Linear traversal query for Namespace#descendants

Linear queries have better performance than recursive CTEs

Changelog: added

app/models/namespaces/traversal/linear.rb

@@ -63,6 +63,12 @@ module Namespaces
         lineage(top: self)
       end
 
+      def descendants
+        return super unless use_traversal_ids?
+
+        self_and_descendants.where.not(id: id)
+      end
+
       def ancestors(hierarchy_order: nil)
         return super() unless use_traversal_ids?
         return super() unless Feature.enabled?(:use_traversal_ids_for_ancestors, root_ancestor, default_enabled: :yaml)

changelogs/unreleased/324747-linear_descendants.yml

+---
+title: Linear traversal query for Namespace#descendants
+merge_request: 59632
+author:
+type: performance

spec/models/group_spec.rb

@@ -441,6 +441,10 @@ RSpec.describe Group do
         it { expect(group.self_and_descendants.to_sql).not_to include 'traversal_ids @>' }
       end
 
+      describe '#descendants' do
+        it { expect(group.descendants.to_sql).not_to include 'traversal_ids @>' }
+      end
+
       describe '#ancestors' do
         it { expect(group.ancestors.to_sql).not_to include 'traversal_ids <@' }
       end
@@ -453,6 +457,10 @@ RSpec.describe Group do
         it { expect(group.self_and_descendants.to_sql).to include 'traversal_ids @>' }
       end
 
+      describe '#descendants' do
+        it { expect(group.descendants.to_sql).to include 'traversal_ids @>' }
+      end
+
       describe '#ancestors' do
         it { expect(group.ancestors.to_sql).to include "\"namespaces\".\"id\" = #{group.parent_id}" }