Move status counters to separate table, count replies (#8104)

* Move status counters to separate table, count replies

* Migration to remove old counter columns from statuses table

* Fix schema file

db/migrate/20180812162710_create_status_stats.rb

@@ -0,0 +1,12 @@
+class CreateStatusStats < ActiveRecord::Migration[5.2]
+  def change
+    create_table :status_stats do |t|
+      t.belongs_to :status, null: false, foreign_key: { on_delete: :cascade }, index: { unique: true }
+      t.bigint :replies_count, null: false, default: 0
+      t.bigint :reblogs_count, null: false, default: 0
+      t.bigint :favourites_count, null: false, default: 0
+
+      t.timestamps
+    end
+  end
+end

db/migrate/20180812173710_copy_status_stats.rb

@@ -0,0 +1,19 @@
+class CopyStatusStats < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    safety_assured do
+      execute <<-SQL.squish
+        INSERT INTO status_stats (status_id, reblogs_count, favourites_count)
+        SELECT id, reblogs_count, favourites_count
+        FROM statuses
+        ON CONFLICT (status_id) DO UPDATE
+        SET reblogs_count = EXCLUDED.reblogs_count, favourites_count = EXCLUDED.favourites_count
+      SQL
+    end
+  end
+
+  def down
+    # Nothing
+  end
+end