Add a query optimisation for fetching group labels

Try a query optimisation for fetching group labels when
transfering a project.

app/services/labels/transfer_service.rb

@@ -50,22 +50,33 @@ module Labels
 
     # rubocop: disable CodeReuse/ActiveRecord
     def group_labels_applied_to_issues
-      @group_labels_applied_to_issues ||= Label.joins(:issues)
-        .where(
+      @labels_applied_to_issues ||= if use_optimized_group_labels_query?
+                                      Label.joins(:issues)
+                                        .joins("INNER JOIN namespaces on namespaces.id = labels.group_id AND namespaces.type = 'Group'" )
+                                        .where(issues: { project_id: project.id }).reorder(nil)
+                                    else
+                                      Label.joins(:issues).where(
                                         issues: { project_id: project.id },
                                         labels: { group_id: old_group.self_and_ancestors }
                                       )
                                     end
+    end
     # rubocop: enable CodeReuse/ActiveRecord
 
     # rubocop: disable CodeReuse/ActiveRecord
     def group_labels_applied_to_merge_requests
-      @group_labels_applied_to_merge_requests ||= Label.joins(:merge_requests)
+      @labels_applied_to_mrs ||= if use_optimized_group_labels_query?
+                                   Label.joins(:merge_requests)
+                                     .joins("INNER JOIN namespaces on namespaces.id = labels.group_id AND namespaces.type = 'Group'" )
+                                     .where(merge_requests: { target_project_id: project.id }).reorder(nil)
+                                 else
+                                   Label.joins(:merge_requests)
                                      .where(
                                        merge_requests: { target_project_id: project.id },
                                        labels: { group_id: old_group.self_and_ancestors }
                                      )
                                  end
+    end
     # rubocop: enable CodeReuse/ActiveRecord
 
     def find_or_create_label!(label)
@@ -88,5 +99,9 @@ module Labels
         .update_all(label_id: new_label_id)
     end
     # rubocop: enable CodeReuse/ActiveRecord
+
+    def use_optimized_group_labels_query?
+      Feature.enabled?(:use_optimized_group_labels_query, project.root_namespace, default_enabled: :yaml)
+    end
   end
 end

config/feature_flags/development/use_optimized_group_labels_query.yml

+---
+name: use_optimized_group_labels_query
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/73501
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/344957
+milestone: '14.5'
+type: development
+group: group::workspace
+default_enabled: false

spec/services/labels/transfer_service_spec.rb

@@ -3,6 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe Labels::TransferService do
+  shared_examples 'transfer labels' do
     describe '#execute' do
       let_it_be(:user) { create(:user) }
 
@@ -106,4 +107,17 @@ RSpec.describe Labels::TransferService do
         end
       end
     end
+  end
+
+  context 'with use_optimized_group_labels_query FF on' do
+    it_behaves_like 'transfer labels'
+  end
+
+  context 'with use_optimized_group_labels_query FF off' do
+    before do
+      stub_feature_flags(use_optimized_group_labels_query: false)
+    end
+
+    it_behaves_like 'transfer labels'
+  end
 end