Optimize page loading of Admin::RunnersController#show

This commit optimizes the page loading of this page in several ways:

1. Remove exact counting of projects on the page. By default, a `SELECT
COUNT(*) FROM projects` would be called, which could take seconds on an
instance with many projects.

2. Preload project/namespace/routes where necessary. This reduces a
significant number of N+1 queries.

3. Remove the use of `becomes(Namespace)` because this causes a SQL
query each time it is called. Instead, use the Rails path helpers to
generate the path directly via `project.namespace`.

Closes https://gitlab.com/gitlab-org/gitlab/issues/119093

app/controllers/admin/runners_controller.rb

@@ -66,7 +66,7 @@ class Admin::RunnersController < Admin::ApplicationController
 
   # rubocop: disable CodeReuse/ActiveRecord
   def assign_builds_and_projects
-    @builds = runner.builds.order('id DESC').first(30)
+    @builds = runner.builds.order('id DESC').preload_project_and_pipeline_project.first(30)
     @projects =
       if params[:search].present?
         ::Project.search(params[:search])
@@ -75,7 +75,8 @@ class Admin::RunnersController < Admin::ApplicationController
       end
 
     @projects = @projects.where.not(id: runner.projects.select(:id)) if runner.projects.any?
-    @projects = @projects.page(params[:page]).per(30)
+    @projects = @projects.inc_routes
+    @projects = @projects.page(params[:page]).per(30).without_count
   end
   # rubocop: enable CodeReuse/ActiveRecord
 end

app/models/ci/build.rb

@@ -172,6 +172,9 @@ module Ci
     scope :queued_before, ->(time) { where(arel_table[:queued_at].lt(time)) }
     scope :order_id_desc, -> { order('ci_builds.id DESC') }
 
+    PROJECT_ROUTE_AND_NAMESPACE_ROUTE = { project: [:project_feature, :route, { namespace: :route }] }.freeze
+    scope :preload_project_and_pipeline_project, -> { preload(PROJECT_ROUTE_AND_NAMESPACE_ROUTE, pipeline: PROJECT_ROUTE_AND_NAMESPACE_ROUTE) }
+
     acts_as_taggable
 
     add_authentication_token_field :token, encrypted: :optional

app/views/admin/runners/show.html.haml

@@ -48,7 +48,7 @@
                   = project.full_name
               %td
                 .float-right
-                  = link_to 'Disable', [:admin, project.namespace.becomes(Namespace), project, runner_project], method: :delete, class: 'btn btn-danger btn-sm'
+                  = link_to 'Disable', admin_namespace_project_runner_project_path(project.namespace, project, runner_project), method: :delete, class: 'btn btn-danger btn-sm'
 
     %table.table.unassigned-projects
       %thead
@@ -70,10 +70,10 @@
             = project.full_name
           %td
             .float-right
-              = form_for [:admin, project.namespace.becomes(Namespace), project, project.runner_projects.new] do |f|
+              = form_for project.runner_projects.new, url: admin_namespace_project_runner_projects_path(project.namespace, project), method: :post do |f|
                 = f.hidden_field :runner_id, value: @runner.id
                 = f.submit 'Enable', class: 'btn btn-sm'
-    = paginate @projects, theme: "gitlab"
+    = paginate_without_count @projects
 
   .col-md-6
     %h4 Recent jobs served by this Runner

changelogs/unreleased/sh-optimize-runners-admin-show.yml

+---
+title: Optimize page loading of Admin::RunnersController#show
+merge_request: 23309
+author:
+type: performance

spec/controllers/admin/runners_controller_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 
 describe Admin::RunnersController do
-  let!(:runner) { create(:ci_runner) }
+  let_it_be(:runner) { create(:ci_runner) }
 
   before do
     sign_in(create(:admin))
@@ -36,6 +36,16 @@ describe Admin::RunnersController do
   end
 
   describe '#show' do
+    render_views
+
+    let_it_be(:project) { create(:project) }
+    let_it_be(:project_two) { create(:project) }
+
+    before_all do
+      create(:ci_build, runner: runner, project: project)
+      create(:ci_build, runner: runner, project: project_two)
+    end
+
     it 'shows a particular runner' do
       get :show, params: { id: runner.id }
 
@@ -47,6 +57,21 @@ describe Admin::RunnersController do
 
       expect(response).to have_gitlab_http_status(404)
     end
+
+    it 'avoids N+1 queries', :request_store do
+      get :show, params: { id: runner.id }
+
+      control_count = ActiveRecord::QueryRecorder.new { get :show, params: { id: runner.id } }.count
+
+      new_project = create(:project)
+      create(:ci_build, runner: runner, project: new_project)
+
+      # There is one additional query looking up subject.group in ProjectPolicy for the
+      # needs_new_sso_session permission
+      expect { get :show, params: { id: runner.id } }.not_to exceed_query_limit(control_count + 1)
+
+      expect(response).to have_gitlab_http_status(200)
+    end
   end
 
   describe '#update' do