Fix N+1 when computing batched job time efficiency

Preload batched_migration when computing time_efficiency for
batched migration jobs.

lib/gitlab/database/background_migration/batched_job.rb

@@ -12,17 +12,6 @@ module Gitlab
         MAX_ATTEMPTS = 3
         STUCK_JOBS_TIMEOUT = 1.hour.freeze
 
-        belongs_to :batched_migration, foreign_key: :batched_background_migration_id
-
-        scope :active, -> { where(status: [:pending, :running]) }
-        scope :stuck, -> { active.where('updated_at <= ?', STUCK_JOBS_TIMEOUT.ago) }
-        scope :retriable, -> {
-          failed_jobs = where(status: :failed).where('attempts < ?', MAX_ATTEMPTS)
-
-          from_union([failed_jobs, self.stuck])
-        }
-        scope :except_succeeded, -> { where(status: self.statuses.except(:succeeded).values) }
-
         enum status: {
           pending: 0,
           running: 1,
@@ -30,7 +19,14 @@ module Gitlab
           succeeded: 3
         }
 
+        belongs_to :batched_migration, foreign_key: :batched_background_migration_id
+
+        scope :active, -> { where(status: [:pending, :running]) }
+        scope :stuck, -> { active.where('updated_at <= ?', STUCK_JOBS_TIMEOUT.ago) }
+        scope :retriable, -> { from_union([failed.where('attempts < ?', MAX_ATTEMPTS), self.stuck]) }
+        scope :except_succeeded, -> { where(status: self.statuses.except(:succeeded).values) }
         scope :successful_in_execution_order, -> { where.not(finished_at: nil).succeeded.order(:finished_at) }
+        scope :with_preloads, -> { preload(:batched_migration) }
 
         delegate :job_class, :table_name, :column_name, :job_arguments,
           to: :batched_migration, prefix: :migration

lib/gitlab/database/background_migration/batched_migration.rb

@@ -113,7 +113,7 @@ module Gitlab
         end
 
         def smoothed_time_efficiency(number_of_jobs: 10, alpha: 0.2)
-          jobs = batched_jobs.successful_in_execution_order.reverse_order.limit(number_of_jobs)
+          jobs = batched_jobs.successful_in_execution_order.reverse_order.limit(number_of_jobs).with_preloads
 
           return if jobs.size < number_of_jobs
 

spec/lib/gitlab/database/background_migration/batched_migration_spec.rb

@@ -336,8 +336,8 @@ RSpec.describe Gitlab::Database::BackgroundMigration::BatchedMigration, type: :m
   end
 
   describe '#smoothed_time_efficiency' do
-    let(:migration) { create(:batched_background_migration, interval: 120.seconds) }
-    let(:end_time) { Time.zone.now }
+    let_it_be(:migration) { create(:batched_background_migration, interval: 120.seconds) }
+    let_it_be(:end_time) { Time.zone.now }
 
     around do |example|
       freeze_time do
@@ -345,7 +345,7 @@ RSpec.describe Gitlab::Database::BackgroundMigration::BatchedMigration, type: :m
       end
     end
 
-    let(:common_attrs) do
+    let_it_be(:common_attrs) do
       {
         status: :succeeded,
         batched_migration: migration,
@@ -364,13 +364,14 @@ RSpec.describe Gitlab::Database::BackgroundMigration::BatchedMigration, type: :m
     end
 
     context 'when there are enough jobs' do
-      subject { migration.smoothed_time_efficiency(number_of_jobs: number_of_jobs) }
+      let_it_be(:number_of_jobs) { 10 }
+      let_it_be(:jobs) { create_list(:batched_background_migration_job, number_of_jobs, **common_attrs.merge(batched_migration: migration)) }
 
-      let!(:jobs) { create_list(:batched_background_migration_job, number_of_jobs, **common_attrs.merge(batched_migration: migration)) }
-      let(:number_of_jobs) { 10 }
+      subject { migration.smoothed_time_efficiency(number_of_jobs: number_of_jobs) }
 
       before do
-        expect(migration).to receive_message_chain(:batched_jobs, :successful_in_execution_order, :reverse_order, :limit).with(no_args).with(no_args).with(number_of_jobs).and_return(jobs)
+        expect(migration).to receive_message_chain(:batched_jobs, :successful_in_execution_order, :reverse_order, :limit, :with_preloads)
+                               .and_return(jobs)
       end
 
       def mock_efficiencies(*effs)
@@ -411,6 +412,18 @@ RSpec.describe Gitlab::Database::BackgroundMigration::BatchedMigration, type: :m
         end
       end
     end
+
+    context 'with preloaded batched migration' do
+      it 'avoids N+1' do
+        create_list(:batched_background_migration_job, 11, **common_attrs.merge(started_at: end_time - 10.seconds))
+
+        control = ActiveRecord::QueryRecorder.new do
+          migration.smoothed_time_efficiency(number_of_jobs: 10)
+        end
+
+        expect { migration.smoothed_time_efficiency(number_of_jobs: 11) }.not_to exceed_query_limit(control)
+      end
+    end
   end
 
   describe '#optimize!' do