Merge branch 'osw-merge-request-metrics-population-scheduler-improvement' into 'master'

Remove unnecessary queries on Merge Request Metrics population scheduler

Closes gitlab-org/release/tasks#13

See merge request gitlab-org/gitlab-ce!16292

db/post_migrate/20171128214150_schedule_populate_merge_request_metrics_with_events_data.rb

@@ -15,8 +15,6 @@ class SchedulePopulateMergeRequestMetricsWithEventsData < ActiveRecord::Migratio
   end
 
   def up
-    merge_requests = MergeRequest.where("id IN (#{updatable_merge_requests_union_sql})").reorder(:id)
-
     say 'Scheduling `PopulateMergeRequestMetricsWithEventsData` jobs'
     # It will update around 4_000_000 records in batches of 10_000 merge
     # requests (running between 10 minutes) and should take around 66 hours to complete.
@@ -25,7 +23,7 @@ class SchedulePopulateMergeRequestMetricsWithEventsData < ActiveRecord::Migratio
     #
     # More information about the updates in `PopulateMergeRequestMetricsWithEventsData` class.
     #
-    merge_requests.each_batch(of: BATCH_SIZE) do |relation, index|
+    MergeRequest.all.each_batch(of: BATCH_SIZE) do |relation, index|
       range = relation.pluck('MIN(id)', 'MAX(id)').first
 
       BackgroundMigrationWorker.perform_in(index * 10.minutes, MIGRATION, range)
@@ -37,32 +35,4 @@ class SchedulePopulateMergeRequestMetricsWithEventsData < ActiveRecord::Migratio
     execute "update merge_request_metrics set latest_closed_by_id = null"
     execute "update merge_request_metrics set merged_by_id = null"
   end
-
-  private
-
-  # On staging:
-  # Planning time: 0.682 ms
-  # Execution time: 22033.158 ms
-  #
-  def updatable_merge_requests_union_sql
-    metrics_not_exists_clause =
-      'NOT EXISTS (SELECT 1 FROM merge_request_metrics WHERE merge_request_metrics.merge_request_id = merge_requests.id)'
-
-    without_metrics_data = <<-SQL.strip_heredoc
-      merge_request_metrics.merged_by_id IS NULL OR
-      merge_request_metrics.latest_closed_by_id IS NULL OR
-      merge_request_metrics.latest_closed_at IS NULL
-    SQL
-
-    mrs_without_metrics_record = MergeRequest
-      .where(metrics_not_exists_clause)
-      .select(:id)
-
-    mrs_without_events_data = MergeRequest
-      .joins('INNER JOIN merge_request_metrics ON merge_requests.id = merge_request_metrics.merge_request_id')
-      .where(without_metrics_data)
-      .select(:id)
-
-    Gitlab::SQL::Union.new([mrs_without_metrics_record, mrs_without_events_data]).to_sql
-  end
 end