Add effective index to subscriptions (#2885)

db/migrate/20170507141759_optimize_index_subscriptions.rb

@@ -0,0 +1,11 @@
+class OptimizeIndexSubscriptions < ActiveRecord::Migration[5.0]
+  def up
+    add_index :subscriptions, [:account_id, :callback_url], unique: true
+    remove_index :subscriptions, [:callback_url, :account_id]
+  end
+
+  def down
+    add_index :subscriptions, [:callback_url, :account_id], unique: true
+    remove_index :subscriptions, [:account_id, :callback_url]
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20170425202925) do
+ActiveRecord::Schema.define(version: 20170507141759) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -288,7 +288,7 @@ ActiveRecord::Schema.define(version: 20170425202925) do
     t.datetime "created_at",                                  null: false
     t.datetime "updated_at",                                  null: false
     t.datetime "last_successful_delivery_at"
-    t.index ["callback_url", "account_id"], name: "index_subscriptions_on_callback_url_and_account_id", unique: true, using: :btree
+    t.index ["account_id", "callback_url"], name: "index_subscriptions_on_account_id_and_callback_url", unique: true, using: :btree
   end
 
   create_table "tags", force: :cascade do |t|