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.

1 files changed, 7 insertions, 0 deletions

diff --git a/spec/lib/gitlab/sql/union_spec.rb b/spec/lib/gitlab/sql/union_spec.rb
index baf8f6644bf..8026fba9f0a 100644
--- a/spec/lib/gitlab/sql/union_spec.rb
+++ b/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