Improve .groups_user_can_read_epics performance

ee/app/models/ee/epic.rb

@@ -196,7 +196,13 @@ module EE
       end
 
       def groups_user_can_read_epics(epics, user)
-        groups = ::Group.where(id: epics.select(:group_id))
+        groups = if ::Feature.enabled?(:optimized_groups_user_can_read_epics_method)
+                   epics_query = epics.select(:group_id)
+                   ::Group.joins("INNER JOIN (#{epics_query.to_sql}) as epics on epics.group_id = namespaces.id")
+                 else
+                   ::Group.where(id: epics.select(:group_id))
+                 end
+
         groups = ::Gitlab::GroupPlansPreloader.new.preload(groups)
 
         DeclarativePolicy.user_scope do

ee/spec/models/epic_spec.rb

@@ -101,7 +101,7 @@ describe Epic do
     end
   end
 
-  describe '.groups_user_can_read_epics' do
+  shared_examples '.groups_user_can_read_epics examples' do
     let_it_be(:private_group) { create(:group, :private) }
     let_it_be(:epic) { create(:epic, group: private_group) }
 
@@ -141,6 +141,24 @@ describe Epic do
     end
   end
 
+  describe '.groups_user_can_read_epics' do
+    context 'with `optimized_groups_user_can_read_epics_method` feature flag enabled' do
+      before do
+        stub_feature_flags(optimized_groups_user_can_read_epics_method: true)
+      end
+
+      include_examples '.groups_user_can_read_epics examples'
+    end
+
+    context 'with `optimized_groups_user_can_read_epics_method` feature flag disabled' do
+      before do
+        stub_feature_flags(optimized_groups_user_can_read_epics_method: false)
+      end
+
+      include_examples '.groups_user_can_read_epics examples'
+    end
+  end
+
   describe '#valid_parent?' do
     context 'basic checks' do
       let(:epic) { build(:epic, group: group) }