Eliminate N+1 queries for pipeline GraphQL endpoint

The 'metadata' relation is the most common N+1 query.
The 'downstream_pipeline' relation is for bridge jobs, not so common.

In this commit, the structure of preloading is also changed.

Changelog: performance

app/graphql/types/ci/stage_type.rb

@@ -52,13 +52,20 @@ def groups(lookahead:)
       # rubocop: disable CodeReuse/ActiveRecord
       def jobs_for_pipeline(pipeline, stage_ids, include_needs)
-        builds_results = pipeline.latest_builds.where(stage_id: stage_ids).preload(:job_artifacts, :project)
-        bridges_results = pipeline.bridges.where(stage_id: stage_ids).preload(:project)
-        builds_results = builds_results.preload(:needs) if include_needs
-        bridges_results = bridges_results.preload(:needs) if include_needs
-        commit_status_results = pipeline.latest_statuses.where(stage_id: stage_ids)
-        results = builds_results | bridges_results | commit_status_results
+        results = pipeline.statuses.latest
+        common_relations = [:project]
+        common_relations << :needs if include_needs
+
+        preloaders = {
+          ::Ci::Build => [:metadata, :job_artifacts],
+          ::Ci::Bridge => [:metadata, :downstream_pipeline],
+          ::GenericCommitStatus => []
+        }
+
+        preloaders.each do |klass, relations|
+          ActiveRecord::Associations::Preloader.new.preload(results.select { |job| job.is_a?(klass) }, relations + common_relations)
+        end
         results.group_by(&:stage_id)
       end

app/models/ci/pipeline.rb

@@ -66,6 +66,7 @@ class Pipeline < Ci::ApplicationRecord
     has_many :processables, class_name: 'Ci::Processable', foreign_key: :commit_id, inverse_of: :pipeline
     has_many :bridges, class_name: 'Ci::Bridge', foreign_key: :commit_id, inverse_of: :pipeline
     has_many :builds, foreign_key: :commit_id, inverse_of: :pipeline
+    has_many :generic_commit_statuses, foreign_key: :commit_id, inverse_of: :pipeline, class_name: 'GenericCommitStatus'
     has_many :job_artifacts, through: :builds
     has_many :trigger_requests, dependent: :destroy, foreign_key: :commit_id # rubocop:disable Cop/ActiveRecordDependent
     has_many :variables, class_name: 'Ci::PipelineVariable'

spec/requests/api/graphql/project/pipeline_spec.rb

@@ -311,6 +311,10 @@ def successful_pipeline
     end
     it 'does not generate N+1 queries', :request_store, :use_sql_query_cache do
+      # create extra statuses
+      create(:generic_commit_status, :pending, name: 'generic-build-a', pipeline: pipeline, stage_idx: 0, stage: 'build')
+      create(:ci_bridge, :failed, name: 'deploy-a', pipeline: pipeline, stage_idx: 2, stage: 'deploy')
+
       # warm up
       post_graphql(query, current_user: current_user)
@@ -318,9 +322,11 @@ def successful_pipeline
         post_graphql(query, current_user: current_user)
       end
-      create(:ci_build, name: 'test-a', pipeline: pipeline, stage_idx: 1, stage: 'test')
-      create(:ci_build, name: 'test-b', pipeline: pipeline, stage_idx: 1, stage: 'test')
-      create(:ci_build, name: 'deploy-a', pipeline: pipeline, stage_idx: 2, stage: 'deploy')
+      create(:generic_commit_status, :pending, name: 'generic-build-b', pipeline: pipeline, stage_idx: 0, stage: 'build')
+      create(:ci_build, :failed, name: 'test-a', pipeline: pipeline, stage_idx: 1, stage: 'test')
+      create(:ci_build, :running, name: 'test-b', pipeline: pipeline, stage_idx: 1, stage: 'test')
+      create(:ci_build, :pending, name: 'deploy-b', pipeline: pipeline, stage_idx: 2, stage: 'deploy')
+      create(:ci_bridge, :failed, name: 'deploy-c', pipeline: pipeline, stage_idx: 2, stage: 'deploy')
       expect do
         post_graphql(query, current_user: current_user)