Optimize use_elasticsearch? call

This MR optimizes the underlying query of the `user_elasticsearch?`
method. The change is behind a default enabled feature flag:
`optimized_elasticsearch_indexes_project`

ee/app/models/ee/application_setting.rb

@@ -135,10 +135,10 @@ module EE
       return false unless elasticsearch_indexing?
       return true unless elasticsearch_limit_indexing?
 
-      return elasticsearch_limited_projects.exists?(project.id) unless ::Feature.enabled?(:elasticsearch_indexes_project_cache, default_enabled: true)
+      return optimized_elasticsearch_indexes_project?(project) unless ::Feature.enabled?(:elasticsearch_indexes_project_cache, default_enabled: true)
 
       ::Gitlab::Elastic::ElasticsearchEnabledCache.fetch(:project, project.id) do
-        elasticsearch_limited_projects.exists?(project.id)
+        optimized_elasticsearch_indexes_project?(project)
       end
     end
 
@@ -290,6 +290,26 @@ module EE
 
     private
 
+    def optimized_elasticsearch_indexes_project?(project)
+      if ::Feature.enabled?(:optimized_elasticsearch_indexes_project, default_enabled: true)
+        indexed_namespaces = ::Gitlab::ObjectHierarchy
+          .new(::Namespace.where(id: project.namespace_id))
+          .base_and_ancestors
+          .joins(:elasticsearch_indexed_namespace)
+
+        indexed_namespaces = ::Project.where('EXISTS (?)', indexed_namespaces)
+        indexed_projects = ::Project.where('EXISTS (?)', ElasticsearchIndexedProject.where(project_id: project.id))
+
+        ::Project
+          .from("(SELECT) as projects") # SELECT from "nothing" since the EXISTS queries have all the conditions.
+          .merge(indexed_namespaces.or(indexed_projects))
+          .exists?
+      else
+        # old behavior
+        elasticsearch_limited_projects.exists?(project.id)
+      end
+    end
+
     def update_personal_access_tokens_lifetime
       return unless max_personal_access_token_lifetime.present? && License.feature_available?(:personal_access_token_expiration_policy)
 

ee/app/models/ee/namespace.rb

@@ -29,6 +29,7 @@ module EE
 
       has_one :namespace_statistics
       has_one :gitlab_subscription, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
+      has_one :elasticsearch_indexed_namespace
 
       accepts_nested_attributes_for :gitlab_subscription
 

ee/spec/models/application_setting_spec.rb

@@ -289,6 +289,53 @@ describe ApplicationSetting do
           expect(setting.elasticsearch_limited_namespaces(true)).to match_array(
             [namespaces.last, child_namespace])
         end
+
+        describe '#elasticsearch_indexes_project?' do
+          shared_examples 'examples for #elasticsearch_indexes_project?' do
+            context 'when project is in a subgroup' do
+              let(:root_group) { create(:group) }
+              let(:subgroup) { create(:group, parent: root_group) }
+              let(:project) { create(:project, group: subgroup) }
+
+              before do
+                create(:elasticsearch_indexed_namespace, namespace: root_group)
+              end
+
+              it 'allows project to be indexed' do
+                expect(setting.elasticsearch_indexes_project?(project)).to be(true)
+              end
+            end
+
+            context 'when project is in a namespace' do
+              let(:namespace) { create(:namespace) }
+              let(:project) { create(:project, namespace: namespace) }
+
+              before do
+                create(:elasticsearch_indexed_namespace, namespace: namespace)
+              end
+
+              it 'allows project to be indexed' do
+                expect(setting.elasticsearch_indexes_project?(project)).to be(true)
+              end
+            end
+          end
+
+          context 'when optimized_elasticsearch_indexes_project feature flag is on' do
+            before do
+              stub_feature_flags(optimized_elasticsearch_indexes_project: true)
+            end
+
+            include_examples 'examples for #elasticsearch_indexes_project?'
+          end
+
+          context 'when optimized_elasticsearch_indexes_project feature flag is off' do
+            before do
+              stub_feature_flags(optimized_elasticsearch_indexes_project: false)
+            end
+
+            include_examples 'examples for #elasticsearch_indexes_project?'
+          end
+        end
       end
 
       context 'projects' do

ee/spec/models/ee/namespace_spec.rb

@@ -14,6 +14,7 @@ describe Namespace do
 
   it { is_expected.to have_one(:namespace_statistics) }
   it { is_expected.to have_one(:gitlab_subscription).dependent(:destroy) }
+  it { is_expected.to have_one(:elasticsearch_indexed_namespace) }
 
   it { is_expected.to delegate_method(:extra_shared_runners_minutes).to(:namespace_statistics) }
   it { is_expected.to delegate_method(:shared_runners_minutes).to(:namespace_statistics) }