Optimize /admin/applications so that it does not timeoutsh-fix-oauth-application-page

On our dev instance, /admin/applications as not loading because:

1. There was an unindexed query by `application_id`.
2. There was an expensive query that attempted to load 1 million
   unique entries via ActiveRecord just to find the unique count.

We fix the first issue by adding an index for that column.

We fix the second issue with a simple SELECT COUNT(DISTINCT
resource_owner_id) SQL query.

In addition, we add pagination to avoid loading more than 20
applications at once.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/67228

8 files changed, 69 insertions, 3 deletions

diff --git a/app/controllers/admin/applications_controller.rb b/app/controllers/admin/applications_controller.rb
index 3648c8be426..22e629ccf59 100644
--- a/app/controllers/admin/applications_controller.rb
+++ b/app/controllers/admin/applications_controller.rb
@@ -7,7 +7,9 @@ class Admin::ApplicationsController < Admin::ApplicationController
   before_action :load_scopes, only: [:new, :create, :edit, :update]
 
   def index
-    @applications = ApplicationsFinder.new.execute
+    applications = ApplicationsFinder.new.execute
+    @applications = Kaminari.paginate_array(applications).page(params[:page])
+    @application_counts = OauthAccessToken.distinct_resource_owner_counts(@applications)
   end
 
   def show
diff --git a/app/models/oauth_access_token.rb b/app/models/oauth_access_token.rb
index 0aa920fa828..9789d8ed62b 100644
--- a/app/models/oauth_access_token.rb
+++ b/app/models/oauth_access_token.rb
@@ -6,6 +6,8 @@ class OauthAccessToken < Doorkeeper::AccessToken
 
   alias_attribute :user, :resource_owner
 
+  scope :distinct_resource_owner_counts, ->(applications) { where(application: applications).distinct.group(:application_id).count(:resource_owner_id) }
+
   def scopes=(value)
     if value.is_a?(Array)
       super(Doorkeeper::OAuth::Scopes.from_array(value).to_s)
diff --git a/app/views/admin/applications/index.html.haml b/app/views/admin/applications/index.html.haml
index 2cdf98075d1..758d722cc63 100644
--- a/app/views/admin/applications/index.html.haml
+++ b/app/views/admin/applications/index.html.haml
@@ -19,7 +19,8 @@
       %tr{ :id => "application_#{application.id}" }
         %td= link_to application.name, admin_application_path(application)
         %td= application.redirect_uri
-        %td= application.access_tokens.map(&:resource_owner_id).uniq.count
+        %td= @application_counts[application.id].to_i
         %td= application.trusted? ? 'Y': 'N'
         %td= link_to 'Edit', edit_admin_application_path(application), class: 'btn btn-link'
         %td= render 'delete_form', application: application
+= paginate @applications, theme: 'gitlab'
diff --git a/changelogs/unreleased/sh-fix-oauth-application-page.yml b/changelogs/unreleased/sh-fix-oauth-application-page.yml
new file mode 100644
index 00000000000..9ac78db2c79
--- /dev/null
+++ b/changelogs/unreleased/sh-fix-oauth-application-page.yml
@@ -0,0 +1,5 @@
+---
+title: Optimize /admin/applications so that it does not timeout
+merge_request: 32852
+author:
+type: performance
diff --git a/db/post_migrate/20190910000130_add_index_on_application_id_on_oauth_access_tokens.rb b/db/post_migrate/20190910000130_add_index_on_application_id_on_oauth_access_tokens.rb
new file mode 100644
index 00000000000..78f7c0ecf0f
--- /dev/null
+++ b/db/post_migrate/20190910000130_add_index_on_application_id_on_oauth_access_tokens.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class AddIndexOnApplicationIdOnOauthAccessTokens < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :oauth_access_tokens, :application_id
+  end
+
+  def down
+    remove_concurrent_index :oauth_access_tokens, :application_id
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 6ddfb8bcb39..342e3a8d623 100644
--- a/db/schema.rb
+++ b/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: 2019_09_05_223900) do
+ActiveRecord::Schema.define(version: 2019_09_10_000130) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -2390,6 +2390,7 @@ ActiveRecord::Schema.define(version: 2019_09_05_223900) do
     t.datetime "revoked_at"
     t.datetime "created_at", null: false
     t.string "scopes"
+    t.index ["application_id"], name: "index_oauth_access_tokens_on_application_id"
     t.index ["refresh_token"], name: "index_oauth_access_tokens_on_refresh_token", unique: true
     t.index ["resource_owner_id"], name: "index_oauth_access_tokens_on_resource_owner_id"
     t.index ["token"], name: "index_oauth_access_tokens_on_token", unique: true
diff --git a/spec/controllers/admin/applications_controller_spec.rb b/spec/controllers/admin/applications_controller_spec.rb
index 9c9f658a0bd..2f3c7da484b 100644
--- a/spec/controllers/admin/applications_controller_spec.rb
+++ b/spec/controllers/admin/applications_controller_spec.rb
@@ -10,6 +10,16 @@ describe Admin::ApplicationsController do
     sign_in(admin)
   end
 
+  describe 'GET #index' do
+    render_views
+
+    it 'renders the application form' do
+      get :index
+
+      expect(response).to have_http_status(200)
+    end
+  end
+
   describe 'GET #new' do
     it 'renders the application form' do
       get :new
diff --git a/spec/models/oauth_access_token_spec.rb b/spec/models/oauth_access_token_spec.rb
new file mode 100644
index 00000000000..0a1c576a5e7
--- /dev/null
+++ b/spec/models/oauth_access_token_spec.rb
@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe OauthAccessToken do
+  let(:user) { create(:user) }
+  let(:app_one) { create(:oauth_application) }
+  let(:app_two) { create(:oauth_application) }
+  let(:app_three) { create(:oauth_application) }
+  let(:tokens) { described_class.all }
+
+  before do
+    create(:oauth_access_token, application_id: app_one.id)
+    create_list(:oauth_access_token, 2, resource_owner: user, application_id: app_two.id)
+  end
+
+  it 'returns unique owners' do
+    expect(tokens.count).to eq(3)
+    expect(tokens.distinct_resource_owner_counts([app_one])).to eq({ app_one.id => 1 })
+    expect(tokens.distinct_resource_owner_counts([app_two])).to eq({ app_two.id => 1 })
+    expect(tokens.distinct_resource_owner_counts([app_three])).to eq({})
+    expect(tokens.distinct_resource_owner_counts([app_one, app_two]))
+      .to eq({
+               app_one.id => 1,
+               app_two.id => 1
+             })
+  end
+end