diff --git a/db/migrate/20140715013018_correct_post_number_index.rb b/db/migrate/20140715013018_correct_post_number_index.rb
new file mode 100644
index 000000000..5aeb03f7a
--- /dev/null
+++ b/db/migrate/20140715013018_correct_post_number_index.rb
@@ -0,0 +1,26 @@
+class CorrectPostNumberIndex < ActiveRecord::Migration
+  def change
+
+    begin
+      a = execute <<SQL
+      UPDATE posts SET post_number = post_number + 1
+      WHERE id IN (
+        SELECT p1.id
+        FROM posts p1
+        JOIN
+        (
+          SELECT post_number, topic_id, min(id) min_id
+          FROM posts
+          GROUP BY post_number, topic_id
+          HAVING COUNT(*) > 1
+        ) pp ON p1.topic_id = pp.topic_id AND
+                p1.post_number >= pp.post_number AND
+                p1.id <> pp.min_id
+      )
+SQL
+    end until a.cmdtuples == 0
+
+    remove_index :posts, [:topic_id, :post_number]
+    add_index :posts, [:topic_id, :post_number], unique: true
+  end
+end