Use linear version of groups_including_descendants_by

Changelog: performance

app/models/group.rb

@@ -823,10 +823,16 @@ class Group < Namespace
   end
 
   def self.groups_including_descendants_by(group_ids)
+    groups = Group.where(id: group_ids)
+
+    if Feature.enabled?(:linear_group_including_descendants_by, default_enabled: :yaml)
+      groups.self_and_descendants
+    else
       Gitlab::ObjectHierarchy
-      .new(Group.where(id: group_ids))
+      .new(groups)
       .base_and_descendants
     end
+  end
 
   def disable_shared_runners!
     update!(

config/feature_flags/development/linear_group_including_descendants_by.yml

+---
+name: linear_group_including_descendants_by
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68835
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339431
+milestone: '14.3'
+type: development
+group: group::access
+default_enabled: false

spec/models/group_spec.rb

@@ -2274,19 +2274,27 @@ RSpec.describe Group do
   end
 
   describe '.groups_including_descendants_by' do
-    it 'returns the expected groups for a group and its descendants' do
-      parent_group1 = create(:group)
-      child_group1 = create(:group, parent: parent_group1)
-      child_group2 = create(:group, parent: parent_group1)
+    let_it_be(:parent_group1) { create(:group) }
+    let_it_be(:parent_group2) { create(:group) }
+    let_it_be(:extra_group)   { create(:group) }
+    let_it_be(:child_group1)  { create(:group, parent: parent_group1) }
+    let_it_be(:child_group2)  { create(:group, parent: parent_group1) }
+    let_it_be(:child_group3)  { create(:group, parent: parent_group2) }
 
-      parent_group2 = create(:group)
-      child_group3 = create(:group, parent: parent_group2)
+    subject { described_class.groups_including_descendants_by([parent_group2.id, parent_group1.id]) }
 
-      create(:group)
+    shared_examples 'returns the expected groups for a group and its descendants' do
+      specify { is_expected.to contain_exactly(parent_group1, parent_group2, child_group1, child_group2, child_group3) }
+    end
+
+    it_behaves_like 'returns the expected groups for a group and its descendants'
 
-      groups = described_class.groups_including_descendants_by([parent_group2.id, parent_group1.id])
+    context 'when :linear_group_including_descendants_by feature flag is disabled' do
+      before do
+        stub_feature_flags(linear_group_including_descendants_by: false)
+      end
 
-      expect(groups).to contain_exactly(parent_group1, parent_group2, child_group1, child_group2, child_group3)
+      it_behaves_like 'returns the expected groups for a group and its descendants'
     end
   end