Add index before going through builds in a migration

db/post_migrate/20180212101928_schedule_build_stage_migration.rb

@@ -15,12 +15,17 @@ class ScheduleBuildStageMigration < ActiveRecord::Migration
   def up
     disable_statement_timeout
 
+    add_concurrent_index(:ci_builds, :stage_id, where: 'stage_id IS NULL',
+                                                name: 'tmp_stage_id_partial_null_index')
+
     Build.where('stage_id IS NULL').tap do |relation|
       queue_background_migration_jobs_by_range_at_intervals(relation,
                                                             MIGRATION,
                                                             5.minutes,
                                                             batch_size: BATCH_SIZE)
     end
+
+    remove_concurrent_index_by_name(:ci_builds, 'tmp_stage_id_partial_null_index')
   end
 
   def down