Introduce linear descendants feature flag

app/models/namespaces/traversal/linear_scopes.rb

@@ -53,7 +53,7 @@ module Namespaces
         end
 
         def self_and_descendants(include_self: true)
-          return super unless use_traversal_ids?
+          return super unless use_traversal_ids_for_descendants_scopes?
 
           if Feature.enabled?(:traversal_ids_btree, default_enabled: :yaml)
             self_and_descendants_with_comparison_operators(include_self: include_self)
@@ -65,7 +65,7 @@ module Namespaces
         end
 
         def self_and_descendant_ids(include_self: true)
-          return super unless use_traversal_ids?
+          return super unless use_traversal_ids_for_descendants_scopes?
 
           if Feature.enabled?(:traversal_ids_btree, default_enabled: :yaml)
             self_and_descendants_with_comparison_operators(include_self: include_self).as_ids
@@ -109,6 +109,11 @@ module Namespaces
           use_traversal_ids?
         end
 
+        def use_traversal_ids_for_descendants_scopes?
+          Feature.enabled?(:use_traversal_ids_for_descendants_scopes, default_enabled: :yaml) &&
+          use_traversal_ids?
+        end
+
         def self_and_descendants_with_comparison_operators(include_self: true)
           base = all.select(
             :traversal_ids,

config/feature_flags/development/use_traversal_ids_for_descendants_scopes.yml

+---
+name: use_traversal_ids_for_descendants_scopes
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/78542
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/350637
+milestone: '14.8'
+type: development
+group: group::authentication and authorization
+default_enabled: true

spec/models/namespace_spec.rb

@@ -361,6 +361,66 @@ RSpec.describe Namespace do
     context 'linear' do
       it_behaves_like 'namespace traversal scopes'
     end
+
+    shared_examples 'makes recursive queries' do
+      specify do
+        expect { subject }.to make_queries_matching(/WITH RECURSIVE/)
+      end
+    end
+
+    shared_examples 'does not make recursive queries' do
+      specify do
+        expect { subject }.not_to make_queries_matching(/WITH RECURSIVE/)
+      end
+    end
+
+    describe '.self_and_descendants' do
+      let_it_be(:namespace) { create(:namespace) }
+
+      subject { described_class.where(id: namespace).self_and_descendants.load }
+
+      it_behaves_like 'does not make recursive queries'
+
+      context 'when feature flag :use_traversal_ids is disabled' do
+        before do
+          stub_feature_flags(use_traversal_ids: false)
+        end
+
+        it_behaves_like 'makes recursive queries'
+      end
+
+      context 'when feature flag :use_traversal_ids_for_descendants_scopes is disabled' do
+        before do
+          stub_feature_flags(use_traversal_ids_for_descendants_scopes: false)
+        end
+
+        it_behaves_like 'makes recursive queries'
+      end
+    end
+
+    describe '.self_and_descendant_ids' do
+      let_it_be(:namespace) { create(:namespace) }
+
+      subject { described_class.where(id: namespace).self_and_descendant_ids.load }
+
+      it_behaves_like 'does not make recursive queries'
+
+      context 'when feature flag :use_traversal_ids is disabled' do
+        before do
+          stub_feature_flags(use_traversal_ids: false)
+        end
+
+        it_behaves_like 'makes recursive queries'
+      end
+
+      context 'when feature flag :use_traversal_ids_for_descendants_scopes is disabled' do
+        before do
+          stub_feature_flags(use_traversal_ids_for_descendants_scopes: false)
+        end
+
+        it_behaves_like 'makes recursive queries'
+      end
+    end
   end
 
   context 'traversal_ids on create' do