Use a UNION ALL for getting merge request notes

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.

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

changelogs/unreleased/merge-request-notes-performance.yml

+---
+title: Use a UNION ALL for getting merge request notes
+merge_request:
+author:
+type: other

lib/gitlab/sql/union.rb

@@ -12,8 +12,9 @@ module Gitlab
     #
     #     Project.where("id IN (#{sql})")
     class Union
-      def initialize(relations)
+      def initialize(relations, remove_duplicates: true)
         @relations = relations
+        @remove_duplicates = remove_duplicates
       end
 
       def to_sql
@@ -25,7 +26,11 @@ module Gitlab
           @relations.map { |rel| rel.reorder(nil).to_sql }.reject(&:blank?)
         end
 
-        fragments.join("\nUNION\n")
+        fragments.join("\n#{union_keyword}\n")
+      end
+
+      def union_keyword
+        @remove_duplicates ? 'UNION' : 'UNION ALL'
       end
     end
   end

spec/lib/gitlab/sql/union_spec.rb

@@ -22,5 +22,12 @@ describe Gitlab::SQL::Union do
       expect {User.where("users.id IN (#{union.to_sql})").to_a}.not_to raise_error
       expect(union.to_sql).to eq("#{to_sql(relation_1)}\nUNION\n#{to_sql(relation_2)}")
     end
+
+    it 'uses UNION ALL when removing duplicates is disabled' do
+      union = described_class
+        .new([relation_1, relation_2], remove_duplicates: false)
+
+      expect(union.to_sql).to include('UNION ALL')
+    end
   end
 end