Base job order on finished_at time

changelogs/unreleased/ab-optimize-batched-migrations.yml

+---
+title: Add index to support execution time order for batched migration jobs
+merge_request: 60133
+author:
+type: other

db/migrate/20210427094931_add_execution_order_index_to_batched_background_migration_jobs.rb

+# frozen_string_literal: true
+
+class AddExecutionOrderIndexToBatchedBackgroundMigrationJobs < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_migration_jobs_on_migration_id_and_finished_at'
+
+  def up
+    add_concurrent_index :batched_background_migration_jobs, %i(batched_background_migration_id finished_at), name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :batched_background_migration_jobs, INDEX_NAME
+  end
+end

db/schema_migrations/20210427094931

+aa0ae491a7f94d99ea0c42250434245a4f23b0084657b709b0aaad0317dfd6b1
\ No newline at end of file

db/structure.sql

@@ -23263,6 +23263,8 @@ CREATE INDEX index_metrics_dashboard_annotations_on_timespan_end ON metrics_dash
 
 CREATE INDEX index_metrics_users_starred_dashboards_on_project_id ON metrics_users_starred_dashboards USING btree (project_id);
 
+CREATE INDEX index_migration_jobs_on_migration_id_and_finished_at ON batched_background_migration_jobs USING btree (batched_background_migration_id, finished_at);
+
 CREATE INDEX index_milestone_releases_on_release_id ON milestone_releases USING btree (release_id);
 
 CREATE INDEX index_milestones_on_description_trigram ON milestones USING gin (description gin_trgm_ops);

lib/gitlab/database/background_migration/batched_job.rb

@@ -15,6 +15,8 @@ module Gitlab
           succeeded: 3
         }
 
+        scope :successful_in_execution_order, -> { where.not(finished_at: nil).succeeded.order(:finished_at) }
+
         delegate :aborted?, :job_class, :table_name, :column_name, :job_arguments,
           to: :batched_migration, prefix: :migration
 

lib/gitlab/database/background_migration/batched_migration.rb

@@ -13,9 +13,6 @@ module Gitlab
         has_one :last_job, -> { order(id: :desc) },
           class_name: 'Gitlab::Database::BackgroundMigration::BatchedJob',
           foreign_key: :batched_background_migration_id
-        has_many :last_jobs, -> { order(id: :desc) },
-          class_name: 'Gitlab::Database::BackgroundMigration::BatchedJob',
-          foreign_key: :batched_background_migration_id
 
         scope :queue_order, -> { order(id: :asc) }
 
@@ -81,7 +78,7 @@ module Gitlab
         end
 
         def smoothed_time_efficiency(number_of_jobs: 10, alpha: 0.2)
-          jobs = last_jobs.succeeded.limit(number_of_jobs)
+          jobs = batched_jobs.successful_in_execution_order.reverse_order.limit(number_of_jobs)
 
           return if jobs.size < number_of_jobs
 

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

@@ -268,7 +268,7 @@ RSpec.describe Gitlab::Database::BackgroundMigration::BatchedMigration, type: :m
       let(:number_of_jobs) { 10 }
 
       before do
-        expect(migration).to receive_message_chain(:last_jobs, :succeeded, :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(no_args).with(no_args).with(number_of_jobs).and_return(jobs)
       end
 
       def mock_efficiencies(*effs)