Use CTE optimization fence for loading projects in dashboard

Certain users experienced Error 500s when loading projects from the
dashboard because the PostgreSQL query planner attempted to scan all
projects rather than just the user's authorized projects. This would
cause the query to hit a statement timeout.

To fix this, we add support for a CTE optimization fence to load
authorized projects first, which can be optionally used by
`ProjectsFinder` via the `use_cte` parameter. To be safe, we only enable
it for the finder call that loads the list of projects behind the
`use_cte_for_projects_finder` feature flag.

Closes https://gitlab.com/gitlab-org/gitlab/issues/198440

app/controllers/dashboard/projects_controller.rb

@@ -66,6 +66,8 @@ class Dashboard::ProjectsController < Dashboard::ApplicationController
     @total_user_projects_count = ProjectsFinder.new(params: { non_public: true }, current_user: current_user).execute
     @total_starred_projects_count = ProjectsFinder.new(params: { starred: true }, current_user: current_user).execute
 
+    finder_params[:use_cte] = Feature.enabled?(:use_cte_for_projects_finder, default_enabled: true)
+
     projects = ProjectsFinder
                 .new(params: finder_params, current_user: current_user)
                 .execute

app/finders/projects_finder.rb

@@ -44,6 +44,8 @@ class ProjectsFinder < UnionFinder
         init_collection
       end
 
+    use_cte = params.delete(:use_cte)
+    collection = Project.wrap_authorized_projects_with_cte(collection) if use_cte
     collection = filter_projects(collection)
     sort(collection)
   end
@@ -177,7 +179,7 @@ class ProjectsFinder < UnionFinder
   end
 
   def sort(items)
-    params[:sort].present? ? items.sort_by_attribute(params[:sort]) : items.order_id_desc
+    params[:sort].present? ? items.sort_by_attribute(params[:sort]) : items.projects_order_id_desc
   end
 
   def by_archived(projects)

app/models/project.rb

@@ -397,6 +397,8 @@ class Project < ApplicationRecord
   scope :sorted_by_stars_desc, -> { reorder(star_count: :desc) }
   scope :sorted_by_stars_asc, -> { reorder(star_count: :asc) }
   scope :sorted_by_name_asc_limited, ->(limit) { reorder(name: :asc).limit(limit) }
+  # Sometimes queries (e.g. using CTEs) require explicit disambiguation with table name
+  scope :projects_order_id_desc, -> { reorder("#{table_name}.id DESC") }
 
   scope :in_namespace, ->(namespace_ids) { where(namespace_id: namespace_ids) }
   scope :personal, ->(user) { where(namespace_id: user.namespace_id) }
@@ -543,6 +545,11 @@ class Project < ApplicationRecord
       )
   end
 
+  def self.wrap_authorized_projects_with_cte(collection)
+    cte = Gitlab::SQL::CTE.new(:authorized_projects, collection)
+    Project.with(cte.to_arel).from(cte.alias_to(Project.arel_table))
+  end
+
   scope :active, -> { joins(:issues, :notes, :merge_requests).order('issues.created_at, notes.created_at, merge_requests.created_at DESC') }
   scope :abandoned, -> { where('projects.last_activity_at < ?', 6.months.ago) }
 

changelogs/unreleased/sh-use-cte-for-projects-finder.yml

+---
+title: Use CTE optimization fence for loading projects in dashboard
+merge_request: 23754
+author:
+type: performance

spec/finders/projects_finder_spec.rb

@@ -28,10 +28,12 @@ describe ProjectsFinder, :do_not_mock_admin_mode do
     let(:params) { {} }
     let(:current_user) { user }
     let(:project_ids_relation) { nil }
-    let(:finder) { described_class.new(params: params, current_user: current_user, project_ids_relation: project_ids_relation) }
+    let(:use_cte) { true }
+    let(:finder) { described_class.new(params: params.merge(use_cte: use_cte), current_user: current_user, project_ids_relation: project_ids_relation) }
 
     subject { finder.execute }
 
+    shared_examples 'ProjectFinder#execute examples' do
       describe 'without a user' do
         let(:current_user) { nil }
 
@@ -202,6 +204,8 @@ describe ProjectsFinder, :do_not_mock_admin_mode do
           current_user.toggle_star(private_project)
 
           is_expected.to eq([public_project])
+          expect(subject.count).to eq(1)
+          expect(subject.limit(1000).count).to eq(1)
         end
       end
 
@@ -234,4 +238,17 @@ describe ProjectsFinder, :do_not_mock_admin_mode do
         end
       end
     end
+
+    describe 'without CTE flag enabled' do
+      let(:use_cte) { false }
+
+      it_behaves_like 'ProjectFinder#execute examples'
+    end
+
+    describe 'with CTE flag enabled' do
+      let(:use_cte) { true }
+
+      it_behaves_like 'ProjectFinder#execute examples'
+    end
+  end
 end

spec/models/project_spec.rb

@@ -3780,6 +3780,25 @@ describe Project do
     end
   end
 
+  describe '.wrap_authorized_projects_with_cte' do
+    let!(:user) { create(:user) }
+
+    let!(:private_project) do
+      create(:project, :private, creator: user, namespace: user.namespace)
+    end
+
+    let!(:public_project) { create(:project, :public) }
+
+    let(:projects) { described_class.all.public_or_visible_to_user(user) }
+
+    subject { described_class.wrap_authorized_projects_with_cte(projects) }
+
+    it 'wrapped query matches original' do
+      expect(subject.to_sql).to match(/^WITH "authorized_projects" AS/)
+      expect(subject).to match_array(projects)
+    end
+  end
+
   describe '#pages_available?' do
     let(:project) { create(:project, group: group) }