Wrap ancestor scopes in CTE

app/models/namespaces/traversal/linear_scopes.rb

@@ -25,15 +25,20 @@ module Namespaces
         def self_and_ancestors(include_self: true, hierarchy_order: nil)
           return super unless use_traversal_ids_for_ancestor_scopes?
 
+          ancestors_cte, base_cte = ancestor_ctes
+          namespaces = Arel::Table.new(:namespaces)
+
           records = unscoped
-            .where(id: select('unnest(traversal_ids)'))
+            .with(base_cte.to_arel, ancestors_cte.to_arel)
+            .distinct
+            .from([ancestors_cte.table, namespaces])
+            .where(namespaces[:id].eq(ancestors_cte.table[:ancestor_id]))
             .order_by_depth(hierarchy_order)
-            .normal_select
 
           if include_self
             records
           else
-            records.where.not(id: all.as_ids)
+            records.where(ancestors_cte.table[:base_id].not_eq(ancestors_cte.table[:ancestor_id]))
           end
         end
 
@@ -150,6 +155,20 @@ module Namespaces
             records.where('namespaces.id <> base.id')
           end
         end
+
+        def ancestor_ctes
+          base_scope = all.select('namespaces.id', 'namespaces.traversal_ids')
+          base_cte = Gitlab::SQL::CTE.new(:base_ancestors_cte, base_scope)
+
+          # We have to alias id with 'AS' to avoid ambiguous column references by calling methods.
+          ancestors_scope = unscoped
+            .unscope(where: [:type])
+            .select('id as base_id', 'unnest(traversal_ids) as ancestor_id')
+            .from(base_cte.table)
+          ancestors_cte = Gitlab::SQL::CTE.new(:ancestors_cte, ancestors_scope)
+
+          [ancestors_cte, base_cte]
+        end
       end
     end
   end

ee/spec/lib/gitlab/group_plans_preloader_spec.rb

@@ -21,6 +21,9 @@ RSpec.describe Gitlab::GroupPlansPreloader, :saas do
 
     shared_examples 'preloading cases' do
       it 'only executes three SQL queries to preload the data' do
+        # Pre-cache `select VERSION()` query to avoid counting.
+        Gitlab::Database::AsWithMaterialized.materialized_supported?
+
         amount = ActiveRecord::QueryRecorder
           .new { preloaded_groups }
           .count

spec/support/shared_examples/namespaces/traversal_scope_examples.rb

@@ -124,6 +124,12 @@ RSpec.shared_examples 'namespace traversal scopes' do
       it { expect(subject[0, 2]).to contain_exactly(group_1, group_2) }
       it { expect(subject[2, 2]).to contain_exactly(nested_group_1, nested_group_2) }
     end
+
+    context 'with offset and limit' do
+      subject { described_class.where(id: [deep_nested_group_1, deep_nested_group_2]).offset(1).limit(1).self_and_ancestors }
+
+      it { is_expected.to contain_exactly(group_2, nested_group_2, deep_nested_group_2) }
+    end
   end
 
   describe '.self_and_ancestors' do
@@ -168,6 +174,19 @@ RSpec.shared_examples 'namespace traversal scopes' do
 
       it { is_expected.to contain_exactly(group_1.id, group_2.id) }
     end
+
+    context 'with offset and limit' do
+      subject do
+        described_class
+          .where(id: [deep_nested_group_1, deep_nested_group_2])
+          .limit(1)
+          .offset(1)
+          .self_and_ancestor_ids
+          .pluck(:id)
+      end
+
+      it { is_expected.to contain_exactly(group_2.id, nested_group_2.id, deep_nested_group_2.id) }
+    end
   end
 
   describe '.self_and_ancestor_ids' do