Move recursive namespace query to INNER JOIN

Move recursive namespace query to INNER JOIN

app/models/namespace.rb

@@ -260,8 +260,13 @@ class Namespace < ApplicationRecord
   # Includes projects from this namespace and projects from all subgroups
   # that belongs to this namespace
   def all_projects
-    namespace = user? ? self : self_and_descendants
-    Project.where(namespace: namespace)
+    return Project.where(namespace: self) if user?
+
+    if Feature.enabled?(:recursive_namespace_lookup_as_inner_join, self)
+      Project.joins("INNER JOIN (#{self_and_descendants.select(:id).to_sql}) namespaces ON namespaces.id=projects.namespace_id")
+    else
+      Project.where(namespace: self_and_descendants)
+    end
   end
 
   # Includes pipelines from this namespace and pipelines from all subgroups

config/feature_flags/development/recursive_namespace_lookup_as_inner_join.yml

+---
+name: recursive_namespace_lookup_as_inner_join
+introduced_by_url:
+rollout_issue_url:
+milestone: '13.10'
+type: development
+group: group::optimize
+default_enabled: false

spec/models/namespace_spec.rb

@@ -897,10 +897,24 @@ RSpec.describe Namespace do
       it { expect(namespace.all_projects.to_a).to match_array([project2, project1]) }
       it { expect(child.all_projects.to_a).to match_array([project2]) }
 
+      context 'when recursive_namespace_lookup_as_inner_join feature flag is on' do
+        before do
+          stub_feature_flags(recursive_namespace_lookup_as_inner_join: true)
+        end
+
         it 'queries for the namespace and its descendants' do
-        expect(Project).to receive(:where).with(namespace: [namespace, child])
+          expect(namespace.all_projects).to match_array([project1, project2])
+        end
+      end
 
-        namespace.all_projects
+      context 'when recursive_namespace_lookup_as_inner_join feature flag is off' do
+        before do
+          stub_feature_flags(recursive_namespace_lookup_as_inner_join: false)
+        end
+
+        it 'queries for the namespace and its descendants' do
+          expect(namespace.all_projects).to match_array([project1, project2])
+        end
       end
     end