Significantly cut memory usage and SQL queries when reloading diffs

By preloading certain models with the diff, we can eliminate many N+1
queries. For a push to the staging GitLab.com www-gitlab-com repository,
this eliminates over 3000 SQL queries and appears to bring down the RSS
usage by several gigabytes.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/49703

1 files changed, 11 insertions, 0 deletions

diff --git a/spec/services/merge_requests/reload_diffs_service_spec.rb b/spec/services/merge_requests/reload_diffs_service_spec.rb
index 21f369a3818..546c9f277c5 100644
--- a/spec/services/merge_requests/reload_diffs_service_spec.rb
+++ b/spec/services/merge_requests/reload_diffs_service_spec.rb
@@ -60,6 +60,17 @@ describe MergeRequests::ReloadDiffsService, :use_clean_rails_memory_store_cachin
 
         subject.execute
       end
+
+      it 'avoids N+1 queries', :request_store do
+        current_user
+        merge_request
+
+        control_count = ActiveRecord::QueryRecorder.new do
+          subject.execute
+        end.count
+
+        expect { subject.execute }.not_to exceed_query_limit(control_count)
+      end
     end
   end
 end