Optimize /admin/applications so that it does not timeout

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

1 files changed, 28 insertions, 0 deletions

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