Use a UNION ALL for getting merge request notesmerge-request-notes-performance

In this particular case the use of UNION ALL leads to a better query
plan compared to using 1 big query that uses an OR statement to combine
different data sources.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/38508 for more
information.

1 files changed, 14 insertions, 8 deletions

diff --git a/app/models/merge_request.rb b/app/models/merge_request.rb
index 8d9a30397a9..15b0c24d6f2 100644
--- a/app/models/merge_request.rb
+++ b/app/models/merge_request.rb
@@ -552,14 +552,20 @@ class MergeRequest < ActiveRecord::Base
     commits_for_notes_limit = 100
     commit_ids = commit_shas.take(commits_for_notes_limit)
 
-    Note.where(
-      "(project_id = :target_project_id AND noteable_type = 'MergeRequest' AND noteable_id = :mr_id) OR" +
-      "((project_id = :source_project_id OR project_id = :target_project_id) AND noteable_type = 'Commit' AND commit_id IN (:commit_ids))",
-      mr_id: id,
-      commit_ids: commit_ids,
-      target_project_id: target_project_id,
-      source_project_id: source_project_id
-    )
+    commit_notes = Note
+      .except(:order)
+      .where(project_id: [source_project_id, target_project_id])
+      .where(noteable_type: 'Commit', commit_id: commit_ids)
+
+    # We're using a UNION ALL here since this results in better performance
+    # compared to using OR statements. We're using UNION ALL since the queries
+    # used won't produce any duplicates (e.g. a note for a commit can't also be
+    # a note for an MR).
+    union = Gitlab::SQL::Union
+      .new([notes, commit_notes], remove_duplicates: false)
+      .to_sql
+
+    Note.from("(#{union}) #{Note.table_name}")
   end
 
   alias_method :discussion_notes, :related_notes