discourse/db/migrate/20121123054127_make_post_number_distinct.rb

class MakePostNumberDistinct < ActiveRecord::Migration
  def up

    Topic.exec_sql('update posts p
set post_number = calc
from
(
	select
		id,
		post_number,
		topic_id,
		row_number() over (partition by topic_id order by post_number, created_at) calc
	from posts
	where topic_id in (
	select topic_id from posts
	  group by topic_id, post_number
	  having count(*)>1
	)

) as X
where calc <> p.post_number and X.id = p.id')

    # automagically does this
    unless rails4?
      remove_index :posts, :forum_thread_id_and_post_number
      add_index :posts, [:topic_id, :post_number], unique: true
    end
  end

  def down
    # don't want to mess with the index ... its annoying
    raise ActiveRecord::IrreversibleMigration
  end
end