (1 for every topic they ever visit) When figuring out what a user has that is unread we need to join the tracking state records to the topic table. This can very quickly lead to cases where you need to scan through the entire topic table. This commit optimises it so we always keep track of the "first" date a user has unread topics. Then we can easily filter out all earlier topics from the join. We use pg functions, instead of nested queries here to assist the planner. 2017-05-26 03:07:12 +08:00			`end`

			`def down`
Revert unread optimisation, has too many edge cases 2017-05-26 21:04:13 +08:00			`raise "Can not be reverted"`
PERF: improve performance of unread queries Figuring out what unread topics a user has is a very expensive operation over time. Users can easily accumulate 10s of thousands of tracking state rows (1 for every topic they ever visit) When figuring out what a user has that is unread we need to join the tracking state records to the topic table. This can very quickly lead to cases where you need to scan through the entire topic table. This commit optimises it so we always keep track of the "first" date a user has unread topics. Then we can easily filter out all earlier topics from the join. We use pg functions, instead of nested queries here to assist the planner. 2017-05-26 03:07:12 +08:00			`end`
			`end`