Improve merge_request graphql endpoint

Fallback to denormalized metrics values if available
instead of calling for gitaly

app/graphql/resolvers/concerns/resolves_merge_requests.rb

@@ -44,6 +44,7 @@ module ResolvesMergeRequests
       author: [:author],
       merged_at: [:metrics],
       commit_count: [:metrics],
+      diff_stats_summary: [:metrics],
       approved_by: [:approved_by_users],
       milestone: [:milestone],
       head_pipeline: [:merge_request_diff, { head_pipeline: [:merge_request] }]

app/graphql/types/merge_request_type.rb

@@ -170,6 +170,12 @@ module Types
     end
 
     def diff_stats_summary
+      metrics = object.metrics
+
+      if metrics && metrics.added_lines && metrics.removed_lines
+        return { additions: metrics.added_lines, deletions: metrics.removed_lines, file_count: object.merge_request_diff&.files_count || 0 }
+      end
+
       nil_stats = { additions: 0, deletions: 0, file_count: 0 }
       return nil_stats unless object.diff_stats.present?
 

changelogs/unreleased/239090-fix-throughput-chart.yml

+---
+title: Optionally use merge request metrics association for merge request diff stats
+  in GraphQL
+merge_request: 44613
+author:
+type: performance

spec/graphql/types/merge_request_type_spec.rb

@@ -38,4 +38,40 @@ RSpec.describe GitlabSchema.types['MergeRequest'] do
 
     expect(described_class).to have_graphql_fields(*expected_fields)
   end
+
+  describe '#diff_stats_summary' do
+    subject { GitlabSchema.execute(query, context: { current_user: current_user }).as_json }
+
+    let(:current_user) { create :admin }
+    let(:query) do
+      %(
+        {
+          project(fullPath: "#{project.full_path}") {
+            mergeRequests {
+              nodes {
+                diffStatsSummary {
+                  additions, deletions
+                }
+              }
+            }
+          }
+        }
+      )
+    end
+
+    let(:project) { create(:project, :public) }
+    let(:merge_request) { create(:merge_request, target_project: project, source_project: project) }
+
+    let(:response) { subject.dig('data', 'project', 'mergeRequests', 'nodes').first['diffStatsSummary'] }
+
+    context 'when MR metrics has additions and deletions' do
+      before do
+        merge_request.metrics.update!(added_lines: 5, removed_lines: 8)
+      end
+
+      it 'pulls out data from metrics object' do
+        expect(response).to match('additions' => 5, 'deletions' => 8)
+      end
+    end
+  end
 end