Use a UNION in MergeRequest.in_projects

The OR condition for source_project_id/target_project_id leads to a
query plan that performs rather poorly on PostgreSQL due to the use of
sub-queries. Because Rails offers no easy alternative for this
particular problem we're forced to using a UNION for both conditions.
The resulting query performs much faster than just using an OR.

1 files changed, 6 insertions, 0 deletions

diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 59c40922abb..8bf68013fd2 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -80,6 +80,12 @@ describe MergeRequest, models: true do
     it { is_expected.to respond_to(:merge_when_build_succeeds) }
   end
 
+  describe '.in_projects' do
+    it 'returns the merge requests for a set of projects' do
+      expect(described_class.in_projects(Project.all)).to eq([subject])
+    end
+  end
+
   describe '#to_reference' do
     it 'returns a String reference to the object' do
       expect(subject.to_reference).to eq "!#{subject.iid}"